TheOracle

The calculator previously did not include the cost of insuring the solar system; I have now added this feature.

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.

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 👏

Excellent the figures are looking far more realistic well done.

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.

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.

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.

@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.

Thanks for the suggestion, Generator fuel cost field added

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.

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.

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.

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.

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.

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

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.

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.

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.

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.

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.

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.

Not sure but that said. I spoke to my brother in Gauteng and he installed a Deye 5kw with a Pylontech US3000C and this is the pinpouts he used with a cable made up by a pc technician.
Deye 5kw. Pylontech US3000C
Inverter 2 -> Pylontech 6
Inverter 4 -> Pylontech 4
Inverter 5 -> Pylontech 5

Before purchasing another CAN cable, strip a piece of network cable use one wire and back probe your existing cable pin 6 to pin 6 with the extra piece of wire check for continuity and see if it solves your comms problem. Then you could decide to purchase a cable with GND. Try Livestainable in Garsfontein Pta.
Livestainable.co.za
3m CAN BMS cable for Solis,sunsynk & Pylontech Lithium ba...
3m Can Bms Cable For Solis,Sunsynk & Pylontech Lithium Batteries

Now just waiting for VE to USB cable.
I wish solar assistant would come back to people I've sent so many emails asking for assistance on this or if this is something in the pipeline etc.
The raspberry pi has Bluetooth soon theory it should be able to talk to the Victron equipment via that protocol too.
 

Good point. For example, the buck transistor (at least one of them) could have shorted. I was thinking that it shot up to 57 V too quickly for that, though. But maybe there was such a short, bringing the voltage up to say 54.5 V, and the BMS decided it had enough of that and disconnected, and that sent the inverter's "battery" voltage to 57 V.
I was also thinking that the battery voltage was jumping around a little too much. Maybe one of the SCC transistors was shorting intermittently, just for seconds at a time, and only when the SoC reaches about 89% does this cause the battery voltage to trigger the BMS to disconnect.
Yes. Overshooting the target voltage that suddenly and that far is not typical Axpert solar charger behaviour.