PowerCalcLab

Battery calculator

Battery Bank Size Calculator

Turn daily energy use into the nominal kWh and amp-hours your bank needs. The model accounts for days without charging, conversion loss, battery health, usable depth of discharge and whole-battery rounding.

Selecting a chemistry loads an editable voltage and usable-depth starting point.

Add the watt-hours used by every load over one representative day.

How long the bank must support the loads without meaningful recharging.

The share of nominal capacity the system is allowed to use before recharging.

Advanced assumptions

Use 100% for direct DC loads; include inverter and wiring losses for AC loads.

Extra capacity for uncertain loads, future growth or imperfect operating conditions.

Use less than 100% when the bank must still meet the target after capacity has faded.

Nominal voltage of one battery used to build the bank.

Rated amp-hour capacity of one battery module.

Results update as you type. The calculator runs entirely in your browser — nothing you enter is sent anywhere.

Minimum nominal bank size3.07 kWh · 239.58 AhBefore rounding to whole 100 Ah batteries
STORED ENERGY90%
90% usable10% reserve
90% of nominal capacity remains inside the modeled usable window after depth-of-discharge and health derating.

Calculation breakdown

Energy for autonomy period720 Wh/day × 3 days2.16 kWh
After design margin15% extra2.48 kWh
Battery-side energy neededAt 90% conversion efficiency2.76 kWh
Whole-battery layout12.8 V nominal string1S × 3P = 3
Installed nominal bank3.84 kWh · 300 Ah
Installed load energyAbout 4.32 days at the entered daily use3.11 kWh
Nominal bank energy = daily energy × days × margin ÷ efficiency ÷ DoD ÷ health
= 720 Wh × 3 × 1.15 ÷ 0.9 ÷ 0.9 ÷ 1
= 3.07 kWh ÷ 12.8 V = 239.58 Ah
  • The whole-battery plan rounds up to 3 batteries; series raises voltage and parallel strings raise amp-hour capacity.
  • The 90% usable-depth assumption is a planning value for LiFePO4 (lithium iron phosphate). Replace it with the battery manufacturer's warranted limit.

Before you rely on this

A bank layout is not an installation design. Confirm the battery manufacturer's series/parallel limits, BMS compatibility, conductor size, overcurrent protection, disconnects and local electrical requirements before assembly.

Method

How the number is reached

Start with measured daily energy and multiply by the number of days the bank must run without charging. A design margin covers uncertainty or future load growth, while conversion efficiency translates load-side energy into what must leave the battery.

Only part of nominal battery capacity is available. Dividing by usable depth of discharge and remaining health turns the battery-side requirement into nominal bank energy. Dividing again by bank voltage gives amp-hours.

Batteries are installed in whole units. Series connections raise voltage without adding amp-hours; parallel strings add amp-hours without changing voltage. The final plan rounds parallel strings up, never down.

Load energy (Wh) = Daily energy × Days of autonomy
Battery-side energy = Load energy × (1 + margin) ÷ conversion efficiency
Nominal bank energy = Battery-side energy ÷ usable DoD ÷ health
Bank capacity (Ah) = Nominal bank energy (Wh) ÷ bank voltage (V)
Battery count = batteries in series × parallel strings

Symbols

Wh
Energy consumed or stored in watt-hours
Ah
Bank charge capacity at its nominal voltage
DoD
Allowed depth of discharge as a fraction of nominal capacity
S / P
Battery count in series / number of parallel strings

Worked examples

The same maths, applied

Example

Three days of RV loads on LiFePO4

Daily use is 720 Wh and the bank must last three days. For a direct-DC planning example, conversion efficiency and design margin are set to 100% and 0%. A 12.8 V LiFePO4 bank is allowed 90% depth of discharge.

Load energy = 720 Wh/day × 3 days = 2,160 Wh
Nominal energy = 2,160 Wh ÷ 90% = 2,400 Wh
Capacity = 2,400 Wh ÷ 12.8 V = 187.5 Ah
Two 12.8 V 100 Ah batteries in parallel provide 200 Ah

Result: 2.4 kWh · 187.5 Ah before rounding; two 100 Ah batteries.

Example

The same loads on lead-acid

The same 720 Wh/day and three-day target uses a 12 V lead-acid bank limited to 50% depth of discharge.

Load energy = 720 Wh/day × 3 days = 2,160 Wh
Nominal energy = 2,160 Wh ÷ 50% = 4,320 Wh
Capacity = 4,320 Wh ÷ 12 V = 360 Ah
Four 12 V 100 Ah batteries in parallel provide 400 Ah

Result: 4.32 kWh · 360 Ah before rounding; four 100 Ah batteries.

Detail

Chemistry defaults used for sizing

The chemistry choice changes usable depth of discharge and the nominal voltage of a 12 V-class battery. Both remain editable because the battery data sheet and warranty define the real limits.

Chemistry starting points for bank sizing
ChemistryNominal 12 V-class voltageTypical usable DoD
LiFePO4 (lithium iron phosphate)12.8 V90%
Lithium-ion (NMC / NCA)10.8 V80%
AGM (sealed lead-acid)12 V50%
Gel (sealed lead-acid)12 V50%
Flooded lead-acid12 V50%

These values populate the form when chemistry changes. Manufacturer limits and warranties take precedence.

Detail

Measure energy before choosing batteries

Daily watt-hours are more useful than a list of appliance wattages. Multiply each load's power by the hours it runs, then add the results. For cycling loads such as refrigerators, measure a complete day when possible.

  • Use critical loads only when sizing a backup bank.
  • Use a representative worst-case day for an off-grid bank.
  • Set conversion efficiency to 100% only when the stated energy is already measured on the battery side or all loads are direct DC.
  • Keep design margin separate from depth of discharge so the reason for every extra kWh stays visible.

Limits of the model

What it assumes, and where it stops

Assumptions

Daily energy use remains similar throughout the autonomy period.

No meaningful charging occurs during the stated days of autonomy.

Conversion efficiency is represented by one average figure.

All batteries in the proposed layout are identical, equally aged and connected with balanced cabling.

The selected nominal module voltages can form the bank voltage in whole series steps.

Not covered

Energy capacity does not size the inverter, cable, fuse, BMS or battery current rating. Peak and surge power must be checked separately.

Cold temperature, high discharge current and ageing can reduce delivered capacity beyond the entered health assumption.

Lead-acid capacity depends on discharge rate; this page does not apply a Peukert correction because the load profile is not specified.

Solar and generator recharge availability are intentionally excluded from an autonomy-without-charging calculation.

Manufacturer limits on series and parallel battery count can be stricter than the mathematical layout.

Sources

Where these figures come from

  • How to Size a Solar System for Your RVVictron EnergyManufacturer sizing walkthrough connecting daily energy, days of autonomy, system voltage and chemistry-dependent usable capacity, with an explicit reminder to include inverter losses.
  • ESS design — Battery bank capacityVictron EnergyExplains that backup battery capacity is determined by required autonomy and why larger banks reduce cycling stress and extend outage coverage.
  • SI UnitsNIST Office of Weights and MeasuresDefinitions of the volt, ampere, watt and joule used for the unit conventions on this site.

Next steps

Related calculators

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  • Wh to Ah Calculator

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Last meaningful update: 2026-08-19. This date changes only when the model, the sources or the guidance change.