Most solar calculators online quote you a payback that never matches reality. They assume you use every kilowatt your panels make, that prices stand still, that your battery lasts forever and that the cash you sank into it could not have earned a cent anywhere else.

So I built one that does not pretend. You put in your real numbers and it shows you two figures side by side - the rosy payback a salesman quotes and the realistic one you are actually likely to get - then it breaks down exactly what eats the gap. Wasted daytime generation, panel degradation, the battery you will replace once down the line, financing interest and the higher fixed charge some municipalities slap on solar homes.

It is free and there is no signup. Set your own tariff off your bill rather than the presets and be honest about how much you use in daylight and you will get a sensible answer.

Anyone interested can have a look here:

https://southafricafacts.co.za/solar-savings-calculator-south-africa/

Keen to hear if the numbers line up with what you are actually seeing. Share your own setup and payback if you are up for it.

1 hour ago, TheOracle said:

Keen to hear if the numbers line up with what you are actually seeing. Share your own setup and payback if you are up for it.

Nice, but there is one serious flaw in your calculations - TAX. Your "Interest you gave up" will be taxed at (say) 35% per year, whilst your saving for not paying for electricity is totally tax free. Run the numbers again with this taken into account and your investment will look much better.

1 hour ago, HennieL said:

Nice, but there is one serious flaw in your calculations - TAX. Your "Interest you gave up" will be taxed at (say) 35% per year, whilst your saving for not paying for electricity is totally tax free. Run the numbers again with this taken into account and your investment will look much better.

Good point, although the first R23,800 interest income per tax year is exempt from income tax, so it will not affect too many people.

Generally love it. Point of feedback, maybe a thought to display the SA map of daily peak sun hours, as an indication of how much sun hours one can expect in different parts of the country. And extend the inputs range to around 6.5 hrs to include major towns in the North-West part of the country like Upington.

Edited June 10Jun 10 by GreenFields

1 hour ago, GreenFields said:

Generally love it. Point of feedback, maybe a thought to display the SA map of daily peak sun hours, as an indication of how much sun hours one can expect in different parts of the country. And extend the inputs range to around 6.5 hrs to include major towns in the North-West part of the country like Upington.

Thanks for the feedback. The range is extended to 6.5. I also added a location dropdown so you can select your location.

Interesting Calculator

Thank you for creating and sharing this tool it’s a great idea and a much-needed step toward more honest solar payback calculations in South Africa.

I fully support the effort to show the difference between optimistic salesman figures and real-world outcomes.I’ve been completely off-grid for more than 5 years with older JA Solar all-black 365W mono panels that are still performing well.

Here’s some constructive feedback based on long-term practical experience:

Current market pricing example (June 2026) for reference: 24 × 460W JA Solar Bi-facial panels + mountings + accessories

2 × 6kW SOLIS Hybrid Tier-1 inverters

2 × 15.3kWh batteries (≈30.6kWh total)

Panels + inverters + batteries + delivery: R113,000 ex VAT

Installation: R30,000 VAT incl.

Batteries alone: ≈R58,000 VAT incl.

A few questions / suggestions on the assumptions:

Battery Replacement

The tool assumes a relatively high replacement cost (around R180k for a 30kWh bank). Current real prices are significantly lower. Would it be possible to allow users to input their own expected future battery prices?

Self-consumption percentage

The “you actually use only 44–61%” figure seems based on typical grid-tied homes. In a well-sized off-grid system this looks very different. After 5+ years off-grid I achieve close to 100% utilisation on a 24-hour cycle. In summer my batteries are often full by midday and all loads (including aircon) run directly off solar. An off-grid mode with higher utilisation options would be very helpful.

Hardware tiers, lifespan & losses

Equipment varies widely in quality and longevity, for solar panels inverters and batteries you have Tier 1 Tier 2 and Budget ratings. Have you considered adding options for different inverter/battery design lives, warranties, cycle counts based on actual loading, and system losses cables, conversion, etc? Cable sizing can factor in a 5% Plus loss or less than 1% loss on the overall system. Inverter efficiency converting PV to Load and Storing PV including conversion from LV DC to AC and battery conversion losses also varies between different manufacturers and voltage ranges LV vs HV.

Bi-facial gains & new technology

Many modern systems benefit from bi-facial panels (5–20%+ extra yield) and improving panel tech. Including these as optional inputs could add value. As an example simply having a reflective surface below a bi facial panel can mean the difference between 5% and 20% or more yield gain.

AI Optimisation

Premium inverters and aftermarket AI systems are already learning usage patterns, automatically switching loads, and improving over time. Does the model have room to factor in these efficiency gains?

Tariff Increases & Real Costs

With recent hikes already in effect and municipalities adding complex fixed/demand charges, your 10% default looks reasonable.

Opportunity Cost

The 8% discount rate is an important realistic factor. In strong off-grid cases it can be partly offset by high self-consumption, bi-facial gains, and rising utility prices.

Tax Incentives

Section 12B still offers companies 100% deduction in year one. A toggle for business vs private use could be useful.

Value of Reliable Power During Outages

Everyone claims load shedding is over, but in reality it continues under new names like “load shifting” and “maintenance”. Many small towns still face outages of 24–48 hours. For businesses, farmers, online workers and industry, the ability to keep operating can prevent significant income loss.

Factoring in the financial value of uninterrupted power (avoided downtime, spoilage, lost productivity, etc.) would make the calculator even more honest and realistic, especially for commercial and off-grid users.

Overall, this is already a very useful calculator that highlights important realities most tools ignore. Adding more flexibility (off-grid mode, hardware tiers, AI, system losses, and reliability benefits) would make it even stronger and applicable to a wider range of users.

My 2 cents

Edited June 10Jun 10 by Powerforum Store

Thanks, this is genuinely one of the most useful bits of feedback I've had, and coming from five years off grid it carries real weight.

A few quick responses. On battery replacement, the figure is actually a field you can edit, it just defaults high and tracks your bank size, but you're right that the default is steeper than today's pricing, so I'll bring it down closer to what you're seeing. The self-consumption point is fair too. The numbers lean toward a typical grid-tied home, and a properly sized off-grid setup like yours sits much higher, so an off-grid mode with a high utilisation option makes a lot of sense.

The Section 12B business toggle and the value of uninterrupted power are both great calls, especially for farmers and anyone running an income off the system. Bi-facial gains and hardware tiers I'll keep on the list, though I want to add them without making the thing a spreadsheet.

Really appreciate you taking the time. I'll work through these and post back when the next version is up.

That battery figure was the one that needed fixing first. I've dropped the default to about R2,500 a kWh, which is much closer to what you're actually seeing, and it's still a field you can edit to your own view. The off-grid self-consumption point is next on my list.

I have made another update. There's now a proper off-grid mode that shows how much of your usage the system actually covers, so a well-sized setup reads close to 100% instead of being marked down for surplus it curtails on sunny days.

Adding the fixed charge section for when off grid dropped my payback quite a bit. That 1485 rand in fixed charges with Citi power was the reason I got disconnected. And your calculator reinforced it, especially with the long-term money back in my pocket. Being Off-grid was one of the best financial decisions I made in the last year. Only spent about 20k on my battery bank for the 17kWh I have so unless prices more than double in the next couple of years your 43k calculation should be good.

Maybe add in for those that are off-grid their monthly fuel expenses etc if they have a generator. I don't use a generator so your calculator is perfect for someone like me.

49 minutes ago, Denns said:

Maybe add in for those that are off-grid their monthly fuel expenses etc if they have a generator. I don't use a generator so your calculator is perfect for someone like me.

Thanks for the suggestion, Generator fuel cost field added

51 minutes ago, Denns said:

@TheOracle This is turning into the best PV cost calculator on the web - well done.

I think one further positive adjustment could be made by having the user input his approximate co-ordinates (latitude and longitude), azimuth (direction that the panels are facing) and panel tilt angle. These three variables will define the actual Global Tilted Radiation GTI value that the solar panels can produce, and will thus bring the solar harvest calculation still closer to actual.

One more thing - Why do you not include the "Panel ageing" cost in your off-grid calculations? Panels will age and need replacement whether you are using grid power or not...

Also, I think the "Panel aging" cost is way to high. My panels' purchase cost was about 16% of the total investment, and according to your calculator replacing my 14 panels will cost 114% of my total initial investment - that surely cannot be correct.

1 hour ago, HennieL said:

I think one further positive adjustment could be made by having the user input his approximate co-ordinates (latitude and longitude), azimuth (direction that the panels are facing) and panel tilt angle. These three variables will define the actual Global Tilted Radiation GTI value that the solar panels can produce, and will thus bring the solar harvest calculation still closer to actual.

Thanks, that's a good suggestion and you're spot on about GTI being the more accurate route. I went with a slightly lighter version of the same idea. There's now an optional "Expected yearly output (kWh per kWp)" box under the sun setting. You pop your address into the free PVGIS tool (it already works out your latitude, panel direction and tilt) and drop the yearly figure straight in, and the calculator uses that instead of the regional sun estimate. Same accuracy you were after, just without asking everyone to type coordinates. Give it a go and let me know if it does the trick.

40 minutes ago, HennieL said:

One more thing - Why do you not include the "Panel ageing" cost in your off-grid calculations? Panels will age and need replacement whether you are using grid power or not...

Also, I think the "Panel aging" cost is way to high. My panels' purchase cost was about 16% of the total investment, and according to your calculator replacing my 14 panels will cost 114% of my total initial investment - that surely cannot be correct.

Good eye on both. Quick answers: panel ageing is in the off-grid numbers, it's just folded into the "Demand not covered" line (as panels age they make less, which widens exactly that gap) rather than shown separately, so it isn't double-counted. And that "Panel ageing" figure isn't a panel replacement cost, the tool never assumes you replace panels, they're warrantied 25 years plus. It's the savings you slowly lose as ageing panels make a little less power each year, added up over 20 years in future rands, which is why it looks big next to what you paid today. I'm relabelling it to make that clearer.

1 hour ago, HennieL said:

One more thing - Why do you not include the "Panel ageing" cost in your off-grid calculations? Panels will age and need replacement whether you are using grid power or not...

Also, I think the "Panel aging" cost is way to high. My panels' purchase cost was about 16% of the total investment, and according to your calculator replacing my 14 panels will cost 114% of my total initial investment - that surely cannot be correct.

To be honest, panel ageing shouldn't be a concern in 20 years of service. There are many utility-scale PV farms from the 90s, over 3 decades old, still outputting over 80% of their initial rating.

And one look at most datasheets would show that they are rated to have at least 80% of their output after 25 years. So in a 20 year span, one should not be considering replacing panels unles damaged from some weather incident, etc.

And batteries also, after 10 years, one would still have some 80% of the capacity on the cells. So if one is able to get the BMS replaced, you should also theoretically get 15 or 20 years? Maybe I am being optimistic on the batteries but I don't see why not as they are rated for 6000 cycles and will still retain 80% of their capacity after that 6000. Factoring temps etc. One should still see 3000/4000 cycles which is 10 years basically and it would be up to them to replace the battery or not but no real reason you cant continue using the battery after that.

59 minutes ago, TheOracle said:

You pop your address into the free PVGIS tool (it already works out your latitude, panel direction and tilt) and drop the yearly figure straight in, and the calculator uses that instead of the regional sun estimate. Same accuracy you were after, just without asking everyone to type coordinates.

Not quite - unless I tell you my actual panel tilt angle and azimuth, someone (PVGIS?) must make an assumption by using the optimum values - and unfortunately there are very few roofs with slopes constructed at exactly the correct angle for optimum solar yield (there wasn't even such a term when my old house was built in the early 1970s...) Also, thanks to clever town planners, many roads do not run directly east-west or north-south, and many houses are built at less than optimum azimuth angles as well. I accept that your town + area location is probably close enough though, but someone placing panels on a roof in Bloemfontein that only has a slope of 10 degrees from horisontal will have a significant loss in solar harvest potential, compared to a roof of (say) 30 degrees - and the same goes for azimuth angle...

5 minutes ago, HennieL said:

I accept that your town + area location is probably close enough though, but someone placing panels on a roof in Bloemfontein that only has a slope of 10 degrees from horisontal will have a significant loss in solar harvest potential, compared to a roof of (say) 30 degrees - and the same goes for azimuth angle...

OK, I quickly ran the numbers via Open-Meteo (A really great site for all things weather related...) using a 20 degree west of north azimuth and a roof/PV panels slope of 15 degrees, and with my 14 panels this gives a solar harvest loss of some 2.9kWh at peak production - see images below (Figures are historic values for Tuesday 9 June 2026)

At my actual 27 degree slope and facing true north (-180 degrees):

At an assumed slope angle of 15 degrees and facing 20 degrees west of true north:

Just on another note added to @HennieL statement.

Your Solar panel calculation degradation what values are you basing that on?

Typically solar panel technology is constantly improving

So old panels Polly type was sitting around 17 to 19 % the mono pushed up to 20/21% then Bifacial improved to 22/ 23% then BC tech improved to 24% and upwards still increasing.

Degradation also improved significantly currently between 87/89% after 30 years before it was 25 years and before that 20 years.

With that said age of the panels and the type of panel will factor in here on your calculator.

Already proven that Bifacial has huge advantages over Mono Facial panels they run cooler and are more efficient and can perform between 5 to 25 % better than a monofacial panel watt for watt if the surface below the Bi Facial is white or reflective they also perform far better in low light and bad pv angles with significant advantages also nano tech coatings on the new panels prevent soiling etc now adding BC tech this further reduces heat and makes the panels even more efficient.

So in essence when creating a new system we always factor degradation of solar panels in initially this would have been 20% larger to compensate for degradation after 20 years however this model has changed significantly because new models now are over 30 years with degradation as low as 12% over 30 years and I believe this will improve further into the future.

The same as battery tech and inverter tech.

I hope this adds to your awesome calculator.

Thanks for all the feedback, really appreciate it.

The 0.5% default comes from the NREL field studies, which track what installed panels actually do over decades, and the median lands right around half a percent a year. The datasheet curves on new Tier 1 panels are lower, around 0.4%, but those are warranty floors rather than measured performance, so I kept the field number as the default.

That said, it's a slider, not a fixed assumption. You can set anything from 0.3 to 0.8, so new BC or bifacial panels can run at 0.3 or 0.4 and older poly at the top end. I've also updated the note under the slider to explain exactly this, what the default is based on and when to move it.

On bifacial gains, the cleanest way to capture those is the yearly output override box. Put in the kWh per kWp your system actually achieves and the gain is baked in, without me having to guess your reflective surface.

Excellent the figures are looking far more realistic well done.

2 hours ago, Powerforum Store said:

Excellent the figures are looking far more realistic well done.

I agree - after inputting the expected solar harvest (kWh/kWp), I get close to the same payback period (6.1 years) that my manual ROI calculation gives (I'm currently standing on 30% of total cost recovered in 1.75 years, extrapolated to 100% = 5.8 years...) - this really is well done 👏