How Long Can a Standby Generator Actually Run During an Outage?
The honest answer depends entirely on fuel type, and it's worth understanding before an outage, not during one.
Natural gas: as long as the outage lasts
A natural-gas standby generator draws from your home's gas line, which draws from the utility's distribution system — there's no on-site tank to run dry. In practice, this means natural gas generators can run continuously for the full duration of an outage, day after day, as long as gas keeps flowing to your meter. That's the appeal, and for most outages (storm-related, hours to a couple of days) it's a real advantage.
The caveat, and it's a real one: natural gas supply itself can fail in extreme conditions. During Winter Storm Uri (see our full explainer), wellhead freeze-offs curtailed natural gas supply in parts of the state — a rare tail risk, but the reason "unlimited runtime" isn't quite the same as "runtime with zero risk of interruption."
Propane: until the tank runs dry — here's the actual math
Propane generators draw down a fixed on-site tank, so runtime is a straightforward calculation once you know two numbers: tank size and consumption rate.
| Generator size | Approx. propane use at half load | 500-gal tank (400 usable) | 250-gal tank (200 usable) |
|---|---|---|---|
| 10–14 kW | ~1.0–1.5 gal/hr | ~11–17 days | ~5–8 days |
| 16–20 kW | ~1.5–2.0 gal/hr | ~8–11 days | ~4–5 days |
| 22–26 kW | ~2.0–2.5 gal/hr | ~7–8 days | ~3–4 days |
These are rough, half-load estimates — running at full load (everything on at once, hottest part of the day) burns noticeably more; running lighter loads (overnight, essentials only) burns less. Tanks are typically only filled to about 80% of rated capacity by code, which is why the "usable" column is lower than the tank's nominal size. Your specific generator's data plate lists its exact consumption rate at different load percentages — use that number, not the table, for real planning.
What actually limits runtime in practice
- Maintenance intervals — most standby units need an oil change around 100–200 hours of run time (check your model's manual), which matters if an outage genuinely runs for a week or more continuously.
- Weekly self-test cycles use a small amount of fuel even when there's no outage — negligible, but it's why a "full" tank checked once a year isn't the same as full right now.
- Propane delivery during a widespread regional event — if the same storm that caused your outage also disrupts local roads, delivery trucks may not reach you to refill mid-event. A larger tank is your buffer against this, not a delivery guarantee.
The practical takeaway
If your area has reliable natural gas service, it's the simpler answer for "how long will this run" — effectively as long as you need, with the rare-event caveat above. If you're on propane, size your tank to your realistic worst-case outage length, not the average one — a 250-gallon tank is fine for typical storm outages, but a household specifically worried about multi-day, Uri-scale events should talk to their installer about a larger tank.