Battery bank size calculator
Calculates the required nominal capacity from daily demand and autonomy — with a cold factor from the chosen chemistry’s characteristic curve instead of a free assumption, and for outdoor installation with your location’s design temperature. That changes the answer substantially: 2,400 Wh/d and two days of autonomy need 329 Ah indoors, 403 Ah outdoors in Berlin — and only the site value triggers the critical LiFePO4 charge cut-off warning below 0 °C. Plus ageing reserve and the C-rate cross-check other calculators skip.
- LiFePO4 must not be charged at -5 °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 |
|---|---|---|---|
| Required nominal capacity | (E_Batterie * D) / (U_sys * DoD * f_temp * f_alter) | 403.23 Ah | exact |
| Usable energy today | C * U_nom * DoD * f_temp | 6,400 Wh | exact |
- Formula
C = (E_bat · d_autonomy) / (U_sys · DoD · f_temp · f_age)- Valid for
- Bank from daily demand, autonomy, depth of discharge, cold factor and ageing reserve. The cold factor comes from the chosen chemistry’s characteristic curve (data/batteries/chemistries.json); for outdoor installation the design temperature comes from the site climate dataset (1 % percentile of daily minima, the same source as calculator 11). Values between curve points are interpolated linearly, beyond them NOT extrapolated. Reference case: 2,400 Wh/d, 2 days, 24 V, LiFePO4 → 329 Ah indoors at 10 °C, 403 Ah outdoors in Berlin at −5 °C.
- Not covered
- Self-discharge, end-of-charge behaviour of individual cells, cell balancing, active battery heating (it shifts the design temperature — the calculator does not model it), ageing as a function of cycles (calculator 34).
- Data sources
- Manufacturer datasheets (Victron, Fronius, BYD) and IEC 61427-1, aggregated · retrieved 2026-06-15
Frequently asked questions
What battery capacity do I need for two days of autonomy?
The required rated capacity is C = (E_battery · autonomy days) / (U_sys · DoD · f_temp · f_age). For 2400 Wh per day, 2 days, 24 V, a DoD of 0.8 and a temperature factor of 0.9 that gives 277.8 Ah; adding an ageing reserve of 0.8 raises it to 347.2 Ah. A lead-acid bank with 0.5 DoD and heavier derating comes to 625 Ah in the reference case — 2.25 times the lithium figure.
Why does the calculator run a C-rate cross-check on the result?
A capacity can be right in energy terms while its maximum discharge current is exceeded by the peak load. The calculator therefore also checks the discharge C-rate P_peak/(U_sys · C) and the charge C-rate I_controller/C against the datasheet limits. With LiFePO4 an overrun means a BMS shutdown in the middle of the night; with lead-acid it means severe voltage sag.
Is it safe to charge a LiFePO4 battery below freezing?
No — charging LiFePO4 below 0 °C causes lithium plating, which damages the cells irreversibly, and many cheap BMS units have no low-temperature charge lockout. The calculator raises a warning whenever the minimum battery temperature is below that threshold, so plan for a heater or a warmer installation spot.
How large must the bank be in the reference case — and what does chemistry change?
For 2,400 Wh/day, 2 days of autonomy, 24 V, DoD 0.8, winter factor 0.9 and ageing reserve 0.8, LiFePO₄ needs 347 Ah. The same task with lead (DoD 0.5, winter factor 0.8): 625 Ah — 2.25 times the nominal capacity, with matching weight and space. Whether the premium still pays off is answered by the chemistry comparison and cycle-cost calculators, with both perspectives: purchase and lifetime.
What does the 14-day simulation show that the autonomy number does not?
“2 days of autonomy” is abstract. The simulation overlays a three-day bad-weather spell at 20% yield and shows whether the state of charge touches the red DoD line — today and in year 10, when the ageing reserve is used up. If the year-10 curve pierces the line you know: the bank is tightly sized, and the bottleneck arrives not at the start but after years. Raise the autonomy until both curves stay above the line.
Why does the calculator additionally check the C-rates?
Because an energetically sufficient bank can still be current-weak: peak load divided by voltage and capacity must stay below the datasheet discharge rate, the controller charge current below the charge rate. With LiFePO₄ an exceeded discharge rate is a BMS shutdown in the middle of the night; with lead, a massive voltage sag. This cross-check is missing from most calculators — here it is two dedicated figures.
Where does the battery design temperature come from?
For outdoor or in-vehicle installation, from the climate dataset for your location — the 1 % percentile of daily minima, the same quantity used to check open-circuit voltage. For Berlin that is −5 °C. If the battery sits in a heated house the site temperature is the wrong number; it then remains your entry, because only you know how cold the cellar gets.
Why was the cold factor previously 0.90?
Because it was an estimate — and it did not even match the temperature set beside it. The LiFePO4 curve gives 0.95 at 10 °C but 0.77 at −5 °C. Both numbers sat independently in the form, so you could combine a cold temperature with a warm factor. The factor now follows from the chosen chemistry's curve.
How much bigger does the cold make the bank?
Substantially, in the reference case: 2,400 Wh per day, two days of autonomy, 24 V, LiFePO4. At 10 °C indoors 329 Ah are needed; outdoors in Berlin at −5 °C it is 403 Ah — 22 % more from temperature alone. With AGM instead of LiFePO4 it is 417 Ah, because its curve falls off slightly more steeply in the cold.
What happens beyond the ends of the curve?
The calculator does not extrapolate. Below the coldest tabulated point its factor continues to apply instead of extending a straight line into nowhere. That is deliberately modest: a claim about −40 °C that the dataset does not support would be worse than a visibly capped value.
Why is the frost warning so emphatic?
Because charging a LiFePO4 cell below 0 °C causes lithium plating and damages it irreversibly — that is not an efficiency matter but damage. Outdoors in Berlin the reference case hits it, so the warning sits above the result and cannot be dismissed. Lead chemistries have no such cut-off; there it remains a capacity loss.