Category Archives: Theory

Theory

Tesla Powerwall Payback Math for South Africa

The Tesla PowerWall is finally becoming available in South Africa, so I thought I’d do some “30% for math” calculations to work out payback periods.

This post is mostly inspired by the comments here – http://mybroadband.co.za/news/energy/154888-here-are-the-tesla-powerwall-systems-you-can-buy-in-south-africa.html

Thanks to MacAfrican for the criticism in my comments, otherwise I’d be too lazy to do this post ๐Ÿ™‚

Onto the math –

Predicted cost at the moment for a daily use PowerWall is $4000.
Rand is currently hovering at R16 (lets hope Zuma doesn’t open his mouth in the near future, as that historically has lead to large drops in Rand values).

Assuming R16 x 4000$, we have a cost of R64,000 for the battery itself.
Obviously thats a large sum of money.

Does it make sense / cents to buy one?

Lets have a look. First up we need to try to work out total lifetime costs.

The warranty for the Tesla on the NaturalSolar.com.au site indicates this

The Tesla Powerwall includes a 10 year limited warranty.
The limited warranty covers defects in parts and workmanship, as well as at least 60% energy retention after 10 years, provided it is registered and used as intended.
and
The Tesla Powerwall is designed for daily use applications like self-consumption of solar and load shifting. Assuming full daily cycles, Tesla Powerwall is designed to provide energy for 3650 full equivalent cycles which is equivalent to 10 years of use.

Thats interesting, as it now gives us an indication of cycle usage.
From that, I can infer that each year we’ll see a drop of around 5% in capacity.

So year 2, we’ll see 95% of original capacity, year 10 down to 60% of original capacity, and at say Year 15, around 30% of original capacity. At year 15, I’d probably want to replace the unit, or have it as a secondary storage device..

With that in mind, we can do some math!

I’ve made a basic spreadsheet using those figures and worked out payback periods for the units.

I can’t predict Eskom pricing, so I’ve gone with current CoCT pricing per KW, and worked with annual % increase’s.
Total lifetime I’ve kept to 15 years, although you could probably scrape another year or two out of the units. I expect battery replacements to at least have halved in current Rand / Dollar terms in 10 years though, so replacement should be cheaper assuming Zuma doesn’t do any more Nene’s..

Below is what it looks like for a 5% annual increase

Screen Shot 2016-02-15 at 11.50.25 AM

You’ll see that it currently doesn’t make sense to use a PowerWall at a yearly 5% increase, even at a 15 year time frame. It comes close, but no cigar..

What happens at 10%?

10% increase

At a 10% annual increase (which might be closer to what Eskom pricing will eventually be than at 5%), we see breakeven in the 12th year of ownership. By 15 years we’re safely into profit.

Lets look at a best case – well, “worst case” scenario with a yearly 15% increase:

15% increase

15% annual increase see’s break even at Year 10.

Its unlikely that we’ll see continued 15% increases though, I guesstimate using thumbsuck that we’ll see continual annual increases of 8%, which leaves us breaking even at around Year 13.

Feel free to play around with the values, I’ve uploaded the Numbers file here (as I’m a larney Mac user), or as an Excel sheet here.

In other news, am fully expecting Rich from HomeBug to critique this, hehe ๐Ÿ˜‰

Some points to note:

The Rand Dollar rate is going to be the main cost influence on whether the PowerWall makes sense. If the rand drops further (and the indications are that it will), then it doesn’t make sense at R20/ dollar. If by some miracle the rand recovers to say R14 or R12 to the dollar, buying a PowerWall is a no-brainer.

NERSA approved increases may or may not beat my guesstimates. Historically we’re much more expensive per KW than 10 years ago by a large factor, so its likely that a moderate value of 10% increase per annum is going to correlate with actual figures. This will also increase once Eskom/ Muni’s introduce further daily connection fee’s and other non tariff increases on top of per KW pricing.
(Actual historical figures can be found here – http://www.eskom.co.za/CustomerCare/TariffsAndCharges/Pages/Tariff_History.aspx )

I don’t calculate round trip values for Electricity in /out of the PowerWall. Tesla documentation indicates that this is 92%, so final KW generation figures probably should be discounted by 8% for further accuracy.

I also assume you’ll be generating electricity to go into the unit from a solar install. Costs for that are not included, as we are looking purely at the viability of the PowerWall. While I can do full system calculations, its already clear that Solar generation is already cheaper than Eskom in South Africa, and has been for a few years no. Rehashing that again is of no interest to me.

Eskom / Municipality vs Solar pricing Maths.

One of the age old questions I get asked is this – Does solar make cents(sic)?
The smartass answer is of course “it depends”.

Eskom is fast turning that answer into “extremely well” though.

Looking at the math, the average household with say 1100KW usage a month or R1800 a month average bill in Cape Town pays these rates:

600KW @ 1.56 = R936
500KW @ 1.86 = R930 (over 600KW is billed at higher rates)

Monthly thats R1866. Lets round that down to R1800 for ease of use.

1100KW / month is 36KW / day.

36KW daily usage = 1.5KW/hr on average. Ouch.

That’s quite high. Lets bring that down. We did.

First steps

Install solar hot water heating for hot water (and pool heating if you have a pool).

That should bring our bill down about 40-50%, as heating water is a major consumer of electricity.
Install gas for cooking. (We didn’t as we don’t cook that often, and it didn’t make sense in our situation)
Install LED lighting instead of power sucking halogens and regular bulbs.
We should be looking at closer to 20KW day now.

You’ll probably have spent up to R30,000 on that.

Good.

Lesson #1
Its ALWAYS cheaper to first reduce costs before going solar.
Our best bang per buck is *always* to reduce our monthly usage first.

In our case, we installed 2 x 150L solar hot water heaters.
Replaced *all* the lighting with LED’s. Don’t forget the outdoor security lighting!
Our gate light was on 12hrs a day. We replaced it with 10W LED lighting.

Outdoor security lighting @ 10W /hr @ 12hrs vs Halogen @ 150W @ 12hrs= *major savings*.
eg 120W usage for 12hrs, vs 1800W!
One 10W LED run for 12Hrs a day = 120W = (monthly 30 x 12Hrs) = R5.4
One 150W Halogen run for 12Hrs a day = R2.7 a *day*, or R81 a month.

Imagine that for all the lights in the house. If you have Halogen downlights get rid of them, and get LED ones. Takes less than a month *per* bulb for payback time…

After all that, our electricity usage went down from 1100KW/ month to about 600KW month.
That’s a 3 year payback on investment if it’s similar to our R30,000 cost.

Sure, but thats got nothing to do with Solar I hear you say.
Well, yes it does. Again, *reduce your footprint* first.

Second steps

Install some PV!

180k will get the average house with 20kw daily usage offgrid including batteries in todays money.
(Say about 5kw panels on the roof, and 30kw of battery, plus a 3kw backup generator to cater for repeated winter outages past 2 days of no sun, and all inverters etc for a single phase household)

Some Math / Justification on that
20KW daily usage = 830W/hr on average.
5KW panels will generate over 15KW in winter, and well over 30KW in summer daily, so deficit is 5KW/day or zero in summer.

Assuming 5KW / day worst case scenario deficit
You need roughly 3 x (3 days of battery) x 2 (50% discharge) for usage.
– Batteries shouldn’t be drained past 50%, so halve the rated value.
– Cater for 3 days of worst case scenario of no sun.
– Add a generator for generation for worst case scenario getting worse, and batteries go below that point of discharge.

With that in mind, deficit is 5KW odd in winter, so 3 x 5 = 15KW for 3 days of discharge (say 3 days of cloudy weather) x 2 (can’t discharge lead acid/agm/crystal batteries more than 50%) = roughly 30KW required in batteries.

28.8KW of battery can be had for a little over R1/Whr eg / 20x12v@120Ah= 28800W which can be run in 24V or 48V easily (battery inverters usually run in 24v or 48v sizing)

12v@120Ah Gel Lead Acid is currently R1500 at retail, or less, which = R30,000 for 28.8KW per 5 years usage worse case.
The good news is that battery prices are headed down, not up.

5KW of panels looks like 18 panels * 300W
300W panels are in the R11/w range retail, so roughly R60,000

Panels + Batteries = R90,000

MPPT PV Inverter should be about R20,000 (or less)
Mounting + 3KW Generator say R10,000
DC -> AC Battery inverter about R20,000 (or less), oversized so that the system is scalable if necessary.

Total so far – R140,000
Add installation, say another R10,000 (1-2 days of work) and replacement batteries in 5 years, and you’re looking at closer to R180,000 for an offgrid system over a 10 year lifespan.

If you start looking at that over 10 year terms, that’s a lot more affordable, even if you cater in replacing batteries every 5 years.

Our monthly bill is only R1000 a month though at 600KW usage / month. (Usage of R936 + other costs), and R180,000 is closer to R1500 a month. There’s a big discrepancy.

R1000 a month for 600KW x 12 = R12,000
R12,000 x 10 = R 120,000.

Our costs are closer to R180,000.

Sure. It doesn’t make sense. Its 50% more expensive!
However, that’s at todays pricing.

Nersa has granted Eskom a 15%+ increase (and Eskom is asking for more now, as the situation is dire).
15% increase on Eskom pricing means that Year 2 monthly rates are now R1150 for our 20KW/day usage.
The municipality is likely going to add a few % on top of that also, as they’ve asked for 7% also (also to be confirmed).

So year one is R12,000/ R1000 monthly
Year two is R13,800 / R1150 monthly (15% increase, which is looking lower than the actual increase will be).
Year three is R15,180 (assuming a meagre 10% increase on year 2), and so on..

Guess what just happened – our costs have (not so magically) equalised with our investment, and fairly quickly at that. Without guessing whats going to happen for year four to year ten, its already looking like a smart decision to have gone solar. We also have a nice equity in a system that has increased our house value, AND we have a system thats more reliable than Eskom is.

I know we’re happy paying a premium for the first year or two just to have electricity 24/7.

Essentially, if you have a R1500 bill a month in electricity now, and you have the capital, and roof space for it, its roughly time to start looking at going completely offgrid, as it will payoff by the 10 year mark.

I’m happy paying that premium to have reliable electricity in house right now, and I’ll guarantee you that the costs will be cheaper for self generation based than you are billed for electricity within 5 years.

Footnote –
You’ll note that I haven’t looked at feeding back to the grid in the above scenario.

Why?

It doesn’t make financial sense (at least for Cape Town). I’ll leave it up to the reader to discover why, and do the math (or look at the comments on previous articles where someone did the math!).

Three Phase

As my system is 3 Phase, I thought I’d talk about some of the different 3 Phase standards for wiring.

3 Phase, as you may or may not know, is better for transmission of power.

Power stations generate electricity at 22 000 volts (3 phase 50Hz). To transmit this power over long distances, Eskom steps up the power to to the following voltages for transmission: 220kV; 275kV; 400kV or 765kV. This electricity now goes into our national grid.

When it gets to the end user it is stepped down. This could be 11kV for a large factory or 400V(380V) for shops/homes. If you take a phase to neutral (single phase voltage) i.e. 400V/sqrt(3) you will get 230V single phase @ 50Hz.

When it gets to the house, it generally gets split up into single phase, and different circuits get each phase. So, the lights might be on one phase, the plugs on another, and heavy equipment may use all three (eg an old 1950’s Oven dating back to the Union of South Africa!).
Plug sockets at home are single phase 230VAC 50Hz.

There are two main connection standards for 3 phase; Delta wiring – which uses 3 wires, and Y wiring (also known as wYe), which uses 4.

Delta has one wire for each phase so 3 wires total.
Y wiring has one wire for each phase, plus a neutral, for 4 wires.

In my house, we have 4 wires, so its the Y standard.

You can check this by looking at how many wires go from the house to the street.
In our case, this is 4 separate wires, as you can see below:

So, what exactly is 3 phase?

AC current runs in a sine wave. This sine wave runs at 50hz for South Africa (50 ups and downs a second). In 3 phase, each phase is run at an offset of 120 degrees, so each phase peaks at an offset of the other. The 3 phases add up to a total of 380v – although these days is more likely to be closer to 400v, as the rest of the world has migrated to that. Either makes no difference, as they’re both values within the margin of error for provision of electricity.

Its extremely important to know what you have with regards to wiring, as it involves large amp circuits, and you don’t want to wire things incorrectly and cause Eskom to come smack you for tripping the street transformer!

Three phase is relatively easy to turn back into 1 phase – I’ll be doing that on one leg to provide an additional “solar” circuit for our laundry room.

Those of you who like the technical aspects should take a look here –
http://ece.k-state.edu/~starret/581/3phase.html

Some basic calculations for 3 phase below. These can be used to work out your maximum load or other important wiring details, like how thick your electrical wire should be if you’re carrying whatever max current your supply provides…

Basic electrical calculation:

Volts = Watt รท Amps
Volts = Ampere x Ohms
Amps = Volts รท Ohms
Amps = Watt รท Volts
Ohms = Volts รท Amps
Volt-Ampere (VA or Watt) = Volts x Ampere

For 3 phase, we need to use the square root of 3 for our calculations as an additional factor.
The square root of 3 is 1.73 (rounded off to 2 digits).

So, to calculate VA its: Volts x Ampere x 1.73

Example calc:

KVA = (Volts x Ampere x 1.73) รท 1000
= (400 volts x 60 amps x 1.73) รท 1000
= 41520 รท 1000
= 41.52 KVA

We can also work out the Amps as below:

KVA = (Volts x Ampere x 1.73) รท 1000
41.52 KVA = (400 volts x A x 1.73) รท 1000
41.52 x 1000 = 400 x A x 1.73
41520 รท (400 x 1.73) = Amps
41520 รท 692 = 60 Amps

If we wanted to convert KVA to KW, we need to use a power factor (this represents losses in transmission). The figure used for this is typically 0.85, so

KW รท 0.85 = KVA
KVA x 0.85 = KW

Simple!

A more detailed explanation of power factor losses is here – http://www.energyaction.com.au/australian-energy-market/power-factor.html

I have the power! (Ripple Control, and load shedding in power systems)

Screen-Shot-2013-05-26-at-2.59.04-PM
As I have an invested interest in consistent electricity back home (see my other recent post on Solar for details) and have been in discussion with the council about net metering and grid tie, I’ve been doing quite a bit of random reading regarding electricity distribution and its various facets.

Not many of us know that the power company / municipality also uses in-line signalling (aka ripple control) to implement power control and load shedding, so I thought I’d do a little writeup on that.

Many of us have noticed that streetlights don’t always come on, or go off when its light or dark โ€“ they appear to be on a timer system.

What most people don’t know is that the timer system controls are actually implemented centrally at substations, and these add signals to the power lines to tell the equipment to turn off / on when instructed.

This is done using ripple control codes.

With ripple control, a small signal is added to the incoming A/C at a distribution location โ€“ eg a substation. This signal is read by a special relay in place on the larger circuits (typically the Geyser), and turns power off or on when the electricity company requires โ€“ usually when power is scarce, and they need to shed some load.

As this signalling can work on multiple channels, each listening relay can be set to listen to a specific channel, and used to power specific things on / off remotely (e.g. Streetlights).

In South Africa, we use DECABIT signalling to tell things to turn off and on, as well as the older K22 signalling standard.

When load shedding needs to occur, the electricity distribution system needs to act fast to avoid system failures. Most things are automated, and happen in order of timing.

Implementations of the protection mechanisms in place have a specific time to occur โ€“ eg a latency. Responses to conditions also have a latency โ€“ eg getting additional idle power plants online to provide more power when needed, so its important to the grid to have multiple control and response mechanisms to respond to loads. Each response mechanism also has a different cost impact, so its also important to the electricity provider to best manage these.

A diagram of this is below (Excerpted from http://www.anime-za.net/tech/literature/Enermet_Farad.pdf ):

Screen-Shot-2013-05-26-at-3.44.21-PM

For light variances in load, frequency changes as generators speed up or slow down to supply enough electricity to the supply. If there isn’t enough supply to meet load, then frequency drops, and large scale equipment will disconnect until load decreases. This happens almost instantaneously โ€“ responses to these issues resolve with a latency of within a few milliseconds to a second. This is called Under-Frequency Load Shedding (UFLS).

Screen-Shot-2013-05-26-at-3.38.27-PM

As seen in the diagram above Eskom implements automated under frequency load shedding in an increasing percentage margin based off frequency rates.

(Additional details are in the PDF below)

http://www.systemoperator.co.nz/f3210,36010947/Appendix_A_-_A_Collation_of_International_Policies_for_Under_Frequency_Load_Shedding.pdf

The next set of load shedding is the one we’re interested in โ€“ ripple signalling. If the system still has too much load after 1 second, then it sends out a signal over DECABIT to turn off more equipment. DECABIT signalling has a latency of about 7 seconds โ€“ a minimum DECABIT signal frame is 6.6 seconds, so this is a second stage response to issues.

As each substation can be connected to up to 20,000 homes/customers (depending on substation load capacity), this allows localized load shedding where its needed, when its needed.

Eskom calls this Demand Market Participation, and has roughly 800MW of systems added into this mechanism. Municipalities are particularly keen on putting loads onto these mechanisms via DECABIT compliant relays, as this saves them peak power fee’s when loads are high โ€“ if they can temporarily cut off power to consumers for 10 seconds โ€“ 10 minutes for non-essential high loads, then they can substantially reduce what power costs them from Eskom, and make additional profits.

A good writeup on Demand Market Participation is below:

http://www.enerweb.co.za/brochures/AMEU%20Conference%20-%20Enerweb%20VPS%20Paper%20-%20201109%20-%20%20V1.0.pdf

Eskom benefits as they can temporarily avoid adding more infrastructure to cope with growth.

This has been the case for a few years now, but it only delays the inevitable โ€“ you do need to invest in infrastructure, not incentivize clients to use less.

Eskom also has a secondary mechanism (using the same theory โ€“ lets encourage you to turn off power) called VPS. They have an additional 50,000MW of connections using this on a contractual basis โ€“ typically industrial users., and are looking to increase this number.

Its only been through introduction of these mechanisms that we’ve been able to stave off grid collapse. Its gotten so bad, that industrial users have been looking closely at what they can do to provide their own power when Eskom can’t.

Other countries โ€“ notably Germany, and the UK, have allowed consumers to become producers, by encouraging localized small scale production of electricity, thus helping the grid without requiring additional investment from the incumbents. This is called net metering โ€“ where both inputs and outputs are metered.

Eg โ€“ if you have a solar system that provides excess power during the day, it can feed into the grid โ€“ (when it needs it most), and they’ll credit you for your participation.

So far, South Africa has been rather reticent to implement this, as the short sighted vision is that its “stealing” from the incumbents profits.

A choice excerpt from that PDF is this –

Residential load can also be incorporated within the VPS, particularly when integrated with Smart Metering systems. Numerous pilot and small scale projects are being undertaken within both Municipalities and Eskom in response to the DOE’s Regulation 773 of 18 July 2008.

The Department of Energies regulation can be found here โ€“

http://www.energy.gov.za/files/policies/Electricity%20Regulations%20on%20Compulsory%20Norms%20and%20standards%20for%20reticulation%20services%2018Jul2008.pdf

These state that all systems over a certain size require that smart metering be installed by 2012. As you may have guessed, quite a few municipalities have not met this deadline, and Eskom has been dragging its feet on that too.

Ironically, introduction of smart metering would actually help the grid here in South Africa, as IPP’s (independant power producers) would make the grid more stable by providing additional energy when needed, and at a lower cost than the incumbents can create it for.

This however does have its issues โ€“ most municipalities generate revenues from Electricity, and so are loath to change the status quo, even when it would benefit the country from a whole.

So, its unlikely to be implemented in the short-medium term, unless the government drags them kicking and screaming through the process.

In summation, this โ€“

486837_586695331355335_1633072872_n
Lawrence.

โ€”โ€”โ€”

References:

http://en.wikipedia.org/wiki/Zellweger_off-peak

http://www.anime-za.net/tech/ripple_index.html

http://mybroadband.co.za/vb/showthread.php/134334-And-so-I-have-proved-the-ripple-control-system-is-buggered

DECABIT Ripple Signal Guide

Thesis on the financial implications of relaxing frequency control as a mechanism.

http://www.energy.gov.za/files/policies/Electricity%20Regulations%20on%20Compulsory%20Norms%20and%20standards%20for%20reticulation%20services%2018Jul2008.pdf โ€“ DoE Regulations

http://www.enerweb.co.za/brochures/AMEU%20Conference%20-%20Enerweb%20VPS%20Paper%20-%20201109%20-%20%20V1.0.pdf โ€“ Demand Market Participation

http://www.systemoperator.co.nz/f3210,36010947/Appendix_A_-_A_Collation_of_International_Policies_for_Under_Frequency_Load_Shedding.pdf โ€“ Load Shedding in International operators

Going Solar

I’ve been interested in going completely solar for a while now back home in South Africa, as pricing for electricity has rapidly increased past the pricing for solar; return on investment is in the 3 year range currently.

It will get close to 1 1/2 year return on investment when Eskom new pricing increases happen, so its a no brainer to install.

I’ve already replaced our geyser (hot water system) with a solar based system, plus all the lighting in the house is already LED based (yay China!), so our base load of electricity is low for the size of the house. I can still improve though by installing solar, to make the electrical costs approach zero, and at some indeterminate point in the future when Eskom allows for legalized grid tie, a profit center!

As I’ll nominally be a 10KW producer (I can add 2 panels to get there), I should be able to at some point pump back into the grid sooner rather than later โ€“ as the trial projects for Cape Town all sit at the 10KW range and upโ€ฆ

From what I read โ€“

Eskom will pay out R 1.20 per kilowatt hour

generated by your solar system for the first three years, 70% immediately after installation

and the balance at 10% for the next three years.

More here โ€“ http://www.capetown.gov.za/en/EnergyForum/Documents/Eskom%20IDM%20small-scale%20renew%20energy%20-%20Lodine%20Redelinghuys_31Aug2012.pdf

(Yes, currently this is only available for commercial use, but I do expect that to change at some point. My system should be in use for at least 25 years, so I should get some benefit at some point in the future.)

โ€”-

Before I get to pricing though, I need to explain how it all works.

For any system, you’ll need some kind of input.

As I’m looking at Solar, thats my input. I can also look at thermal or wind based. Wind based is a distinct possibility in Cape Town, but I have been advised that its probably too windy to use! (turbines can’t run during extremely windy weather or you break the turbines).

So, I’m going with Solar.

There are 2 types of solar panel out there. Monocrystalline and Polycrystalline.

Monocrystalline is more expensive per watt, as its a more difficult process to make panels from.

Mono panels are also slightly smaller per watt of output. On average mono panels are about 14% smaller. They also work better in hot climates.

Aside from those differences, they’re fairly similar.

Panels are typically rated in watt terms.

A 300w panel will give you 300W of power at peak output (eg mid-day).

This 300w of power is at DC voltage though, and for house use, we need A/C

The 300w panels I’ve been looking at give 36V @ 8.3A.

I’ll probably go with polycrystalline, as the pricing isn’t really worth the extra 30% for mono crystalline for my needs.

Panel info below โ€“ (click me for pdf)

Screen-Shot-2013-05-13-at-5.55.14-PM

Basic calculation for power output is P = V * A

This works out to 300W a panel (303W = 37.6V * 8.06A)

I’ll be getting 30 panels, as thats about the max I can fit on my roof in theory.

Screen-Shot-2013-05-13-at-5.19.09-PM

(My brother hasn’t gotten me the exact sizing yet).

To use this, we need an inverter though, as something has to convert the DC power into AC.

In my house, I have 3 phase power, and an antique metering system.

Screen-Shot-2013-05-13-at-5.08.01-PM

3 Phase is good, as i have sufficient power for my needs, but its bad as I need a more advanced inverter to give me 3 phase.

I could use 3 x single phase inverters, but for simplicity, I’ll be going with a single 3 phase inverter.

If you see the cabling here โ€“ you’ll see we have 3 phases + 1 neutral = 4 cables.

Screen-Shot-2013-05-13-at-5.09.15-PM

To work out what sort of inverter I can use, I need to do some basic math.

I’ll have 30 Panels total.

Each panel gives out 36V @ 8A., and that will give me approximately 9KW output. As the smallest *decent* 3 phase inverters I could find are 10KW ones, thats a good size.

I have a choice of running the panels in series or in parallel.

If I run them in series, then the Voltage increases.

Eg 1 panel = 37v, 2 panels = 74v @ 8Aโ€ฆ

If I run them in parallel, then the Ampage increases.

Eg 1 panel = 8A, 2 panels = 16A @ 37v

If you’ve ever seen welding cables or car battery cables, you’ll see what sort of cabling is required for high Amps. So, everyone wires using DC voltage.

My inverter of choice is probably going to be this: Growatt10000UE

Screen-Shot-2013-05-13-at-5.12.58-PM

That 3 phase inverter has the following characteristics.

It will power up from 300V (min voltage to run), and accepts voltage up to 1000V.

It also has 4 inputs for panels.

Generally each input is called a “string”.

As I’ll have 30 panels, I’ll probably be balancing them out in 2 x 15 piece strings -> the inverter.

Each string will work like this

37V * 15 = 564V DC * 8A (4.54KW of power)

37V * 15 = 564V DC * 8A (4.54KW of power)

This will give me a rough total of 9W peak power.

As conversions are never perfect, and panels can output more during peak than they are rated for, I’m getting a 10KW inverter. This will allow for some small headroom in future if I need to expand slightly.

It also is fine for something I haven’t talked about yet โ€“ open circuit. The panels I’m looking at run at 36v open circuit (i.e. before they kick in), the inverter also needs to be able to work without issue at open circuit voltages. As the inverter supports 1000v, open circuit of 564v isn’t an issue.

So far, costs are:

30 Panels = 720RMB / poly panel = 21,600 (mono panels are about 900-1000 per piece). Poly panels are physically 1.9M x 1M @ 300w / 28KG , Mono 1.9M x 1M @ 300W / 25kg

10KW 3 Phase inverter = 9,000RMB

Weight = 1000KG with packing.

Shipping + clearance โ€“ roughly 15,000 + duties @ 20%

Total landed in Cape Town = 45,000RMB / R60,000

That gives me a rough pricing of R6.6 a watt *installed*.

It also gives me a system that I can hook into the grid (illegally currently!), but won’t provide for power in case of failure.

I actually don’t need something that size, but sadly, due to the cost of clearance being a complete rip off, it doesn’t make sense to ship less

Currently our power bill sits at about 700-1000 rand a month, over a year this is around R12,000 using worst case scenario maths

My intended system will cost me about R60,000 + install labour. At current electricity pricing, I should see a complete payback for the system in about 5 years. Given that electricity prices are going to be *doubling* over the next 5 years in Cape Town, this will actually be achieved in about 3 years or so.

Not too shabby!

Our current monthly electricity usage looks like this for those who may be interested.

Screen-Shot-2013-05-13-at-5.25.54-PM

You’ll note that electricity use spikes on certain days (mainly weekends) โ€“ this generally ties into when the maid is there, as then the washing machine, dish washer etc get run, or on the rare occasion that my brother actually cooks

Initially I’ll be feeding excess power back into the grid, and using that as a “battery”.

How will that work?

Well, as I have an older meter, it can run backwards. So, daytime when I have _substantial_ excess, i’ll be running backwards, and nighttime, when the solar panels are not generating, I’ll be running forwards.

Essentially, using the grid as my battery..

Eskom will be benefiting from all this, as I’ll be a net producer far over what I consume โ€“ so they’ll get all the free electricity I’ll be generating.

Its also safe โ€“ as the inverter will not feed back into the grid if its offline โ€“ eg when we have one of our rather too regular power outages (3 in the last month from my logs).

Longer term I’ll be installing a battery system to allow for complete off grid, but funds don’t currently stretch to that yet..

Do note that the above is for my needs โ€“ your needs might not be my needs!

I need a 3 phase system. Most people _don’t_. I’m also going grid tied for the moment due to funding available. Others might find it better to have a hybrid grid tie/ battery system. If I could afford it, I’d go that route!

I’m also *heavily* overspeccing the output โ€“ clearance costs are substantial for South Africa (highest in the world almost), so it doesn’t make sense for me to ship a small system, as there is only a marginal cost for what I’m speccing.

A suitably sized system for us would be 8 panels, and a 3kw inverter. I’d be crazy to ship that though, as the clearance is more than the cost of the system. So, I’m heavily overspeccing on requirements so that it makes sense. Long term its also a no-brainer for me, as I’ll have substantial excess I can sell back to the grid.

In case anyone is interested how I’ll retrofit this sort of system with a battery backup โ€“ here is a diagram of a single phase implementation โ€“ I’d be doing something similar:

MAGNUM-AC-COUPLED-LINE-DIAGRAM_large

That said, I do have another easier solution โ€“ I’ll probably go cheap โ€“ stick the things that may not lose power(tm) circuit on a 2KV UPS, and have an isolator switch in circuit for when the grid goes down so its isolated from Eskom. This will accomplish the same thing pretty much, and should tide us over for the average 3-4hour outages we seem to experience every few weeks. It will also sit nicely in the computer rack that will contain the media side of the house and data storage needs