The combination of rooftop solar and an electric vehicle is now the most impactful energy upgrade a homeowner can make. Done right, you drive essentially for free on sunshine. Done poorly — with an undersized solar system, or one that was never designed with EV charging in mind — you simply shift your electricity bill from petrol station to utility company without capturing the real potential savings.
Start with Your Annual Mileage
Before designing a solar system that includes EV charging, you need to know how much energy your vehicle will actually consume. The calculation is straightforward:
Annual EV energy consumption (kWh) = Annual kilometres ÷ Vehicle efficiency (km/kWh)
Modern EVs consume between 14 kWh/100 km (efficient compact models like the Tesla Model 3 RWD) and 25 kWh/100 km (larger SUVs). A useful planning figure is 18 kWh/100 km for a mainstream family EV.
Example: 15,000 km/year at 18 kWh/100 km = 2,700 kWh/year = 7.4 kWh/day
Account for Charging Efficiency
Not all electricity drawn from your solar panels makes it into the car battery. AC Level 2 charging loses 10–15% to charger conversion inefficiency and cable losses. On-board charger (OBC) efficiency varies by vehicle model. Plan for a charging round-trip efficiency of 85% (more conservative than manufacturers often state, accounting for real-world conditions).
Adjusted daily solar requirement for EV: 7.4 kWh ÷ 0.85 = 8.7 kWh/day from solar.
→ Numbers speak louder: calculate your annual solar return and take the guesswork out.
Add Household Loads
Your solar system needs to cover both household consumption and EV charging. Add the two together before applying the sizing formula.
Example total daily load: 15 kWh (household) + 8.7 kWh (EV) = 23.7 kWh/day
Apply the universal sizing formula (see our solar size calculator guide): 23.7 ÷ peak sun hours ÷ efficiency factor
For a northern European location (3.2 PSH, 0.77 efficiency): 23.7 ÷ 3.2 ÷ 0.77 = 9.6 kWp For a southern European location (5.0 PSH, 0.78 efficiency): 23.7 ÷ 5.0 ÷ 0.78 = 6.1 kWp
This confirms an important insight: in lower-irradiance climates, adding an EV to your household load can substantially increase the required system size — from a 4–5 kWp system covering household loads alone to a 9–10 kWp system.
The Timing Challenge
Solar panels generate power during the day. Most EV charging happens overnight (when the car is parked at home). This timing mismatch is the central challenge of solar+EV integration.
Three strategies address it:
1. Daytime charging: Charge the EV during working hours using a scheduled or smart charger. This maximises solar self-consumption but requires the car to be home during the day — practical for those working from home or for second cars.
2. Battery storage bridge: Store midday solar surplus in a home battery, then use that stored energy to charge the EV in the evening. This adds battery cost but maximises self-sufficiency.
3. Smart overnight charging: Accept that some EV charging will use grid power overnight, but optimise by scheduling charging during off-peak tariff periods. Track your solar self-consumption separately from grid usage and size the solar system large enough to cover the total annual energy balance.
Sizing the System for Different Strategies
| Strategy | Additional kWp vs Household-Only | Battery Required | Self-Consumption Rate |
|---|---|---|---|
| Daytime EV charging (WFH) | +20–40% | Optional | High (60–80%) |
| Battery bridge (evening charging) | +30–50% | Yes (10–15 kWh) | High (65–80%) |
| Smart overnight grid backup | +20–30% | No | Moderate (45–60%) |
Inverter and Charger Compatibility
The solar inverter and EV charger need to communicate — or at least co-exist without interference — for optimised charging. Three levels of integration are possible:
Basic (no communication): The charger draws power from the home circuit without any awareness of solar generation. Simple and reliable, but no solar self-consumption optimisation.
CT clamp integration: A current transformer at the grid connection point tells the charger whether the house is exporting surplus power. When export is detected, the charger ramps up. Wallbox Pulsar Plus, Easee, and Ohme all support this mode.
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Direct inverter integration: The charger communicates directly with the inverter (via Modbus, SMA Sunny Home Manager, SolarEdge Energy Hub, or manufacturer-specific API). This enables precise real-time optimisation of charging speed based on instant solar generation and home load.
A Practical Recommendation
If you are buying solar and an EV simultaneously, size your solar system to cover your complete annual energy balance including EV charging — even if you initially plan to charge mostly overnight. Energy tariff structures are changing rapidly, and a larger solar system gives you flexibility to shift to daytime charging or add a battery later without the expense of expanding the array.
A 10–13 kWp system covers the combined needs of an average European household plus one EV in northern climates. In southern Europe or Australia, 7–9 kWp typically achieves the same coverage.