Category Archives: 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 –

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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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.


So, thats what I intend to implement.

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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. Continue reading

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

As I have a vested 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. Continue reading

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!

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