Enter your daily energy in watt-hours, the days of autonomy you want, your system voltage and the usable depth of discharge to size the battery bank in amp-hours and kilowatt-hours.
Results are estimates for planning and education, based on your inputs and standard engineering values (AWG resistance, NEC ampacity, resistivity). Electrical work can be dangerous and is governed by the NEC and your local code — verify all sizing with a licensed electrician and your authority having jurisdiction (AHJ). Not a substitute for professional design.
Calculator
Battery capacity
600 Ah
Usable storage
7.20 kWh
Formula
Battery Ah = (daily Wh × days of autonomy) / (system V × depth of discharge). Energy in kWh = Ah × system V / 1000.
Worked example
For 1,200 Wh/day, 3 days of autonomy, a 12 V bank and a 0.5 depth of discharge (lead-acid): Ah = (1200 × 3) / (12 × 0.5) = 600 Ah, which is 600 × 12 / 1000 = 7.2 kWh of nominal capacity.
A battery bank is sized by three things: how much energy you use, how many days of autonomy you want for cloudy or high-demand stretches, and how deeply you can discharge the chemistry without harming it. That last factor is decisive — lithium tolerates deep, frequent cycling while lead-acid should rarely drop below half, so a lead bank needs roughly twice the nominal capacity for the same usable energy. This tool works back from usable watt-hours to the amp-hours you must install at your system voltage. Temperature matters too: cold cuts available capacity, so a bank in an unheated space needs headroom beyond the paper figure.
Frequently asked questions
What is depth of discharge and why does it matter?
Depth of discharge, or DoD, is the fraction of a battery you can use before recharging without shortening its life. Flooded and AGM lead-acid batteries are usually limited to about 50 percent, so a 100 Ah lead-acid battery gives roughly 50 usable amp-hours. Lithium iron phosphate (LiFePO4) tolerates 80 percent or more, giving 80 usable amp-hours from the same nominal 100 Ah. Because the calculator divides by DoD, a lower DoD demands a larger nominal bank to deliver the same usable energy. Always size against usable capacity, not the nameplate number.
How is amp-hours different from kilowatt-hours?
Amp-hours measure charge at a given voltage, while kilowatt-hours measure energy regardless of voltage. To convert, multiply amp-hours by the system voltage and divide by 1,000: a 600 Ah bank at 12 V is 7.2 kWh, and so is a 300 Ah bank at 24 V. Energy is what powers your loads, so kWh is the fairer way to compare banks built at different voltages. Amp-hours still matter for choosing batteries and sizing charge current, but two banks with equal kWh store the same energy.
Does a higher system voltage shrink the bank?
It shrinks the amp-hours, not the energy. Doubling the voltage from 12 V to 24 V halves the required amp-hours for the same stored energy, because amp-hours times voltage is constant. The kilowatt-hours stay the same. Higher voltage is attractive for bigger systems because it lowers current, allowing thinner cables and smaller fuses and reducing losses. The energy you must store is set by your loads and autonomy, and no amount of rewiring changes that.
Should I add margin beyond the calculated size?
A modest margin is wise. Cold temperatures reduce usable capacity, especially for lead-acid, and batteries lose a little capacity as they age. Inverter standby draw and small phantom loads also nibble at the bank. Sizing 10 to 20 percent above the calculated figure, or rounding up to standard battery sizes, gives breathing room. Do not oversize wildly, though: an array that cannot keep an oversized bank charged leads to chronic undercharging, which itself shortens battery life.
Why does battery chemistry change my usable energy?
Different chemistries can be safely discharged to different depths and have different round-trip efficiency. Lead-acid prefers shallow cycling and loses more energy to heat when charging and discharging, while lithium accepts deep cycles and is more efficient. The depth-of-discharge input is where you capture this: enter 0.5 for typical lead-acid and 0.8 for LiFePO4. The result is the nominal bank needed so that the usable portion covers your daily energy across the autonomy you set.
Source: Usable-energy balance: Ah = (daily Wh × days) / (system V × depth of discharge); energy kWh = Ah × V / 1000. · All sources