Solar panels and an electric car go well together, though you have to arrange a few things for it. Your panels produce during the day. Your car is usually at work then, or only gets plugged in in the evening, when the roof has stopped giving anything.
Why it doesn’t happen automatically
Your installation feeds the house first and sends what is left to the grid. An ordinary wallbox knows nothing about that. Plug in at night and the power comes from the grid, even if your roof gave half of it away at 1 pm.
If you are home during the day, timing solves part of the problem. Start the charger on a sunny Saturday afternoon and your charging overlaps with your production. You do have to keep an eye on the sky yourself, though. If you want the charger to do that for you, you need solar-aware charging.
Solar-aware charging on the wallbox
A smart wallbox with solar control measures how much your panels have left over and adjusts the charging power every few seconds. A cloud passes and the power drops. The sun returns and it climbs again. Check the specifications to see whether your wallbox can do this and which meter it needs. More on choosing one in how to choose a wallbox.
The minimum charging power
A wallbox won’t charge below a minimum current, usually 6 amps. On a single phase that is roughly 1.4 kW (6 A times 230 V). On three phases it is roughly 4.1 kW.
Two situations to make it concrete:
- You have a single-phase charger and 1 kW of surplus. That is less than 1.4 kW. The charger stays idle, or it charges anyway and takes the missing 0.4 kW from the grid. It depends on your settings.
- You have a three-phase charger and 3 kW of surplus. That is below 4.1 kW, so again a part is missing. Some wallboxes can switch down to one phase, others can’t.
Many wallboxes let you choose what they do when the surplus is too small. Check this before you buy, because that choice decides how much of your own power really ends up in the car.
A worked example
Say your car uses 18 kWh per 100 km and you drive 200 km a week. That is 36 kWh a week, or 7.2 kWh for a 40 km trip.
On a sunny afternoon you have 2 kW left over. Charge for five hours at 2 kW and 10 kWh goes into the battery. At 18 kWh per 100 km that is about 55 km of range. For your 36 kWh a week you therefore need 18 hours with 2 kW to spare. In a good summer week that works. In December it doesn’t.
Now the euros, with assumptions you should replace with your own figures. Say you pay 0.30 euro per kWh for grid power and receive 0.05 euro per kWh for what you feed in. Charge those 36 kWh from surplus and you give up 1.80 euro of feed-in income. Take them from the grid and you pay 10.80 euro. You come out 9 euro ahead per week. Tariffs and feed-in rates differ per supplier and change over time. The rules on feed-in and meters have also been adjusted in recent years. Check your own contract and your grid operator to see what applies to your installation.
If you are out during the day
Then you have three routes:
- A home battery stores your surplus for the evening. How that works with your car is in home battery and EV.
- You charge mostly on days you are at home, for example at weekends or when working from home.
- You take a dynamic contract and charge in the cheapest hours. That gives you no solar power, only lower costs. More in dynamic energy tariff and charging.
If you are away from 8 to 6 every working day and have no battery, solar-aware charging gains you little. Work out honestly whether a smart wallbox pays for itself.
What about winter?
In winter the days are short and panels give much less. Expect to charge mostly from the grid. A low sun on a clear, cold day still yields something, a grey week almost nothing. Because a car also uses more in the cold, the gap grows.
Common mistakes
- You pick a wallbox without solar control and assume it will use your surplus by itself.
- You forget the minimum charging current and wonder why the charger stops in light cloud.
- You count on last year’s feed-in rate, while your contract or the rules have changed since.
- You charge at full power on a sunny afternoon while your heat pump and tumble dryer are running too.
The capacity tariff still applies
If you still draw power from the grid while charging, it counts towards your highest quarter-hour peak of the month. A wallbox with load management limits the total consumption of your home and avoids needlessly expensive peaks. You can read the basics in home charging.