Charging an electric car from photovoltaics: how to use rooftop surplus

The electric car is the biggest battery in the house. Charge it with photovoltaic surplus and you drive almost for free while the PV system pays back sooner. We explain how the wallbox detects surplus, why phase switching matters and how much you really save.

Why charge the car from the roof

Home photovoltaics produce most during the day, when the house is often empty and consumption is low. The surplus goes to the grid at a feed-in price that in 2026 is around EUR 0.04 to 0.08 per kWh in the Czech Republic, while buying electricity costs EUR 0.16 to 0.28 per kWh depending on the tariff. Every kWh you charge into the car instead of selling saves the difference between those two prices. An EV battery also holds 40 to 100 kWh, five to ten times more than a typical home battery, and it is free because you already own it.

Example: an 8 kWp system produces about 8,000 kWh a year in Central Europe. The household directly uses about a third. Of the remaining 5,000 kWh of surplus, 2,500 to 3,500 kWh can realistically go into the car, which is 15,000 to 20,000 km of driving. At a price difference of EUR 0.16 per kWh that is a saving of EUR 400 to 560 a year.

How it works technically

The wallbox has to know how much energy is flowing to the grid right now and adjust the charging current smoothly. The e:car WALL SMART does it with its own meter at the main supply. It measures the real flow of electricity on site, so it sees both the export to the grid and the house's own consumption, and adjusts power every few seconds. The e:car WALL SMART works in three modes:

An important detail: three-phase charging has a minimum current of 6 A per phase, about 4.1 kW. Smaller surpluses could not be used. Smart stations therefore switch to single phase (minimum 1.4 kW) at low surplus and use weaker sun too. Check that the station can switch phases, otherwise you will miss most of the spring and autumn surplus.

Why the inverter brand does not matter

Most smart wallboxes have to talk to the inverter, which leaves you tied to a list of supported brands. The e:car WALL SMART takes a different route: it measures at the main supply and works out the surplus itself, so it runs with photovoltaics from any manufacturer, including installations where the inverter has been replaced or where there are two of them.

If you run your own home automation, the station covers that too: both e:car WALL SMART and PREMIUM can be controlled over Modbus TCP, so they fit into Home Assistant, Loxone or a building energy management system. The installer sets it up; the user then switches modes in the IONT.info app. The full overview is on the page Wallbox and photovoltaics.

Wallbox or home battery?

How much you save: a model calculation

1. Annual mileage 15,000 km, consumption 18 kWh per 100 km = 2,700 kWh.
2. The car is at home during the day three days a week; the sun covers about 50 % of consumption = 1,350 kWh.
3. Difference between purchase (EUR 0.22) and feed-in (EUR 0.06) = EUR 0.16 per kWh.
4. Annual saving EUR 216 from the sun alone; the rest is charged at night on a low tariff.
5. The price difference between SMART and BASIC (about EUR 280) pays back in under two years. If you work from home, the saving doubles.

A day in Eco mode: what it looks like in practice

The key is the automatic phase switching between 9 and 11 a.m. and again in the afternoon. A station without this function would not charge at all before 11 a.m. and after 3 p.m., because the three-phase minimum of 4.1 kW would not be covered by the surplus. Over a year this makes a difference of 30 to 40 % of usable surplus.

Three myths about solar charging

• "I need a home battery, otherwise it makes no sense." On the contrary, the car is the battery. Home storage helps in the evening, but you do not need it to charge with surplus.
• "Slow surplus charging harms the battery." Variable power of 1.4 to 11 kW is the gentlest way to charge. Frequent fast charging and long periods at 100 % are what harm it.
• "My inverter brand cannot do it." With the e:car WALL SMART the inverter brand is irrelevant. The station measures at the house's main supply, works out the export itself and never has to speak to the inverter at all.

What to watch during installation

• The meter belongs at the main supply. That is the only place where the real export to the grid is visible. Metering at the inverter shows production, not what the house is failing to use.
• Network connection. The app and any Modbus TCP control need a network. Cable is more reliable than Wi-Fi; the e:car WALL can be fitted with a LAN module.
• Breaker and RCD. Solar charging still requires a dedicated residual current device per charging point in line with IEC 60364-7-722. In IONT tech wallboxes it is integrated.
• Minimum current and hysteresis settings. So the station does not stop at every cloud, a response delay is configured. The installer handles this.

Checklist before buying

• Does the wallbox work out the export from its own meter at the main supply, or does it depend on talking to the inverter?
• Does it switch automatically between one and three phases to use surpluses below 4 kW?
• Does it have surplus-only and full-power modes switchable from the app?
• Is the solution independent of the inverter brand? Otherwise it may stop behaving as expected after the PV system is replaced.
• Does it have LAN or stable Wi-Fi at the installation site?

Summary in five points

1. PV surplus sells for EUR 0.04 to 0.08; charging it into the car replaces electricity at EUR 0.16 to 0.28.
2. You need a smart wallbox with its own meter and an Eco mode, i.e. the SMART version; a basic station knows nothing about PV.
3. Phase switching decides whether you also use spring and autumn sun.
4. Annual saving EUR 200 to 560 depending on mileage and how often the car is home during the day.
5. In the Czech Republic the PV plus SMART wallbox combination is also the condition of the CZK 10,000 charging-point grant.

Frequently asked questions