UPS runtime estimator
How long the batteries hold at a given load, and whether that is long enough for everything to shut down cleanly.
The battery and the load
Per cent. Lead-acid is normally held to 50%.
Per cent.
Watts, not VA. W = VA × power factor.
Minutes, longest machine first.
How long it holds
- Runtime at this load
- —min
- Energy in the bank
- —Wh
- Of that, actually usable
- —Wh
- High-rate derating applied
- —%
| If the load were | Runtime |
|---|
What this does and does not know
- Batteries deliver less when you pull harder. A bank rated over a 20-hour discharge gives noticeably less than its rating at a one-hour rate. That is modelled here with a Peukert-style exponent — 1.2 for sealed lead-acid, 1.05 for lithium — applied as the ratio between the 20-hour discharge power and your actual load. It is an approximation, and a good one for planning; it is not a datasheet.
- Depth of discharge is a lifespan decision, not a limit. Lead-acid will hand over more than half its charge. Doing that routinely is how a three-year battery becomes a one-year battery.
- Nameplate VA is not watts. Multiply by the power factor, usually 0.6 to 0.9 for a UPS. Use the real figure from the UPS, not the one on the box, if you have it: upsc reports load as a percentage of the nameplate rating.
- Age is the variable that matters most and the one nothing here can see. A four-year-old lead-acid bank can hold half its rated energy while still reporting a full charge. Test with a real discharge once a year; the number this page gives you is for a healthy battery.
- Plan for the shutdown, not the outage. The useful question is whether everything can stop cleanly before the batteries do, which usually means starting the shutdown early rather than buying more battery.
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