Does a battery pay off?
A battery doesn't make your power cheaper. It moves it: from midday, when you would otherwise sell it cheaply, and from the night, when it's cheap, to the evening, when it's expensive. Here is what that's worth, calculated with real figures from our own system.
It can, when there is power to move from midday and night to the evening. Our own battery, a Growatt WIT with 16.8 kWh in DK1, saved DKK 9.98 a day after wear from 23 September to 2 October 2026. Multiplying September up gives DKK 3,000 to 3,600 a year after wear, but a year is not September times twelve: in winter there is less sun to save and a bigger difference between night and evening. The payback time is the battery's price divided by what it saves a year.
What does a battery save on?
Four things, and the first two matter most.
- Solar power in the evening. Solar power you would otherwise sell for around DKK 0.36 in the middle of the day can be used from 17:00 to 21:00, when the same power costs around DKK 2.30 to buy. See the calculation.
- Cheap night, expensive evening. In winter, the grid tariff from 17:00 to 21:00 at most grid companies costs 45 to 85 øre more per kWh than at night, before VAT. See your grid company.
- No selling at a loss. At negative prices , the solar power can be stored instead of being sold. See how many quarter hours had negative prices last month.
- Selling from the battery. When the evening price is high, the battery can sell. It only pays off when the price is higher than what the power would otherwise be worth in your own house later.
Our own battery in figures
A Growatt WIT with a 16.8 kWh battery in DK1, 23 September to 2 October 2026. GridZeno controlled the battery from the evening of the 23rd.
| Power from the battery to the house, 77.45 kWh | DKK 182.68 |
|---|---|
| Sold from the battery, 9.81 kWh | DKK 21.79 |
| Charged from the grid, 27.76 kWh | DKK −39.40 |
| Solar power in the battery that would otherwise have been sold, 79.88 kWh | DKK −52.15 |
| The battery's gain | DKK 112.92 |
| Estimated wear, 87.26 kWh discharged | DKK −13.09 |
| After wear | DKK 99.83 |
Power to the house is calculated at the buying price in the quarter hour it was used, and sold power at the selling price in the quarter hour it was sold, with the grid company's grid tariff as it applied that day. Wear is DKK 0.15 per kWh that comes out of the battery.
That is DKK 11.29 a day before wear and DKK 9.98 after. 32 kWh of the power to the house was used from 17:00 to 21:00, when it's most expensive, and the battery ran the equivalent of 5.2 full charges and discharges. During the period we tested four different strategies and on 2 October some individual features with artificial prices, so the battery didn't always do what paid off best.
In the first 19 days of September, GridZeno didn't yet control the battery, and the inverter ran on its own settings. Calculated the same way, the battery then gave DKK 9.41 a day before wear. The weather wasn't the same, so the difference isn't a measure of what the control is worth.
How much does a battery save a year?
A year isn't September times twelve.
Multiplying September up gives DKK 3,400 to 4,100 a year before wear and DKK 3,000 to 3,600 after. But in winter there's less sun to store and a bigger difference between night and evening, and in summer it's the other way round. We'll show a full year when we have measured it.
The payback time is the battery's price divided by what it saves per year. A worked example: if a battery costs DKK 40,000 installed and saves DKK 3,000 a year, it takes over 13 years to earn back. So also check how many years of warranty the battery has.
How much power does a battery lose?
Not all the power that goes into the battery comes out again.
- Tab
- Around 10 to 15% for a round trip in and out of the battery, measured on our own system.
- Wear
- A battery can only be charged and discharged a certain number of times. GridZeno uses a price per kWh for the wear, which you can change yourself under Automatic.
- Charging from the grid
- Only pays off when the power is worth more later than the buying price plus the loss and the wear. GridZeno requires it to save at least 25 øre.
If you buy power at DKK 1.50 at night and lose 15%, each kWh you get back out costs DKK 1.76. With DKK 0.15 in wear , it is DKK 1.91. So the power has to be worth more than that in the evening before it earns anything.
When doesn't a battery pay off?
A battery only earns when there's something to move.
- You already use most of your solar power yourself , so there isn't much to store.
- Your grid company has the same grid tariff around the clock, so only the difference in spot price is left.
- You don't use much power in the evening and at night.
- The battery is large compared with what you can move. A battery that's rarely filled or emptied doesn't earn more than a smaller one.
- The electricity tax is low in 2026. That makes solar power you store for the evening 89 øre less valuable per kWh with VAT than it was in 2025.
How GridZeno uses the battery
The plan decides for every quarter hour what the battery should do, and why.
- The plan
- Charges, holds, uses or sells based on your prices, the solar forecast and your consumption, a day ahead.
- Hold the battery
- Stores the power while it's cheap until it becomes expensive. Smart and Pro.
- Selling from the battery
- Only when the evening price pays for the loss and the wear. Pro.
- Savings
- The Savings page in the app shows for each day what the solar panels and the battery have saved.
The plan is carried out by the GridZeno Dongle, which sits at your inverter. Work out what GridZeno can mean for your system.
Work out what a battery can shift for you.
Enter your own battery size and price difference, and see what it gives over a year.