Sizing for How Long — Runtime, Fuel, and Watt-Hours
Last verified: September 2026
Once you know how much power you need, plan how long you'll need it. Runtime depends on your load and your fuel or stored energy — lighter loads run longer. Plan fuel or watt-hours for the outage you expect, plus a reserve. Rather talk it through? My number is (330) 200-8042.
Sizing for How Long, at a Glance
Sizing has two dimensions: how much power (watts) and how long you'll need it (hours). Runtime depends on your load and on your fuel or stored energy — lighter loads run longer, because an inverter generator throttles down and a battery's watt-hours stretch further. For a generator, plan fuel for the outage you expect plus a reserve, stored safely. For a battery, its watt-hours divided by your load gives roughly the hours, and recharging extends it. Refuel only with the engine off and cool. This guide keeps every figure general, with a printable version up top.
Common Questions
1 What decides how long my generator or battery will run?
Two things: how heavy a load you're drawing, and how much fuel or stored energy you have. The lighter the load, the longer you last — an inverter generator throttles down to sip fuel, and a battery's watt-hours stretch further. Plan for both how much and how long.
2 How do I figure out how many hours a battery will last?
Roughly, take its watt-hours and divide by the load you're drawing in watts, then trim a little for losses — that's about your hours. A lighter load lasts longer; a heavy one drains it fast. Our watts-versus-watt-hours guide walks through the two numbers.
3 How much fuel should I store?
Enough for the outage you realistically expect, plus a reserve for a longer one — more in the country, where outages run longer. Store it safely in approved containers, cool and well ventilated, away from living space. Our guide on storing generator fuel safely covers the how.
4 Can I refuel a generator while it's running?
No — never. Shut it off and let it cool first. Gasoline splashed on a hot engine can catch fire. It's one of the most important habits with any fuel-burning unit: engine off, engine cool, then refuel.
5 How can I make my power last longer?
Run a lighter load: cycle the fridge and freezer, run heavy things one at a time, and switch off what you're not using. A battery can also be recharged from the wall, from solar, or from a car. Our managing-loads guide covers stretching a run.
6 How long should I plan for in a rural outage?
Longer than you'd think — often days rather than hours. Country outages take longer to restore, so plan a bigger fuel reserve or a rechargeable battery alongside a generator, and keep stored water on hand. Estimate from your own past outages.
Would You Rather I Just Help?
Planning how long your power needs to last — and how much fuel or battery to keep on hand — is easy to get wrong in both directions. I'll help you estimate your outages, plan a sensible reserve, and set up safe fuel storage, with nothing to sell you. $99 flat for a visit, 30-day come-back-free.
Would you rather I just do this for you?
No shame in it — plenty of folks would rather hand it off, and that's what I'm here for. I'll come to your home, set it up right, and make sure it's working before I leave. Flat $99 per visit — no jargon, no upsell, and every visit is 30-day come-back-free.
Not a hardware job? I can often fix it remotely for $49 — a secure screen-share, up to 2 hours, that you watch the whole time and can end whenever (included free for $39/mo Support Plan members, and it counts toward the $99 if it turns out I need to come out).