An EV already has a big battery. So why install another one? The answer depends on two things: when your panels produce power and when your car is plugged in.
What does a home battery do?
A home battery stores power you do not need at that moment, usually solar power from the daytime, and gives it back to the house later. You send less to the grid and draw less from it in the evening. Some energy is lost when charging and discharging, so you never get back everything you put in.
Your car normally does not do this. It takes in power and keeps it. A few combinations of car and charger can send power back to the house, see bidirectional charging. Do not count on it for a standard installation.
A worked example
Say your panels produce 20 kWh on a sunny day and the house uses 5 kWh during the day. That leaves 15 kWh. Without a battery it goes to the grid.
In the evening the house uses another 6 kWh, and the car needs 20 kWh for the next few days.
- With a 10 kWh battery (usable capacity) you capture 10 of the 15 kWh surplus. That covers the house in the evening (6 kWh), and the remaining 4 kWh go to the car.
- The other 16 kWh for the car come from the grid, or from a sunny day later on.
- The other 5 kWh of surplus have already gone to the grid, because the battery was full.
A home battery is small compared with a car battery of, say, 60 kWh. It mainly helps the house through the evening.
When the car is home during the day
Do you work from home, or is the car on the driveway during the day? Then it soaks up the surplus itself. In the example it charges 15 kWh straight from your panels. That is a large part of what you would otherwise put in a battery, with no extra investment and no storage losses.
For that you need a wallbox that adjusts its charging power to the solar surplus. Many chargers have a minimum power to start. On a cloudy day you may not reach it, and part of the power then comes from the grid. Check the manual to see how low your wallbox can go, or first read how to choose a wallbox.
When does a home battery make sense?
- You have a lot of solar panels and a big surplus you cannot use during the day.
- You mostly charge in the evening or at night, when the panels produce nothing.
- The gap between what you pay for power taken from the grid and what you get for power fed back is large.
- You want power during an outage. Not every home battery provides backup power, so ask about it explicitly.
When is it better to skip it?
A home battery is a big investment. If your surplus is small, or the car is home during the day, you will struggle to earn it back.
You do not always need a battery for the Flemish capacity tariff either. If you mainly want to stop an 11 kW charging session from pushing up your quarter-hour peak, a wallbox with load management can do the same. It is cheaper.
On a dynamic energy contract you simply set charging to the cheap hours with the car’s or wallbox’s timer. For the car alone a battery is not needed there. More in charging on a dynamic tariff.
How to work it out
- Check your digital meter or app for how many kWh you feed back per year and when.
- Subtract what your car can take in itself during the day.
- Whatever is left, a battery could in theory store. Say that is 2,000 kWh a year.
- Multiply that by the gap between your purchase price and your feed-in price. What you get for feeding back differs per contract and changes over time, so take the figures from your own contract.
An example with made-up numbers: if a kWh from the grid costs 0.30 euro and you get 0.05 euro per kWh fed back, every stored kWh is worth 0.25 euro. At 2,000 kWh that is 500 euro a year. If the battery costs 5,000 euro, it takes ten years to pay off. If the gap drops to 0.15 euro, you save 300 euro a year and it takes well over sixteen years.
Have quotes work out the saving from your own consumption, not from an average in a brochure. A battery also has a limited lifespan and loses capacity over the years.
Common mistakes
- Buying a battery to charge the car, when the car could be home during the day.
- Looking only at capacity and not at the power the battery can deliver. At low power it cannot run your hob and washing machine at the same time.
- Calculating with fixed prices for the next fifteen years, while tariffs and compensation change.
More on combining solar panels and a car in the piece on charging with solar panels. To see what a charge costs you, try the charging calculator.