UPS Battery Runtime Estimate

Make a nominal battery-energy runtime estimate with explicit usable-capacity and inverter-efficiency factors.

How this calculation works

This estimate converts the complete battery bank's nominal voltage and amp-hour capacity into stored DC energy. Enter bank-level values: series connections add voltage rather than amp-hours, while parallel strings add amp-hour capacity. A usable-capacity fraction represents the intended reserve and expected condition, and battery-to-load efficiency accounts for conversion losses. Applying both factors gives estimated deliverable AC energy, which is divided by the constant AC load to estimate runtime. Approximate discharge current is calculated separately from load, nominal voltage and conversion efficiency. Real capacity depends on discharge rate, cutoff voltage, temperature and aging, none of which is modeled dynamically. Ah-based estimates can substantially overstate short-duration performance, so required backup time must be checked against the UPS manufacturer's discharge or runtime curves.

Inputs and units

Complete battery-bank nominal voltage (V)
Complete bank capacity at rated discharge (Ah): Series strings add voltage, not Ah; parallel strings add Ah
Usable fraction of nominal capacity (1)
Battery-to-load conversion efficiency (%)
Constant AC load (W)

Method and formula

Enominal = Vbank × Ahbank; EAC = Enominal × usable fraction × efficiency/100; runtime = 60EAC/load in minutes; IDC ≈ load/(Vbank × efficiency/100).

Worked example

Example inputs

Complete battery-bank nominal voltage: 48 V
Complete bank capacity at rated discharge: 100 Ah
Usable fraction of nominal capacity: 0.8 1
Battery-to-load conversion efficiency: 90 %
Constant AC load: 1000 W

Calculation steps

Calculate nominal DC energy: 48 × 100 = 4800 Wh.
Apply usable fraction and conversion efficiency: EAC = 4800 × 0.8 × 90/100 = 3456 Wh.
Divide by the 1000 W AC load: runtime = 3456/1000 = 3.456 h; 3.456 × 60 = 207.36 min.
Estimate battery discharge current: IDC = 1000/(48 × 90/100) = 23.148148 A.

Example results

Nominal DC battery energy: 4800 Wh
Estimated deliverable AC energy: 3456 Wh
Nominal estimated runtime: 207.36 min
Approximate battery discharge current: 23.14814815 A

Example notes

Use the UPS manufacturer’s discharge/runtime curves for required backup time; Ah-based estimates can substantially overstate short-duration runtime

Assumptions

Constant nominal voltage, capacity factor, conversion efficiency and load
Usable fraction includes the intended reserve and expected condition

Limitations

Does not model Peukert behavior, cutoff voltage, temperature, aging, surge or inverter overhead
Not a UPS selection or life-safety backup guarantee

Sources

Victron Energy: Battery Capacity and Peukert Exponent: Ah capacity, discharge rate and nonlinear available capacity