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 standard is Modbus TCP: the wallbox reads data from the inverter or from a meter at the house's main supply and adjusts power every few seconds. The e:car WALL SMART works in three modes:
| Mode | What it does | When to use it |
|---|---|---|
| Eco | Charges only from PV surplus. When clouds come, power drops or charging pauses. | The car is at home during the day and you are not in a hurry. |
| Normal | Charges at full available power but protects the house's main fuse and backs off at consumption peaks. | Regular overnight charging when PV does not produce. |
| Boost | Charges at maximum power regardless of PV. | You need to leave soon. |
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.
Compatibility with inverters and home automation
IONT tech SMART wallboxes communicate via Modbus TCP with Solax, GoodWe, Fronius and other inverters that support the protocol, and can be integrated into Home Assistant or other home automation. 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?
| Charging the car with surplus | Home battery storage | |
|---|---|---|
| Capacity | 40 to 100 kWh (car battery) | 5 to 15 kWh |
| Purchase price | SMART wallbox from CZK 21,900 (about EUR 880) excl. VAT | EUR 6,000 to 12,000 |
| Use of surplus | only when the car is at home during the day | every day, including the evening after coming home |
| Payback | 1 to 2 years at 15,000 km a year | 8 to 12 years |
| Ideal combination | both: the battery covers the evening, the car takes the big surpluses |
How much you save: a model calculation
- Annual mileage 15,000 km, consumption 18 kWh per 100 km = 2,700 kWh.
- The car is at home during the day three days a week; the sun covers about 50 % of consumption = 1,350 kWh.
- Difference between purchase (EUR 0.22) and feed-in (EUR 0.06) = EUR 0.16 per kWh.
- Annual saving EUR 216 from the sun alone; the rest is charged at night on a low tariff.
- 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
| Time | PV output 8 kWp | House consumption | Surplus | What the SMART wallbox does |
|---|---|---|---|---|
| 7:00 | 0.5 kW | 1.2 kW | 0 | waits, car plugged in |
| 9:00 | 3.2 kW | 0.8 kW | 2.4 kW | charges single-phase at 2.4 kW |
| 11:00 | 6.5 kW | 0.9 kW | 5.6 kW | switches to three phases, charges at 5.6 kW |
| 13:00 | 7.1 kW | 2.5 kW (washing machine) | 4.6 kW | reduces to 4.6 kW, the washing machine has priority |
| 15:00 | 4.0 kW | 0.7 kW | 3.3 kW | back to single phase, 3.3 kW |
| 17:00 | 1.1 kW | 1.5 kW | 0 | pauses charging |
| 22:00 | 0 | 0.6 kW | 0 | Normal mode: finishes the rest on the low tariff |
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." Most inverters have Modbus TCP, and where it is missing, a meter at the house's main supply solves it: the wallbox reads the export independently of the inverter brand.
What to watch during installation
- Metering at the right point. The wallbox needs to know the export to the grid, i.e. a meter at the main supply, not just the inverter's production.
- Network connection. Modbus TCP runs over LAN or Wi-Fi. Cable is more reliable; 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
- Can the wallbox read grid export (Modbus TCP from the inverter or a meter at the supply)?
- 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 your inverter on the compatibility list, or will the manufacturer confirm integration via a meter?
- Does it have LAN or stable Wi-Fi at the installation site?
Do you have or plan photovoltaics?
Tell us your inverter type. We check compatibility and prepare a quote for a SMART wallbox with installation. We ship across the EU.
Summary in five points
- PV surplus sells for EUR 0.04 to 0.08; charging it into the car replaces electricity at EUR 0.16 to 0.28.
- You need a smart wallbox with Modbus TCP and an Eco mode; a basic station knows nothing about PV.
- Phase switching decides whether you also use spring and autumn sun.
- Annual saving EUR 200 to 560 depending on mileage and how often the car is home during the day.
- In the Czech Republic the PV plus SMART wallbox combination is also the condition of the CZK 15,000 subsidy.
Často kladené otázky
Can I charge an electric car directly from photovoltaics?
Yes. You need a wallbox with dynamic load management and communication with the inverter or a meter at the supply, such as the e:car WALL SMART. The station monitors grid export and charges only with surplus.
How much PV power do I need to charge a car?
For meaningful surplus charging at least 5 kWp. Three-phase charging starts at 4.1 kW, so it is important that the wallbox can switch to single phase with a 1.4 kW minimum at lower surplus.
Does an IONT tech wallbox work with Solax, GoodWe or Fronius inverters?
Yes. SMART wallboxes communicate via Modbus TCP with Solax, GoodWe, Fronius and other inverters supporting the protocol, and can be connected to Home Assistant.
Is solar charging worth it if I am at work during the day?
Less, but still yes. You use surplus at weekends and on holiday, and charge the rest at night on a low tariff. Combining with a home battery increases the use.
Is there a subsidy for a wallbox with photovoltaics?
In the Czech Republic New Green Savings supports a charging point with CZK 15,000 precisely in combination with photovoltaics, on condition of dynamic load management. The programme reopens points from September 2026.
Is solar charging gentle on the car battery?
Yes. It is slow AC charging with variable power up to 11 kW, which stresses the battery less than DC fast charging.