Calculator
EV Charging from Solar Calculator
An electric car is the largest controllable load most households own, and it is the single most effective way to raise self-consumption. The catch is timing: the car is often away when the sun is out.
This calculator estimates how much of the annual charging demand a given array can realistically cover, in kilowatt-hours, kilometres and money.
Enter the vehicle and array
AC-DC conversion and battery thermal management.
Generation not already consumed by the house.
Energy needed at the meter
2,800 kWh/yr
2,520 kWh at the wheels plus charging losses
Solar energy the car can absorb
1,260 kWh/yr
Share of charging from solar
45%
Solar kilometres per year
6,300 km
Saving versus home grid charging
€239
Against the export income given up
Saving versus public rapid charging
€731
Grid energy still required
1,540 kWh/yr
About 170 hours a year of solar charging at 7.4 kW
How the calculation works
Start at the wheels: distance divided by 100, multiplied by the vehicle's consumption figure. Then add charging losses, because the meter records more than the battery receives — 8 to 12% for AC charging, more in cold weather when the pack is preconditioned.
The solar contribution is capped twice: by the surplus the array actually exports, and by the share of charging that can physically happen during daylight. A car that leaves at 07:30 and returns at 18:30 on weekdays can only charge from solar at weekends and on days it stays home.
The financial saving is the spread between what the energy would have cost and the export revenue given up, exactly as with a battery. Against public rapid charging the spread is far larger, which is why home solar charging is transformative for high-mileage drivers.
The formula
E_wheels = km/100 × c_veh E_meter = E_wheels / (1 − L_charge) E_solar = min(Surplus, E_meter × share_daylight) Saving = E_solar × (p_alternative − p_export) km_solar = E_solar × (1 − L_charge) / c_veh × 100
Assumptions behind the numbers
- The surplus figure is annual and available at the times the car is present; a monthly analysis will show winter shortfalls this model averages away.
- Charging losses are constant. In practice they rise sharply below 5°C and at low charging currents.
- Solar-matched charging control is assumed, so the charger modulates to follow available surplus rather than drawing at full power.
- No time-of-use night tariff is modelled. Where a cheap overnight rate exists, the value of solar charging falls to the difference between that rate and the export price.
- Vehicle consumption is the real-world figure including heating and motorway driving, not the WLTP number.
How to read the result
A household with a car at home during the day can commonly cover 50 to 70% of its charging from solar. A commuter household without weekday access typically manages 20 to 35% without a battery.
Solar-matched charging matters more than array size. A 7.4 kW charger pulling full power from the grid at 15:00 on a bright day does not use solar; a modulating charger following a 2 kW surplus does.
For high-mileage drivers who would otherwise use public rapid charging, the saving per kilowatt-hour is four to eight times the home-tariff case, and the array frequently pays for itself on vehicle charging alone.
Worked example
A household drives 14,000 km a year in a car using 18 kWh/100 km, with 10% charging losses, 3,200 kWh of annual solar surplus and about 45% of charging able to happen in daylight. Import is €0.26/kWh, export €0.07/kWh and public rapid charging €0.65/kWh.
Energy at the wheels is 2,520 kWh, or 2,800 kWh at the meter. Daylight-available charging is 1,260 kWh, well inside the 3,200 kWh surplus, so solar covers 45% of the charging — about 6,300 solar kilometres a year.
Against home grid charging that saves 1,260 × €0.19 = €239 a year. Against public rapid charging the same energy is worth €731. The remaining 1,540 kWh still has to come from the grid, ideally on a night tariff.
Mistakes that skew the answer
- Using the WLTP consumption figure, which understates real energy use by 15 to 25% in winter.
- Ignoring charging losses, so the array appears to cover more kilometres than it does.
- Assuming a 7 kW charger uses solar simply because the sun is shining — without surplus-following control it draws mostly grid power.
- Averaging over the year and missing that December surplus is close to zero.
- Comparing solar charging against the retail tariff instead of against the export price forgone, which overstates the saving.
Questions readers ask
- How many solar panels do I need to charge an EV?
- Around 2 to 3 kWp of additional capacity covers a typical 14,000 km a year in southern Europe, and 3.5 to 4.5 kWp in northern Europe. That is capacity dedicated to the car, on top of whatever the household already consumes.
- Can I charge only from surplus solar?
- Yes, with a charger that supports solar-matched or eco charging. It monitors the export meter and modulates the charging current down to the minimum the vehicle accepts, usually 1.4 kW single-phase, pausing when surplus falls below that.
- Is a battery better than charging the car?
- Usually the car is better, because the vehicle battery is already paid for and is far larger. Direct solar charging avoids one round-trip conversion. A home battery only wins when the car is regularly absent during daylight.
- Does vehicle-to-home change the picture?
- Substantially, where it is available. A bidirectional charger turns a 60 kWh vehicle pack into the household's largest storage asset, but it requires a compatible vehicle, a certified bidirectional charger and grid-code approval, and it adds cycles to a warranty you cannot replace cheaply.
- What about charging at work?
- It removes the timing problem entirely and shifts the value to whoever owns that array. If workplace charging is free or cheap, home solar charging saves less, and the array is better sized to household load.