Reading the result
Loss is calculated as load x (1 / efficiency minus 1). At 200 W and 90% efficiency, the UPS draws about 222 W from the wall and turns 22 W into heat. Over a year, running continuously, that is about 195 kWh.
Efficiency figures on spec sheets are usually measured at or near full load. Most home and small-office UPS units run at 20 to 40% load, where efficiency is lower, so treat published numbers as a best case.
Size affects efficiency too
A large UPS lightly loaded wastes a larger share of its throughput than a right-sized one, because its fixed losses (control board, fans, charger) are spread over less load. Oversizing "for safety" has a running cost that shows up here.
For a full discussion of the trade-offs, see UPS efficiency and energy cost and line-interactive vs online UPS.
Frequently asked questions
How much electricity does a UPS use?
A line-interactive UPS in normal mode typically loses about 2 to 5% of the power passing through it, plus a few watts for its own electronics and battery charging. An online double-conversion UPS commonly loses 6 to 12% at partial loads. At 200 W that is roughly 4 to 24 W, running all day.
Is an online UPS worth the extra running cost?
For sensitive equipment on poor or generator power, yes. For a home office on stable utility power, a good line-interactive unit protects equally well in practice at lower running cost and noise. Many online units also offer an eco or high-efficiency mode.
Does a UPS use power when nothing is plugged in?
Yes. It keeps its battery charged and its control electronics running, typically a few watts for small units and more for large online units. Switch it off fully if you will not use it for a long period, but recharge the battery every few months.
Where can I find a UPS's efficiency?
Spec sheets list efficiency in normal (online or line) mode, often at full load. Partial-load efficiency is usually lower. ENERGY STAR certified UPS models must meet minimum average efficiency levels.