About Battery Storage Sizing Calculator
The battery storage sizing calculator determines how large a battery bank an off-grid or backup system needs to cover a daily energy demand for a chosen number of days of autonomy. Starting from the usable energy required (daily load times days of autonomy), it grosses that up for the depth of discharge, the round-trip charge/discharge efficiency, and a temperature derate to obtain the nominal capacity that must actually be installed. It then converts the nominal energy to amp-hours at the system voltage and works out how many batteries are needed in series and in parallel.
Enter the daily energy demand in kilowatt-hours, the days of autonomy, the depth of discharge (DoD), the round-trip efficiency, a temperature derate, the DC system voltage, and the rated capacity and voltage of a single battery. The tool reports the usable and nominal capacity in kWh and Ah, the series count needed to reach the bus voltage, the parallel strings needed for capacity, the total battery count, the installed capacity, and the C-rate for a given charge or discharge current. A chart shows how the battery count grows with each extra day of autonomy.
How It Works
- Enter the daily energy demand (kWh/day), days of autonomy, depth of discharge (0-1), round-trip efficiency (0-1), and a temperature derate (0-1, 1 = none).
- Usable energy = daily load x days of autonomy; required nominal capacity = usable energy / (DoD x round-trip efficiency x temperature derate).
- Required amp-hours = required energy (Wh) / system voltage; batteries in series = ceil(system voltage / unit voltage); parallel strings = ceil(required Ah / unit Ah).
- Total battery count = series x parallel; installed capacity = parallel x unit Ah; C-rate = charge/discharge current / installed Ah.
Worked Example
An off-grid cabin uses 10 kWh per day and needs 2 days of autonomy on a 48 V DC bus using 200 Ah, 12 V batteries, charged at 50 A. The usable energy is 10 x 2 = 20 kWh. With a 50% depth of discharge, 90% round-trip efficiency and no temperature derate, the required nominal capacity is 20 / (0.5 x 0.9 x 1.0) = 44.4 kWh. In amp-hours that is 44,444 / 48 = 926 Ah. Reaching 48 V needs ceil(48 / 12) = 4 batteries in series, and 926 Ah needs ceil(926 / 200) = 5 parallel strings, so 4 x 5 = 20 batteries giving 1000 Ah and 48 kWh installed. The C-rate at 50 A is 50 / 1000 = 0.05 C.
Formulas
- Usable energy
E_usable = dailyLoad * daysAutonomy- Required nominal capacity
E_required = E_usable / (DoD * eta_rt * tempDerate)- Required amp-hours
Ah_required = E_required * 1000 / systemVoltage- Battery configuration
series = ceil(V_sys / V_unit) ; parallel = ceil(Ah_req / Ah_unit) ; count = series * parallel- C-rate
C = current / installedAh
Standards & References
- IEEE 1013 (sizing lead-acid batteries for stand-alone PV systems)
- IEEE 485 (sizing lead-acid batteries for stationary applications)
- IEC 61427 (secondary cells/batteries for renewable energy storage)
- NREL stand-alone PV system design guidance
Frequently Asked Questions
Why divide by depth of discharge and round-trip efficiency?
You can only safely use a fraction of a battery rated capacity (the depth of discharge) without shortening its life, and some energy is lost charging and discharging (round-trip efficiency). Dividing the usable energy by both, plus a temperature derate, gives the larger nominal capacity you must actually install.
How do series and parallel connections differ?
Batteries in series add their voltages to reach the system bus voltage, so series count = ceil(system voltage / unit voltage). Batteries in parallel add their capacities, so parallel strings = ceil(required Ah / unit Ah). Total batteries = series x parallel; the bank both meets the voltage and the capacity.
What is the C-rate and why does it matter?
The C-rate is the charge or discharge current divided by the installed amp-hour capacity, in units of 1/hour. A 0.05 C charge rate means a 20-hour full charge. Staying within the battery recommended C-rate avoids overheating and capacity loss; lead-acid banks typically charge near 0.1-0.2 C.
Why apply a temperature derate?
Battery usable capacity falls in cold conditions; a derate factor below 1 (for example 0.8 at low temperatures) increases the required capacity so the bank still delivers the load on the coldest expected day. Set it to 1.0 when batteries are kept at a controlled temperature.