Case study
What an EV and solar package actually costs a Gauteng household
This is a worked example, not a real client. No client is named, and no client has given permission for their details to be published. Every figure below is computed by the same calculator this site uses, from the stated assumptions — change the assumptions and the answer changes.
The household we modelled
Sunlight for this roof averages 144.5 kWh per kWp a month, between 127 in the weakest month and 162 in the strongest, fetched from PVGIS (EU JRC) monthly yield for the worked-example site, fetched 21 Oct 2026.. Finance terms are the calculator's starting figures — 10% over 72 months for solar, 11.75% over 72 months with no balloon for the cars — not lender-quoted offers.
The four monthly budgets
The first comparison holds the monthly car repayment the same for both cars (R7 000), so it isolates fuel, electricity and solar finance. That is the view the calculator opens on.
Same monthly repayment for both cars
| Situation | Car | Solar | Fuel, grid & public charging | Monthly total |
|---|---|---|---|---|
| Petrol/diesel + grid | R7 000 | R0 | R6 630 | R13 630 |
| EV + grid | R7 000 | R0 | R4 378 | R11 378 |
| Petrol/diesel + solar | R7 000 | R2 508 | R3 529 | R13 038 |
| EV + solar | R7 000 | R2 508 | R733 | R10 241 |
EV + solar saves R3 388 a month less than petrol + grid (24.9%)
Now finance each car in full
The example EV costs R200 000 more than the petrol car. Borrow that difference over 72 months at 11.75% and it adds R3 884 a month on its own.
Each car financed in full at its listed price
| Situation | Car | Solar | Fuel, grid & public charging | Monthly total |
|---|---|---|---|---|
| Petrol/diesel + grid | R8 763 | R0 | R6 630 | R15 392 |
| EV + grid | R12 647 | R0 | R4 378 | R17 024 |
| Petrol/diesel + solar | R8 763 | R2 508 | R3 529 | R14 800 |
| EV + solar | R12 647 | R2 508 | R733 | R15 888 |
EV + solar costs R496 a month more than petrol + grid (3.2%)
Why the answer flips
- Running the example EV on home-charged grid electricity costs about R1 163 a month against R3 403 for petrol — an energy saving of about R2 240.
- Solar on its own, with the petrol car kept, saves about R592 a month — real, but modest.
- Financing the example EV's higher price costs about R3 884 a month, which is larger than the energy saving. That is why the second table tips negative.
- So the honest summary is: solar helps, and a cheaper EV helps more. An EV that costs the same to buy as the petrol car it replaces is where the saving is real.
Checking it hour by hour
The tables above assume a share of solar is used on site. Modelling the car's charging against each hour of the day's solar output is stricter, because the car is often away when the sun is up.
Down from R3 388 on the simpler model — the saving is still there, just smaller.
The gap widens: on this stricter view the example EV + solar package costs more than keeping the petrol car on the grid.
What would change this answer
- A cheaper EV. Anything under about R370 346 changes the second table's direction.
- Your tariff. This roof pays Ekurhuleni's flat R3.8755/kWh. A lower tariff shrinks the solar saving; a higher one grows it.
- Your roof. Shading, a south-facing slope or a small array all cut the 144.5 kWh/kWp this example enjoys.
- Your driving. At 500 km a month the energy saving is a third of this; at 3,000 km it is double.
- Backup power. Keeping the charger running through a load-shedding outage needs a backed-up circuit and enough battery — extra cost that isn't in these tables.
Put your own roof and car in the picture.
The calculator runs these same four budgets on your numbers, and the quote page sizes solar for your roof.
Figures are estimates from the stated assumptions and the sources named on the calculator. They are not a quote, a financing offer or advice. Vehicle prices, petrol prices and finance terms are the calculator's starting assumptions, not live lender quotes.