All calculators

LoadCairn / UPS battery sizing

UPS battery sizing calculator.

Work backwards from the runtime you need. Get the watts each battery must deliver for that many minutes, then check a battery's datasheet against it.

Inputs

Illustrative example · editable

Starts with a 40,000 W load, 15-minute target, 95% efficiency, a 1.25 aging factor, and a 40-battery series string. These are editable planning examples, not site operating data.

Load and runtime

Use the real-power load and runtime the UPS must support.

UPS and planning factors

Apply the UPS efficiency, aging reserve, and temperature correction.

1.25 means the bank still meets the load when the batteries are down to 80% of rated capacity, the usual replacement point for lead-acid (IEEE 485 / IEEE 1184 practice).

1.00 at 77°F (25°C). Use the battery maker's factor for colder rooms.

Battery bank

Describe the bank wiring, cell count, and optional manufacturer rating.

6 for a 12 V lead-acid block. Change for other batteries.

From the manufacturer's constant-power table for your runtime and end voltage.

Planning estimate

Battery bank requirement

Each battery must deliver at least 1,316 W for 15 minutes

219 W per cell

Add a battery datasheet rating to compare bank capability, margin, and strings needed.

Calculation details

Estimate formula

  1. DC power at battery = connected load ÷ (efficiency ÷ 100)
  2. Required battery power = DC power × aging factor × temperature correction factor
  3. Minimum W per battery = required power ÷ (series batteries × parallel strings)
  4. Minimum W per cell = W per battery ÷ cells per battery

Current calculation

  1. 40,000.00 W ÷ (95.00% ÷ 100) = 42,105.26 W DC power at battery
  2. 42,105.26 W × 1.25 × 1.00 = 52,631.58 W required
  3. 52,631.58 W ÷ (40 batteries × 1 strings) = 1,315.79 W per battery
  4. 1,315.79 W ÷ 6 cells = 219.30 W per cell

Assumptions

  • Constant-power discharge for the full runtime; batteries start fully charged.
  • The datasheet value must be at the same runtime and end voltage your UPS uses.
  • Aging and temperature factors are applied as straight multipliers.
  • Doesn't model battery chemistry curves, recharge time, or the UPS's own DC limits.

Sources

  • IEEE 485, Recommended Practice for Sizing Lead-Acid Batteries for Stationary Applications
  • IEEE 1184, Guide for Batteries for Uninterruptible Power Supply Systems
  • Battery manufacturer constant-power (W per cell / per battery) discharge tables

Planning estimate only. Final battery selection should use the manufacturer's sizing data and a qualified engineer's review.

Calculated in your browser · No values leave this page