Usable battery capacity calculator
Converts rated capacity into energy you can actually use — accounting for depth of discharge, temperature, ageing and the discharge path. All factors multiply; they do not add.
- LiFePO4 must not be charged at 0.6 °C. Lithium plating damages the cells irreversibly. A battery heater or a BMS with low-temperature charge cut-off is required.
LFP_CHARGE_BELOW_FREEZING
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: 2026-06-15
| Step | Formula | Value | Provenance |
|---|---|---|---|
| Discharge path efficiency | eta_cable * (f_DC + f_AC * eta_inv) | 0.931 | assumed |
| Usable energy | C * U_nenn * DoD * f_temp * f_alter * eta_Entladepfad | 652.87 Wh | exact |
| Required nominal capacity | E_gefordert / (U_nenn * f_gesamt) | 459.51 Ah | exact |
- Formula
E_usable = C · U_nom · DoD · f_temp · f_age · η_path- Valid for
- E_usable = C·U·DoD·f_temp·f_age·η_discharge, multiplicative (the additive value stands beside it as comparison). The cold factor follows the chemistry’s curve at the installation temperature — outdoors at the site monthly mean (PVGIS TMY), indoors at the user entry; frost and derating warnings use the same temperature. Reference case: 100 Ah LiFePO4 outdoors in Berlin in January → 0.6 °C, factor 86 %. Round trip deliberately NOT included (charging side — double counting).
- Not covered
- Recovery effects and Peukert (calculator 36), temperature gradient inside the case, active heating, cell spread within the block, calendar ageing as a trajectory (the age factor is a point value; the cost view is calculator 34).
- Data sources
- Manufacturer datasheets (Victron, Fronius, BYD) and IEC 61427-1, aggregated · retrieved 2026-06-15
Frequently asked questions
How much energy can I actually draw from a 100 Ah battery?
Usable energy is rated capacity times nominal voltage times depth of discharge times temperature factor times age factor times discharge-path efficiency. The key point is that these factors multiply rather than add: 1.00 · 0.80 · 0.90 · 0.80 · 0.90 comes to about 0.52, not the 0.40 an additive estimate suggests, and the calculator shows that naive figure for comparison.
Should round-trip efficiency be subtracted from usable capacity?
No — round-trip efficiency belongs on the charging side, not in this calculation. Applying it here as well double-counts it and makes the result 8 to 15 percent too pessimistic.
How deep can I safely discharge a lead-acid battery?
For AGM, gel and flooded batteries the calculator warns once depth of discharge exceeds 50 percent, and for LiFePO4 it warns about charging below 5 °C. Not covered are the energy content along the real discharge curve, capacity fade over the battery's life and cell imbalance — nominal voltage is used as an approximation of the discharge curve.
Why do the deductions not simply add up?
Because each factor acts on the REMAINDER, not on the nominal value: 0.80 (DoD) × 0.90 (temperature) × 0.80 (ageing) × 0.90 (inverter) = 0.518 — 51.8% remains. Adding the deductions instead (20 + 10 + 20 + 10 = 60 percentage points) would give 40% — a different and wrong number. The depletion bar shows both figures side by side.
How does the reverse mode work?
You enter the required usable energy at the load output — say 3,000 Wh — and the calculator solves for nominal capacity: C_nom = E / (U_nom × total factor). It also names the factor with the biggest lever. That is almost always the ageing reserve or temperature — and then putting the battery into a heated compartment is often cheaper than buying more amp-hours.
Why does round-trip efficiency not appear here?
Because it belongs on the CHARGING side: it describes how much of the charged energy comes back out, and is therefore part of charge and array sizing. Applying it on the discharge side as well counts it twice and calculates 8–15% too pessimistically. On the discharge side only cable and inverter losses count — proportional to the AC load share.
Where does the cold factor come from?
From the chosen chemistry's characteristic curve at the installation site's temperature — outdoors at your location's monthly mean, indoors at your entry. Previously factor and temperature sat side by side unconnected: you could enter −5 °C and still compute with 0.9. For Berlin in January (0.6 °C) the LiFePO4 curve gives 86 % — and only this real temperature also triggers the frost charging warning.
Why do the factors act multiplicatively, not additively?
Because each factor acts on what the previous ones leave: 0.80 DoD × 0.86 cold × 0.80 age × discharge path yields a good half — computed additively it would come out considerably lower, and methodically wrong. The calculator shows the additive value deliberately as a comparison — the same core error as in the loss chain.
What does the dominant factor mean in practice?
It shows where an improvement pays most — start there instead of buying bigger. If cold leads, a warmer location or a battery heater beats 20 % extra capacity; if the discharge path dominates, look at AC share and inverter. The inverse mode computes alongside what nominal capacity the required energy would otherwise need.
Why is round-trip efficiency absent from this calculation?
Because it acts on the charging side and is counted there (array and charge-time calculators). A full battery has its charging losses behind it — subtracting round trip here again computes 8 to 15 % too pessimistically. The same double-counting error in the other direction would be applying the discharge path to charging.
Does the per-chemistry depth-of-discharge guidance apply here too?
Yes: discharging lead types beyond 50 % costs cycles far beyond proportion — the calculator then warns rather than forbids. LiFePO4 tolerates 80 to 90 %. Depth of discharge deliberately remains your choice, being a trade between usable energy today and lifetime tomorrow; the cycle-cost page prices exactly that trade in cents.