Generator Fuel Math: How Long Yours Will Actually Run After a Hurricane

Everyone on the Mississippi Coast owns some version of the same memory: the hum of generators for days after a storm, the gas-station lines, the neighbor asking to plug a freezer in. After Katrina, parts of the Coast waited weeks for power; after an ordinary strong hurricane, five to ten days of outage is a normal planning number. The question that decides whether your generator is a lifeline or a lawn ornament is brutally simple: how many hours of fuel do you actually have?

This guide does the arithmetic most spec sheets avoid — real consumption numbers for gasoline portables, propane standbys, and natural gas systems, what load management changes, and the safety math that matters more than any of it. The numbers are round and conservative; your exact figures depend on the unit and the load, but these are the right numbers to plan against.

Gasoline portables: the 24-hour problem

A typical 5,000–7,500-watt portable burns roughly three-quarters of a gallon per hour under real load. Run it around the clock and that is 15 to 18 gallons a day. The 15 gallons most prepared households have stored — three red cans in the garage — is one day. And the post-storm refill plan has a flaw everyone discovers at once: gas stations without power cannot pump, and the ones with generators draw lines measured in hours.

Portables still have a place: run intermittently — a few hours morning and evening to hold the fridge and freezer, charge phones, run fans — the same 15 gallons stretches to four or five days. That is the honest portable strategy: rationed hours, rotated loads, fuel treated like water. Stabilize stored gasoline and rotate it twice a year, or the fuel that was supposed to save you will varnish the carburetor instead.

Propane standbys: tank size is runtime

A whole-home standby in the 20–26kW range burns roughly 2 to 3.5 gallons of propane per hour depending on load. Against that, tank sizes translate directly into days:

• 120-gallon tank (about 96 usable gallons): roughly 1 to 2 days continuous — a weekend unit, not a hurricane unit.
• 250-gallon tank (about 200 usable): roughly 2.5 to 4 days at heavy load; 5–6 days with disciplined load management.
• 500-gallon tank (about 400 usable): the Coast standard — roughly 5 to 8 days continuous, more with management, which covers the restoration window of most storms.
• Buried tanks near the water should be anchored/ballasted — a properly installed buried tank rides out surge; an empty above-ground tank can float away.

Natural gas: 'unlimited' with an asterisk

Where natural gas service exists — most of Gulfport, Biloxi, and the denser Coast neighborhoods — a standby on gas runs as long as the utility keeps flowing, which through most hurricanes it does; buried gas mains shrug off wind that wrecks the electric grid. That makes natural gas the closest thing to set-and-forget runtime on the Coast.

The asterisk: in extreme events, utilities can curtail or lose pressure regionally, and Katrina did interrupt gas service in surge-devastated areas. The practical read is that natural gas is the right choice wherever it is available, and the households that want belt-and-suspenders resilience near the beach pair it with realistic expectations rather than a backup tank.

Load management: the multiplier nobody advertises

Fuel math is really load math. Central air conditioning is half or more of a Coast home's generator load in September — which is exactly when you will be running it. A system with load management (smart breakers that shed and alternate big loads) can hold the AC, refrigeration, and essentials on a smaller unit and stretch every tank number above by 30 to 50 percent. Cycling the AC a few degrees warmer, running pool pumps never, and doing laundry in daylight hours all buy real days.

This is also the honest argument against oversizing. A generator loafing at 25% load burns disproportionately more fuel per useful watt than one working at 60–70%. Sizing to your managed load — not to your panel's theoretical maximum — is cheaper twice: at purchase and every hour it runs.

The safety math that outranks all of it

Carbon monoxide kills more people after hurricanes than the storms' winds do in many years, and nearly every case is a portable generator in a garage, carport, or too close to a window. The rule has no exceptions: portables run outdoors, at least 20 feet from any door, window, or vent, exhaust pointed away, with battery CO alarms inside. Never in the garage, even with the door open. Not once, not overnight.

The wiring version of the same rule: never backfeed a portable through a dryer outlet. It can kill the lineman working your street and it is illegal everywhere. Portables feed appliances through heavy-gauge cords or through a properly installed transfer switch or interlock; standbys switch automatically and safely by design — one of the quieter reasons they are worth the money on a coast that does this every few years.