A worked example
A 65 W laptop and a 12 W router make 77 W. For 4 hours, with an inverter efficiency of 0.85 and using half the battery, you need 77 x 4 / 0.85 / 0.5 = 724.7 Wh. On a 12 V battery that's 60.4 Ah. With our margin of 1.25, the inverter should be at least about 96 W.
The sum is watts x hours / efficiency / depth of discharge. Amp hours are watt hours divided by the battery's volts.
Why we start at half the battery
A battery lasts longer when you don't run it flat. Victron Energy, a battery maker, says that past about 80% depth of discharge "the aging process advances disproportionately fast". Your battery's own sheet may give different limits, so change the box to match it. The 0.85 and 0.5 are our starting assumptions, not rules.
The labels on chargers show the most they can draw, so real use is often lower. That gives you a little room. We show no prices and no brands.
Next steps
Read our guide to power and internet backup, and check how much data your calls use. Say what you've got on your CV. Then browse remote roles.
Questions people ask
What size UPS do I need for a router?
Type your router's watts from its label and the hours you need. A router alone is often a small load, so the result shows how little you may need.
Can a 100Ah battery run my laptop and router?
At 12 V a 100 Ah battery holds about 1,200 Wh. With our starting assumptions, the same 77 W load would run for about 6.6 hours. Type your own numbers to check.
This is an estimate. It isn't electrical advice, so ask an electrician to do any mains wiring. Reviewed on 2 November 2026.