Calculator
Home Battery Payback and Value Calculator
A battery generates nothing. It moves energy from a moment when it is cheap to a moment when it is expensive, loses some in the process, and charges you for the service. Whether that is worthwhile depends on the size of the price gap and on how many days a year the battery is actually full.
This calculator works out the realistic annual throughput, the value of each shifted kilowatt-hour, and the cost per cycle over the warranted life.
Enter the storage case
Usable, not nominal — check the depth of discharge.
Winter surplus rarely fills the battery.
Energy cycled per usable day
9 kWh
Limited by the smallest of capacity, surplus and load
Delivered to the house per year
1,879 kWh
Tariff spread captured
€0.19/kWh
Annual financial benefit
€339
Simple payback
19.2 years
Inside the warranted life
Equivalent full cycles per year
214
Warranty exhausted in about 28 years
Storage cost per delivered kWh
€0.123/kWh
Cheaper than the spread it captures
How the calculation works
Figure — DC versus AC coupled storage
The energy a battery can cycle on any day is the smallest of three limits: its usable capacity, the solar surplus available to charge it, and the evening load available to discharge into. Sizing above the binding limit adds cost and no throughput.
The utilisation factor accounts for the season. In northern Europe a battery may cycle fully from March to October and barely at all in December, so annual throughput is typically 55 to 75% of the theoretical 365 full cycles.
The value of each cycle is the retail price avoided on discharge minus the export revenue given up on charge. That spread — not the retail price alone — is what a battery earns.
The formula
E_day = min(C_usable, Surplus, Load / η) E_charge= E_day × 365 × u E_deliv = E_charge × η Value = E_deliv × p_import − E_charge × p_export Payback = Cost / Value €/kWh = Cost / (Cycles_warranty × C_usable × η) η round-trip efficiency · u utilisation factor
Assumptions behind the numbers
- Usable capacity is entered directly. A 10 kWh nominal battery at 90% depth of discharge has 9 kWh usable.
- Round-trip efficiency covers the battery and its converter. Manufacturer figures usually exclude standby consumption, which costs a further 30 to 100 kWh a year.
- No time-of-use arbitrage from the grid is modelled. Where a cheap night tariff exists, grid charging can add materially to the value.
- Battery capacity fade is not modelled year by year; the warranted cycle count is the proxy for life.
- Backup value during outages is excluded, as is any grid-services income from aggregation.
How to read the result
A payback shorter than the warranted life means the battery earns its cost back. Under current European prices that usually needs a spread above roughly €0.15/kWh and high utilisation.
The cost-per-delivered-kilowatt-hour line is the cleanest test. If storage costs €0.11/kWh to deliver and the spread is €0.19/kWh, the battery is economic; if the spread is €0.08/kWh, no amount of optimisation rescues it.
Batteries improve dramatically when export payments are low or zero, because the forgone revenue term collapses. In markets that still pay near-retail for exports, storage rarely pays on economics alone.
Worked example
A €6,500 battery with 10 kWh usable capacity sits in a house with 9 kWh average daily surplus and 8 kWh of evening load, 88% round-trip efficiency, importing at €0.26/kWh and exporting at €0.07/kWh, cycling fully on 65% of days, warranted for 6,000 cycles.
Daily throughput is limited by the load: 8 ÷ 0.88 = 9.1 kWh charged, capped by the 9 kWh surplus. Over 237 effective days that is 2,133 kWh charged and 1,877 kWh delivered. Value is 1,877 × 0.26 − 2,133 × 0.07 = €488 − €149 = €339 a year.
Simple payback is 19 years against roughly 28 years of warranted cycles at 213 cycles a year. Storage costs €0.123 per delivered kilowatt-hour against a €0.19 spread, so the battery is marginally economic — and would become clearly economic if export payments fell or the battery cost dropped below €5,000.
Mistakes that skew the answer
- Valuing every stored kilowatt-hour at the full retail tariff and ignoring the export revenue that charging gives up.
- Using nominal rather than usable capacity, which overstates throughput by 5 to 20%.
- Assuming 365 full cycles a year. Real systems reach 200 to 280 in most European climates.
- Forgetting standby and conversion losses, which quietly consume 3 to 5% of the benefit.
- Sizing the battery to the solar surplus when the evening load is smaller, which leaves capacity permanently unused.
Questions readers ask
- Do home batteries pay for themselves?
- Sometimes. With a wide import-export spread, high utilisation and a battery under about €600 per usable kilowatt-hour, payback falls inside the warranty. With near-retail export payments or a small evening load, it does not, and the battery is bought for resilience or independence rather than return.
- How many cycles will my battery do?
- In a solar self-consumption role, 200 to 280 equivalent full cycles a year is typical in Europe. Adding cheap-rate grid charging in winter can push that above 320, which improves the economics considerably where a suitable tariff exists.
- Is a bigger battery better value?
- Only up to the binding constraint. Once capacity exceeds either the daily surplus or the evening load, extra kilowatt-hours sit idle most of the year. The cost per delivered kilowatt-hour rises steeply beyond that point.
- What round-trip efficiency should I assume?
- For a modern lithium iron phosphate system with a hybrid inverter, 86 to 90% including conversion. DC-coupled systems save one conversion step and typically run two to four points higher than AC-coupled ones on solar charging.
- Does backup capability change the numbers?
- It adds value that this model does not price. If outages are frequent, the avoided cost of spoiled food, lost work or a generator can dominate the arbitrage case. Note it separately rather than inflating the energy value to justify the purchase.