UPS and Battery Backup Runtime Calculator
Work out how long a battery bank will carry a given load through a power cut.
Results
Backup time
1.80
h
Minutes
108
min
Usable energy
540.0
Wh
Stored energy
1200
Wh
The dashed line marks the value you entered.
What this tool does
A battery bank holds amp-hours, but what a load consumes is watts, and the bridge between them is the bank voltage. Multiply amp-hours by volts and you have watt-hours; take off the part of the charge you are not willing to use, take off the inverter's losses, and divide by the load. Two of those five numbers are choices rather than measurements: depth of discharge is how hard you are prepared to treat the battery, and inverter efficiency is usually 85–95 % for a decent unit under a reasonable load.
Formula
time = capacity × voltage × (depth of discharge ÷ 100) × (efficiency ÷ 100) ÷ load
Variables
| Symbol | Meaning | Unit |
|---|---|---|
ah | Battery capacity | Ah |
v | Voltage | V |
load | Load power | W |
dod | Usable depth of discharge | % |
eff | Inverter efficiency | % |
H | Backup time | h |
M | Minutes | min |
UE | Usable energy | Wh |
SE | Stored energy | Wh |
Worked example
- Battery capacity100 Ah
- Voltage12 V
- Load power300 W
- Usable depth of discharge50 %
- Inverter efficiency90 %
- Backup time1.80 h
- Minutes108 min
- Usable energy540.0 Wh
- Stored energy1200 Wh
Limitations
- A battery does not give its full rated capacity at a heavy load: the higher the current, the less energy you actually get out. This calculation is linear, so treat it as an upper bound and use the efficiency field to allow for the difference.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- The calculation runs at full precision and only the display is rounded. If you copy an intermediate value and retype it, small differences can appear.