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Size a UPS battery bank by dividing the connected load in watts by the battery voltage, efficiency, and usable-capacity factors.
Most UPS sizing mistakes trace back to one wrong input: the VA rating on the box, which only equals watts when the load is perfectly resistive. To calculate battery capacity for a UPS, start with the actual watts the connected equipment draws, choose the backup time you need, and identify the battery bank’s DC voltage. Those three numbers, adjusted for efficiency and depth of discharge, give the minimum amp-hour capacity.
The rest of this page steps through the formula with real inputs, a worked example, and the mistakes that inflate battery size.
Calculating UPS Battery Capacity Starts With Watts
The sizing formula is Ah = (W × h) / (V × η × DoD). W is the load in watts, h is the target runtime in hours, V is the bank’s DC voltage, η is inverter efficiency, and DoD is the share of battery capacity you can safely use before recharging. In energy terms: Wh = W × h, and usable watt-hours equal Wh / (η × DoD). Divide usable watt-hours by the bank voltage to get amp-hours.
The table shows where each input comes from, because that step causes more miscalculations than the math itself.
| Input | What It Represents | How To Find It |
|---|---|---|
| Load watts (W) | Real power draw, not the UPS VA rating | Watt meter or the watts shown by UPS monitoring software |
| Runtime (h) | Hours the load must stay up | Your requirement; divide minutes by 60 |
| Bank voltage (V) | DC voltage the UPS expects from its batteries | UPS nameplate or manual; common values are 12 V, 24 V, 48 V, 96 V, 120 V, and 240 V |
| Efficiency (η) | Conversion losses inside the inverter | Spec sheet, or the 0.85–0.95 range used by typical calculators |
| Depth of discharge (DoD) | Usable fraction of battery capacity | Battery datasheet; roughly 0.5 for lead-acid and 0.8 for lithium |
| Battery configuration | Series or parallel strings change bank voltage and total amp-hours | UPS manual; keep the string voltage inside the UPS limits |
| Temperature | Cold lowers usable capacity | Manufacturer ratings usually reference 77°F (25°C) |
Until those values pass through η and DoD, the capacity printed on a battery label is not usable capacity.
Putting The Numbers Through The Formula
Run the calculation in this order, or enter the same inputs into a UPS runtime calculator — the numbers underneath are identical.
- Measure or estimate the connected load in watts. List every device the UPS must support and add their running watts. A watt meter gives the real figure when nameplates use VA instead of watts.
- Decide the backup time, in hours or minutes. Thirty minutes becomes 0.5 hours before it enters the formula.
- Confirm the UPS battery bank voltage. Consumer units often run at 12 V or 24 V, while larger rack and three-phase systems use 48 V, 96 V, 120 V, or 240 V banks.
- Apply efficiency and depth of discharge. Typical calculators assume an inverter efficiency between 0.85 and 0.95 and a DoD matched to the battery chemistry.
- Add headroom, then verify. Oversize slightly for aging, cold rooms, or future load growth, and check the result against the manufacturer’s runtime chart — discharge rate and cutoff voltage change usable capacity.
Worked example: a 600 W load that must run 2 hours on a 48 V bank. With η set to 0.9 and DoD at 0.8, the bank needs 600 × 2 / (48 × 0.9 × 0.8) ≈ 34.7 Ah minimum. Round up to the next standard capacity your UPS supports.
Ace Battery’s UPS sizing guide runs through the same watt-hour-to-amp-hour path with worked numbers if you want a second pass on the math.
What If Your Equipment Is Rated In VA, Not Watts?
Convert volt-amps to watts with power factor before sizing, because VA only becomes watts when the power factor is applied. Many UPS runtime calculators ask for a power factor for exactly this reason.
Look for the power factor on the device nameplate or power supply label before using VA as a stand-in for watts.
Mistakes That Throw Off The Number
Four mistakes account for most oversized or undersized UPS batteries.
- Sizing from the UPS VA rating. The VA number describes what the UPS can supply, not what your equipment actually draws. The load wattage decides battery capacity.
- Treating label amp-hours as fully usable. Battery ratings assume slow discharge and a full cycle; efficiency losses and DoD limits cut deeply into that number.
- Mixing one-battery values with whole-bank values. In a series string the same current flows through every battery; in parallel strings the amp-hours add. Keep each value on the same side of that line.
- Ignoring discharge rate and cutoff voltage. Usable capacity shrinks as discharge gets faster, and the inverter stops at its cutoff voltage — not at zero charge. Manufacturer runtime charts beat formula-only estimates on critical loads.
When the amp-hour number points toward a replacement, look over our roundup of best UPS batteries before you buy.
FAQs
Should I size UPS capacity from watts or volt-amps?
Use watts. The VA figure is the maximum apparent power the inverter can handle, while watts reflect real power after power factor. Most loads carry a power factor below 1, so sizing from VA builds in error. Watts equal VA multiplied by the device’s power factor, and battery capacity should follow the watt figure.
How long will a 12 V 7 Ah battery run a 100 W load?