New tool: an honest solar payback calculator for South Africa

Personal Finance
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A little while back Johan and MarkD asked for a solar payback calculator that actually works for South African conditions, instead of the rosy maths the installers quote. I have built it, and here it is.

The whole point of this one is honesty. Most solar calculators assume you use every single kilowatt your panels make, that electricity never gets more expensive, and that your battery lasts forever. None of that holds in real life. So this tool gives you two payback figures side by side, the optimistic one a salesman tends to quote, and the realistic one you are actually likely to get, then it lists exactly what eats the gap between them.

The big honesty levers are all in there.

The power you waste. Without a battery you only save on what you use during daylight, and any surplus is gone unless your municipality pays you to export it, which most do not. The tool sets the export credit to zero by default for that reason.

Panel ageing and battery replacement. Panels slowly make a little less each year, and a lithium battery needs replacing once during the life of the system, so both are built in.

Tariff increases, the one factor that works in your favour, because every unit you make yourself is worth more as prices climb.

The hidden fixed charge. Some municipalities move you onto a higher monthly charge once you register as a solar home, so there is a box for that too.

You set your own tariff off your bill, your panel and battery size, and roughly how much power you use in daylight, and it does the rest, including a twenty year cash flow chart and a clear self consumption figure.

A couple of honest caveats. It is an estimate to sanity check a quote, not financial advice, and the self consumption is worked out with a simple daily approximation rather than hour by hour data. The tariff presets are rough starting points, so please set your own off your bill.

MarkD, you have sized a few of these setups yourself, so I would really value your eye on whether the self consumption side feels right. If anyone spots a number that looks off, tell me and I will tune it. The borehole running cost calculator Johan asked for is next on my list, as a separate tool.

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Right, I had a proper go at this over coffee this morning and put my own setup into it, eight panels feeding a 5kW hybrid inverter with one battery, and the numbers landed about where I expected. That is more than I can say for most of these calculators, which usually have me retired on a yacht by year four.

The self consumption side is spot on, and that is the bit nobody wants to talk about. Folk see a few kilowatts on the roof and reckon they are minting money, but if the wife is at work and the geyser has already had its fill by ten in the morning, a good chunk of that midday sun just goes up in smoke unless you have somewhere to put it. Your tool actually shows that loss instead of pretending it away, which is the whole honesty of it.

One thing from my side of the trade, the higher fixed charge once you register is very real down here in the Cape, people get a fright when that lands. Good that you kept it as a lever and left export on zero, because that is the truth for most municipalities right now.

Now when is the borehole one coming, my brother in Worcester has been nagging me about his pump for months.

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Thank you for building this, it is genuinely refreshing to see honest maths for a change. The angle I keep wishing more of these tools captured is the financing reality. Most of us aren’t paying cash for a full system, so the monthly saving on the bill quietly gets swallowed by the loan or access bond repayment, and the real payback only starts once that is settled. Does your calculator let you factor in a financed install, or is it built around an upfront cost?

The other thing worth flagging for younger professionals in Joburg is that plenty of us live in complexes or sectional title schemes where you simply can’t bolt panels onto a shared roof without trustee approval, and that conversation can take a year. So even with the numbers stacking up beautifully, the admin is its own hurdle. Tools like this still help though, because walking into a body corporate meeting with realistic figures beats arguing with an installer’s brochure.

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Thank you Thandi, that financing point is the honest gap in the tool as it stands, and you have put your finger right on it. At the moment the calculator is built around an upfront cash cost, so the payback year it gives you is the moment the system has paid for itself in pure electricity terms, with no loan sitting on top.

In real life, as you say, the saving on the bill quietly goes to the repayment first, so the clock you actually care about only starts once the finance is settled. The way to read the current result in the meantime is to treat the monthly saving it works out as the money that services your loan, then ask whether that saving comfortably covers the repayment. If it does, the system is carrying itself while you pay it off, which is the position you want to be in. The access bond route usually softens this, because bond money is cheaper than a dedicated solar loan, so the gap is smaller, though the principle is exactly as you describe it.

A proper finance mode is worth adding, a box for the amount financed, the rate and the term, so the chart can show the repayment line against the saving line and give you the real break even rather than the cash one. Let me put it on the list behind the borehole tool, and if you would find it genuinely useful I will bump it up.

On the sectional title side you are completely right, and that is a hurdle no calculator can solve. The most I can hope for is that walking into a trustees meeting with a realistic figure, rather than an installer trying to close a sale, makes the case land better. If there is appetite for it I could even put together a short companion piece on the body corporate approval process itself, because that admin trips up as many people as the maths does.

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Honestly this is the kind of straight talking I wish we had years ago, because the installer who came to our place in Durban had us saving thousands a month on a slide that conveniently skipped the loan part. Thandi’s point hit home for me, our budget runs on the actual repayment, not the theoretical saving, so a tool that tells you the truth before you sign is worth a lot.

We haven’t taken the plunge yet, mostly because every rand at the moment goes to school fees and the grocery bill, but load shedding has us seriously thinking about it again. One thing I’d love to see, does it let you bump up the Eskom tariff a bit each year? Our bill never sits still, it climbs like clockwork, so a system that looks slow to pay back today might look very different once you factor in those increases. :slightly_smiling_face:

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Thanks Ayesha, and yes, that is exactly what it does. There is a yearly tariff increase box, so you set how much you reckon Eskom will climb each year and the chart works it in for you. It is honestly the one lever that helps you, because every unit you make yourself is worth more as the price goes up, so a system that looks slow to pay back today can land a good bit sooner once you allow for those increases.

Set it off your own bill history rather than a round guess if you can, that gives the truest picture. And sort the school fees first, the tool will still be here when the time is right.

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Right, the self consumption point is the one I keep coming back to, because that is where the real money quietly hides. Running Home Assistant on a Pi 4 at home, once you start logging your actual draw against generation you realise how much just feeds back for almost nothing on most municipal tariffs.

A few things that moved my payback closer in practice:

  • Shifting the geyser and dishwasher to mid morning when the panels are pumping
  • Pre heating the geyser off solar instead of letting it pull at 6pm on grid
  • Keeping an eye on battery cycle count, since that quietly sets your replacement clock

On Thandi’s financing question, it might be worth splitting access bond rates from personal loan rates in the model, because the gap is massive. Bond money near prime is a totally different animal to an 18 percent personal loan, and it changes the real payback year hugely. Any chance of even a rough toggle for that?

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Quick update, took the feedback from this thread on board. The tool now has a finance mode (deposit, rate and term, so it shows the real break even, not the cash one), an off grid toggle, and it now counts the interest your cash could have earned. Also a nudge on the tariff field so people use the energy charge per unit, not the whole bill divided by units. Have another look.

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That finance mode is the bit that turns this from interesting into properly useful. I have watched plenty of folk sign on the strength of the cash payback the salesman waves about, then get a fright when the loan repayment lands and the saving is already spoken for. Showing the real break even with the deposit and term worked in is the honest version, and that is the number people should be staring at before they put pen to paper.

The tariff nudge is a good catch too. Half the people I talk to work it off the whole bill divided by units, which is miles out once you account for the fixed charges and the basic charge you pay whether you make your own power or not. Use the energy charge per unit like you say and the maths suddenly starts behaving itself.

Counting the interest the cash could have earned is a nice touch as well, since money sitting in the bank is not doing nothing either. Well sorted, this is the version I would actually hand to someone before they sign.