Tool
UPS sizing & runtime
Two questions, honestly answered: what size do I need, and how long will it actually run. Every figure shows its working, because most calculators in this corner of the internet quietly print a naive number that is wrong by a factor of two at the loads people care about.
1 · What's on the UPS
| Device | Watts | Qty | Keep on battery? |
|---|
Use measured draw, not the PSU label. A NAS with a 750 W supply typically draws 40–60 W. Across ENERGY STAR certified desktops the median power supply is rated 360 W against 16.6 W measured idle — 21× over. Sizing from the sticker is the classic over-buy.
2 · What you need to buy
3 · How long it will run
Show the working
4 · What unplugging something buys you
Across certified small-office units the median runtime at half load is 3.57× the runtime at full load — not 2×. Batteries give up proportionally more capacity the harder you drain them, so load you remove is worth more than capacity you buy.
Why the naive formula is wrong
The obvious runtime calculation is usable watt-hours divided by load. It is also optimistic exactly where it matters, because lead-acid batteries deliver less total energy the faster you discharge them — the Peukert effect. At a trickle you get close to the rated capacity; at a heavy draw you can lose a third of it.
So this tool derates. In battery mode it applies a Peukert correction against the discharge rate, checked against published APC runtime charts for a 1500 VA / 900 W unit: this model gives 4 / 10 / 16 / 37 minutes at 900 / 450 / 300 / 150 W, against a published ~4 / 10 / 16 / 37. In model mode it does something better — it fits the curve between the two runtimes your manufacturer already publishes for that exact unit, so the answer is theirs rather than ours.
VA is not watts, and the gap is bigger than you think
A UPS carries two ratings and has to satisfy both. Everyone quotes VA because it is the larger number. Across 407 certified units rated 1500 VA or under, the measured median is 0.625 W per VA — and for line-interactive units, the type most people actually buy, 0.600. A 1500 VA unit is a 900 W unit. Size against watts and the VA takes care of itself.
Where the numbers come from
Model runtimes, watt ratings and the power-factor figures are from ENERGY STAR's certified product data, published by the US EPA and in the public domain. Device wattages are measured where ENERGY STAR covers the category and clearly-labelled estimates where it doesn't — there is no public measured dataset for NAS, prosumer network gear or access points, and I would rather say so than invent a number.
There's a plainer-English version of this aimed at small businesses on my company site: how long will my UPS last in a power cut.