Ahmedonics

Energy

Battery backup runtime calculator

Estimate how long a battery bank will run a load through a UPS or inverter, and the capacity needed for a target runtime, accounting for depth of discharge and inverter efficiency.

Load and battery bank
Total power drawn from the inverter output. Add up the appliances that will run during the outage.
The Ah rating at the 20-hour rate (C20), as printed on the battery.
How much of the capacity you allow to be used. 50 % is common for lead-acid, 80–90 % for LiFePO4.
85 % is typical for a mid-range inverter at partial load; check the datasheet.
Leave as is to see the capacity needed for this many hours.

Runtime

Enter your values and press Calculate.

What the calculator does

It converts the battery bank into usable energy (watt-hours), applies the inverter's efficiency, and divides by the load to estimate runtime. It also reports the discharge current and the C-rate, which tells you whether the battery's printed capacity is realistic at that current, and the capacity you would need for a target runtime.

Formula

Usable energy (Wh) = Vbank × Ahtotal × DoD
Runtime (h) = Usable energy × ηinverter ÷ Load (W)

Discharge current (A) = Load ÷ (ηinverter × Vbank)
C-rate = Discharge current ÷ Ahtotal

Capacity for target (Ah) = Load × hours ÷ (Vbank × DoD × ηinverter)

Ahtotal is the capacity per battery multiplied by the number of batteries or strings in parallel. Batteries in series raise the bank voltage but not the Ah.

Worked example

A 300 W load on a 12 V inverter with one 150 Ah flooded battery, 50 % depth of discharge and 85 % inverter efficiency.

  1. Usable energy = 12 × 150 × 0.5 = 900 Wh.
  2. Runtime = 900 × 0.85 ÷ 300 = 2.55 h, about 2 h 33 min.
  3. Discharge current = 300 ÷ (0.85 × 12) = 29.4 A, a C-rate of 0.196 (roughly C/5).
  4. For 4 hours: 300 × 4 ÷ (12 × 0.5 × 0.85) = 235 Ah, so two 150 Ah batteries in parallel (300 Ah) would be needed.

Why the printed Ah is not the whole story

Lead-acid capacity is rated at a slow 20-hour discharge (C20). Draw the same battery down in 5 hours and it delivers noticeably less than its rating; in 1 hour, much less. This is the Peukert effect. The calculator flags C-rates above C/5 (0.2 C) for lead-acid, where the shortfall becomes significant, and above 0.5 C for LiFePO4, which is far less affected. Use the flag as a prompt to add capacity or check the manufacturer's discharge curves; the calculator does not apply a Peukert correction itself.

Assumptions and limitations

  • Constant load. Real loads vary; refrigerators and pumps cycle, and motors draw several times their running current at start.
  • Healthy, fully charged batteries at moderate temperature. Capacity falls with age, sulphation and cold.
  • Efficiency is treated as constant. Inverters are least efficient at light load; a 2 kW inverter running a 100 W load may be well under 80 % efficient. The inverter's own idle draw is not included: add it to the load if the datasheet gives it.
  • No Peukert correction. High-rate discharges deliver less than the rating; see above.
  • Not a charging or solar sizing tool. Recharge time, charger current and solar array sizing are separate calculations.

Frequently asked questions

Why not use 100 % depth of discharge?

Because it destroys lead-acid batteries quickly. Cycle life falls steeply with deeper discharge; 50 % is a common compromise between runtime and battery life. LiFePO4 tolerates 80–90 % routinely.

Does a 24 V or 48 V bank last longer than 12 V?

Not by itself: energy is volts × amp-hours, so the same batteries rearranged in series hold the same energy. Higher voltage does reduce current and cable losses, and inverters are often more efficient at 24 V or 48 V.

How do I find my load in watts?

Read it from each appliance label, or measure with a plug-in power meter. For appliances rated in amps, multiply by the mains voltage (e.g. 0.5 A × 230 V = 115 W). Add a margin for start-up surges on motors and compressors.

References

  • IEC 61427-1:2013, Secondary cells and batteries for renewable energy storage — general requirements and methods of test; context for capacity ratings and cycling
  • Peukert, W. (1897), "Über die Abhängigkeit der Kapazität von der Entladestromstärke bei Bleiakkumulatoren" — original description of capacity falling with discharge current

Last reviewed 2026-09-19.