Is a home battery worth it?
Checks whether a battery pays — against three limits instead of one: datasheet cycles, calendar ageing and the surplus actually available. The last is computed from your location and household profile, not assumed. In the reference case calendar ageing binds, throughput cost lands at 18.52 ct and the margin at 4.88 ct per kilowatt-hour — against the 11.83 ct the datasheet promises.
The formulas behind the calculator
Every number above can be recomputed: the full calculation path, all assumptions and the data source with retrieval date — plus cross-validation against independent references. Disclosed, not claimed.
An estimate based on the stated assumptions. The final design must be checked by a qualified professional against the rules that apply where you are.
Data as of: 2026-08-06
| Step | Formula | Value | Provenance |
|---|---|---|---|
| Spread between retail and feed-in | p_grid - v_feed | 23.4 ct/kWh | measured |
| Throughput per datasheet | n_cycles * C * DoD * eta_rt | 43,200 kWh | assumed |
| Throughput, calendar-limited | years * cycles/a * C * DoD * eta_rt | 27,000 kWh | exact |
| Governing throughput | min(datasheet ; calendar ; surplus) | 27,000 kWh | exact |
| Margin per kilowatt-hour | spread - c_throughput | 4.8815 ct/kWh | exact |
- Formula
margin = (p_power − v_feedin) − K / D · D = min(datasheet cycles · C_use · DoD · η_rt, life · cycles/a · C_use · η_rt, surplus · years)- Valid for
- The specification's reference case is reproduced (10 kWh, €5,000, 6,000 cycles at 80% DoD, η_rt 0.90, 15 years, 250 cycles/a): datasheet throughput 43,200 kWh, calendar 27,000 — the lifetime binds, cost 18.52 ct, margin 4.88 instead of 11.83 ct. Feed-in per the official German EEG rates 08/2026 (7.70 ct). The available surplus arrives from the self-consumption calculator via handoff.
- Not covered
- Backup power and its value, dynamic tariffs and arbitrage, capacity degradation over life, subsidy programmes, the self-consumption gain itself (calculator 43 computes it; here the economics per cycled kWh count).
- Data sources
- Bundesnetzagentur SMARD electricity price analysis, StromAuskunft price index (as of 2026-08-06) and EEG remuneration rates per § 48 EEG 2023 · Q3/2026 (EEG-Sätze H2/2026) · retrieved 2026-08-06
- Manufacturer datasheets (Victron, Fronius, BYD) and IEC 61427-1, aggregated · retrieved 2026-06-15
Frequently asked questions
Is a home battery economically worth it in 2026?
Barely — and only with honest maths: in the reference case (10 kWh, €5,000, spread 23.40 ct) about 4.9 ct of margin per stored kilowatt-hour remains once calendar life is accounted for. The datasheet maths promises 11.8 ct — 59% too much. Whether your battery pays depends mostly on the price per kWh of capacity and on whether there is enough surplus for the cycles.
Why do most battery calculators come out too cheap?
Because they compute throughput from the datasheet cycle count: 6,000 cycles sound like 43,200 kWh. At a realistic 250 full cycles per year those would only be reached after 24 years — longer than the cell lives. What binds is usually 15 years × 250 cycles = 27,000 kWh. The calculator checks all three limits (datasheet, calendar, surplus) and uses the minimum.
What is the spread, and why is it half the answer?
The spread is the power price minus the feed-in tariff — the value of every kilowatt-hour the battery shifts from exporting to self-consuming: currently 31.1 − 7.70 = 23.40 ct at the existing-customer price (EEG rate as of 08/2026). If the spread is below the throughput cost, the battery cannot pay off no matter how often it cycles. In the chart that is the diagonal.
How do I know my available surplus?
From the self-consumption calculator: it simulates your household profile hour by hour against the system and hands the annual surplus (export without a battery) over automatically. If the surplus does not cover the assumed cycles, the calculator warns — an oversized battery cycles too rarely and makes every kilowatt-hour dearer.
What battery size makes economic sense?
As a rule of thumb from the simulation: about one kilowatt-hour of usable capacity per 1,000 kWh of annual consumption — enough for the night, small enough for daily cycling. Bigger is rarely better: self-sufficiency barely rises, while every extra kilowatt-hour of capacity spreads the purchase over less throughput.
What about backup power, dynamic tariffs and subsidies?
Deliberately excluded: backup capability is a safety value, not an economic one; arbitrage on dynamic tariffs needs different hardware and its own model; subsidy programmes vary regionally. The calculator shows the bare battery economics — deciding on those keeps you on the safe side.
Is a home battery worth it at all?
In the reference case, barely: 10 kWh usable for 5,000 euros gives 18.52 ct of throughput cost against a spread of 23.40 ct — leaving 4.88 ct per kilowatt-hour. The datasheet with its 6,000 cycles promises 11.83 ct of margin; calendar ageing more than halves it. What matters is which of the three limits binds first, and it is rarely the cycle count.
When does a battery NOT pay?
When too little surplus arises. A small array with a high-consumption household — 3 kWp and 5,000 kWh working from home — leaves only 1,362 kWh of annual surplus in the Berlin region. Then ageing no longer binds but available energy does: throughput cost rises to 24.48 ct and the margin turns negative at −1.08 ct. The battery then costs money instead of saving it. A calculator with an assumed surplus can never show that.
Why is the datasheet cycle count misleading?
Because it describes a ceiling almost never reached in a household. 6,000 cycles at 250 cycles a year would be 24 years — but calendar ageing ends after about 15. The battery dies of age, not of throughput. The calculator therefore places all three limits side by side and marks the binding one; the margin drops 59 percent against the datasheet calculation.
How much spread do I need for a battery to pay?
At least the throughput cost plus a safety margin. In the reference case that is 18.52 ct; with a spread of 23.40 ct (31.1 ct grid price minus 7.7 ct feed-in) a thin margin remains. If the electricity price falls or the feed-in tariff rises, the calculation tips. That is why the calculator reports the margin rather than just a yes or no.
What is round-trip efficiency and how much does it matter?
It states how much of the stored energy comes back out — typically 90 percent for lithium. The missing 10 percent directly raises the cost of every stored kilowatt-hour, because you must store 1.11 for one usable kilowatt-hour. Importantly, this loss must not be counted twice if it is already in the system loss chain — the calculator applies it exactly once.