Author Archives: lawrence

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 🙂

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).

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Being efficient – Roof insulation – keep it cool in summer, and hot in winter!

Energy efficiency is something everyone can aim for – its typically only a minor expense and long term you save. Heck, given our electricity costs, short term we save!

Today, I look at roof insulation.

In Cape Town, we have hot summers, and coldish winters. Sure, it *feels* cold to us, but I’ve spent many a winter in far colder places.
The worst being Harbin at -40c!
I can safely say:
#1 it does freeze before it hits the ground.
#2 don’t eat the yellow snow 🙂

Back to warmer climes though –

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Off-grid

I’ve “almost” completed my house in Noordhoek, and its completely offgrid power-wise.

Currently I have 6 x 300W panels, 2 x 220Ah GEL Lead Acid batteries, an FM80, and a Victron 3KW Multiplus running, which gives me about 2KW of usable battery +- in 16 hr period, or in easier to understand terms – about 125W / hr of usage outside of daytime.

Panels come in from the roof, in 2 strings of 3 panels each.
The FM80 can support 150V / 64A input max, so I have to put 3 panels max per string for safety reasons.
36V x 3 @ 8A x2 / 108V /8A per string.
For a 108V / 16A into the FM80.

This then comes out of the FM80 to a 24V battery bank.
The FM80 charges the in the early mornings, and keeps it topped up during the day if necessary.
The 24V battery bank is connected to a Victron Multiplus 3000/24, which then provides AC out to the house.

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Net Metering and Solar in South Africa

Great overview on Solar Refit issues in South Africa here –

http://www.ee.co.za/wp-content/uploads/2014/11/AMEU-Convention-2014-p44-50.pdf
It doesn’t cover what I consider to be an upcoming major issue that will be faced if the Muni’s / Eskom don’t stop putting roadblocks in the way – complete off-grid migration, but it covers everything else.

Worth reading.

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.

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Getting into Hot water (underwater heating systems)

I’ve just bought a plot of land over near Capri (De Oude Weg), and intend to build a house on it.

One of the things I’d like to do is install an underwater heating system (hydronic heating) for the house. I almost installed one in my apartment in Shanghai a few years ago, and regret not doing it, especially when I visited friends who had done it!

Underfloor heating has literally been around for thousands of years, so its not a new option.
As the romans and greeks noted, heating is best situated at the ground level, as heat rises, and the area’s that we live in will stay warm.

aniradi

So, thats what I intend to implement.

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Going off-grid with a grid-tied inverter aka a Hybrid System

As Eskom has again gone down the load shedding route, and it looks like there won’t be much respite until Medupi et al come online, I thought I’d look at how to take my grid-tied setup to the next level, and incorporate off-grid + batteries so that we have power when the rest of the suburb does not.

I haven’t done that so far, as thats by far the most expensive part of going off-grid!

A flash file explaining a possible setup below:

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Further reading on Solar for those in Cape Town

Some info on what City of Cape Town has been doing regarding feeding back into the grid, for single phase users (hint, not much). As for three phase – nothing..
WDC_project_048

Info on why net generators get charged a base fee (NERSA!), why council has been dragging their feet on renewables, feeding back, and other issues (by the very helpful Brian Jones) –
Brian Jones_Challenges to get RE going in municipalities_CoCT

Latest SSEG application form Grid tied form SSEG

Update

The system has been running nicely for the last 3 months without issue, and we’ve generated a little over 2MW so far!

We still don’t have all the panels on the roof – only the 16 ones we put up last year!

Quite impressed with the yield, and the inverter. Continue reading →