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SolarCalcWorks

Battery Bank Sizing Calculator

Work out the battery capacity your system actually needs, from your daily consumption outwards. Depth of discharge and round-trip efficiency are visible and adjustable here rather than hidden, because they are the assumptions that change the answer most.

  • Amp-hours and kWh
  • LiFePO4, AGM and flooded presets
  • Days of autonomy
  • Battery count for your unit size

Battery Bank Sizing Calculator

179 Ah8.6 kWh at 48 V
Inputs
Bank voltage
Battery bank8.58 kWh nameplate

179Ah at 48 V

2 × 100 Ah batteries

200 Ah installed, of which 6.52 kWh is usable across 2 days. Rounding up is deliberate — you cannot buy a fraction of a battery.

Nameplate capacity
179 Ah
8.58 kWh
Usable energy
6.52 kWh
80 % of nameplate
DC draw per day
3,261 Wh
after inverter losses
Batteries
2
200 Ah installed
  • NoteDepth of discharge and round-trip efficiency are manufacturer conventions rather than code requirements. Use the figures on your own datasheet where you have them.
Calculation

1What the bank has to deliver

3,000 Wh AC ÷ 0.92 inverter = 3,261 Wh DC per day

The inverter draws more from the battery than it delivers to the load, so the efficiency divides.

2Across the autonomy period

3,261 Wh × 2 days = 6,522 Wh usable

Autonomy is how long you can run with no useful charging — a stretch of overcast days, or snow on the array.

3Turning usable into nameplate

6,522 Wh ÷ 0.80 depth ÷ 0.95 round trip = 8,581 Wh
÷ 48 V = 179 Ah

You cannot use all of a battery, and you do not get back everything you put in. Both losses expand the nameplate you have to buy. This is where a calculator that quietly assumes 100 % depth of discharge produces an answer half the size it should be.

Battery capacity is not governed by the NEC. Depth of discharge and round-trip efficiency are manufacturer conventions; the code covers the wiring and protection around a bank, not its size. Sources and limitations.

NextSize the array that refills this bankA bank is only as useful as the array that recharges it before the next night.

Work backwards from what you use

Every number in an off-grid system derives from daily energy consumption, and it is the one figure worth measuring rather than estimating. List every load with its watts and the hours per day it actually runs:

Load Watts Hours/day Wh/day
12 V compressor fridge 45 10 450
LED lighting, 6 fixtures 30 5 150
Laptop and phones 60 4 240
Water pump 60 0.5 30
Induction hob 1500 0.5 750
Total 1620 Wh

A clamp meter or a shunt-based battery monitor beats any spreadsheet, because measured consumption is invariably higher than estimated consumption.

The chain from load to nameplate

DC energy per day = AC energy ÷ inverter efficiency
usable energy = DC per day × days of autonomy
nameplate = usable ÷ depth of discharge ÷ round-trip efficiency
amp-hours = nameplate ÷ bank voltage

Two of those divisions are where calculators quietly diverge. You cannot use all of a battery, and you do not get back everything you put in. Both losses expand the nameplate you have to buy, and a tool that assumes 100 % depth of discharge produces an answer roughly half the size it should be.

Choosing a bank voltage

Bank voltage does not change how much energy you need. It changes every current downstream, and therefore what your cables and devices cost.

Bank Suits Watch out for
12 V Under about 1500 W continuous. Vans, small cabins, anywhere 12 V appliances dominate. Cable cost climbs fast. A 2000 W inverter pulls 185 A.
24 V Up to roughly 3 kW. Mid-size cabins and larger RVs. You will need a DC-DC converter for 12 V appliances.
48 V 3 kW and above, and anything that might grow. Widest modern component choice; server-rack LiFePO4 and hybrid inverters are built around it.

Chemistry, honestly

LiFePO4 costs more per nameplate amp-hour and less per usable amp-hour, because you can use 80 % or more of it and it survives thousands of cycles doing so. It is lighter, needs no ventilation, and holds voltage far flatter under load. Its weakness is charging below freezing, which a good BMS blocks and which matters in an unheated space.

AGM is sealed, needs no maintenance, tolerates being mounted in more orientations, and is cheaper up front. Designed to 50 % depth, its usable capacity per dollar is worse than it looks.

Flooded lead-acid is the cheapest per nameplate amp-hour and the most demanding: ventilation for hydrogen, periodic watering, and an equalisation charge. In a stationary bank with someone willing to maintain it, the economics can still work.

What this calculator does not decide

Maximum continuous discharge current is a battery specification, not a capacity calculation. A 100 Ah LiFePO4 pack rated at 100 A continuous cannot feed a 3000 W inverter on a 12 V bus, which needs about 280 A, no matter how many watt-hours it stores. Check that figure on the datasheet alongside the capacity you calculate here, and size the battery-side conductors and protection to it.

What to calculate next

NextSize the array that refills itA bank is only as useful as the array that recharges it.

Frequently asked questions

How many batteries do I need for a 2000W inverter?

That question cannot be answered from the inverter rating, and any calculator that tries is guessing. An inverter rating describes peak power, not energy. A 2000 W inverter running a microwave for six minutes uses 200 Wh; the same inverter running a 1500 W heater for four hours uses 6000 Wh — thirty times the battery. Size from watt-hours per day, which is what this calculator asks for. The inverter rating does matter for maximum discharge current, which is a separate limit on your datasheet.

What depth of discharge should I use?

80 % for LiFePO4 and 50 % for lead-acid are the conventional design points, and they are conventions rather than standards. Lead-acid cycle life falls sharply with depth, so 50 % is the long-standing compromise. Most LiFePO4 packs tolerate deeper cycling and many are rated for 100 %, but designing to 80 % leaves margin for BMS cutoffs, cold-weather capacity loss and ageing. Use the figure on your own datasheet where you have one.

How many days of autonomy do I need?

One day is a floor for grid-backup. Two to three is normal for genuinely off-grid living, since it covers a typical run of overcast weather. Beyond about three days, extra array or a generator is usually cheaper per kWh of reliability than extra battery, because the additional capacity sits unused most of the year and still ages.

Why does the calculator ask for round-trip efficiency?

Because you do not get back everything you put in. Charging and discharging both lose energy to heat and, on lead-acid, to gassing during absorption. LiFePO4 returns around 95 % of what it accepts; flooded lead-acid closer to 80 %. That loss expands the nameplate capacity you have to buy, and a calculator that ignores it understates the bank.

Is it better to have one big battery or several small ones?

Fewer, larger units are generally better. Every parallel string of batteries must see equal cable lengths and an equal number of connections, or current divides unevenly and one unit works harder and ages faster than the rest. Four parallel units is a common practical ceiling before that becomes hard to guarantee. Series connections do not have this problem, which is part of why higher bank voltages are easier to build well.

Last reviewed 2026-08-19. Calculations reference NFPA 70 (NEC) 2023 where a code section applies. Sources, and what these tools deliberately do not model.