Where natural gas doesn't reach, a standby generator runs on the propane in your yard — which makes the tank a runtime decision, not a plumbing detail. Here's the math, the fill rule nobody explains, and the tank sizes Baldwin County storms actually call for.
Get My Free Generator QuoteA generator burns fuel at a rate set by its size and load. Generac's specification sheet for its 22 kW air-cooled model — the most common whole-home size here — lists propane consumption of about 2.53 gallons per hour at half load and 3.90 gallons per hour at full load. Real homes cycle between the two as the AC, well pump, and water heater come and go, so the half-load column is the standard planning case; the full-load column is your floor. Smaller units burn proportionally less, larger ones more — your model's spec sheet has its own figures.
Propane tanks are never filled to their nameplate capacity. Suppliers fill to about 80%, leaving headspace for the liquid propane to expand safely as temperatures change. So a "500-gallon tank" holds about 400 usable gallons, and every runtime figure below already accounts for that. It's also why the gauge on a freshly filled tank reads 80%, not 100% — that's full.
| Tank (nameplate) | Usable gallons | At ~half load (2.53 gal/hr) | At full load (3.90 gal/hr) |
|---|---|---|---|
| 120 gal | ~96 | ~38 hours (1.6 days) | ~25 hours |
| 250 gal | ~200 | ~79 hours (3.3 days) | ~51 hours (2.1 days) |
| 500 gal | ~400 | ~158 hours (6.6 days) | ~103 hours (4.3 days) |
| 1,000 gal | ~800 | ~316 hours (13.2 days) | ~205 hours (8.5 days) |
Planning estimates computed from Generac's published 22 kW consumption figures and the 80% fill rule — the same math behind the runtime chart in our cost guide. Last updated: October 2026.
Now hold those numbers against Baldwin County's record: five days after Hurricane Sally, roughly 18,000 Baldwin EMC meters were still dark (see our outage history). A 250-gallon tank runs out in the middle of that story. A 500-gallon tank finishes it. That's the whole sizing argument for most coastal homes:
Every figure above assumes the generator is the only mouth at the tank. If the same tank also feeds a furnace, water heater, range, or dryer, those appliances draw from the same usable gallons — in heating season, meaningfully. Homes that heat with propane should size up a class or accept shorter generator endurance, and say so on the quote request so the installer plans for it.
Tank size also decides placement, under NFPA 58, the national propane code your supplier and installer work to:
Local rules and your propane supplier's judgment govern the final placement; the supplier typically sells or leases the tank and sets it, and the installer coordinates the fuel line from there. On an itemized quote, the tank work should appear as its own line — if it doesn't, ask.
A big tank only helps if it's full when the storm arrives. Two habits close that gap: schedule a top-off when hurricane season opens (our readiness checklist builds it in), and consider a cellular tank monitor — especially for second homes and rentals, where nobody's home to read a gauge. Suppliers also offer automatic-delivery programs that track usage and fill before you run low; after a landfall, when delivery routes are chaos, starting full is the only guarantee.
If street gas reaches your home — Daphne Utilities serves much of the Eastern Shore, the City of Fairhope provides gas in Fairhope, and Riviera Utilities provides Foley's — the whole tank question disappears: no tank, no refills, no runtime ceiling, and a cheaper install (our FAQ walks the gas-or-propane fork by area). Where the gas main doesn't reach — much of inland Baldwin County — propane with a properly sized tank is the standard, proven answer. It just has to be sized for the outages we actually get.
Sources: Generac Guardian Series specification sheet for the 22/19.5 kW model (fuel consumption, as cited in our cost guide); propane tank separation distances per NFPA 58, summarized at LegalClarity's NFPA 58 separation-distance summary. Your propane supplier and installer make final sizing and placement calls for your property.
A local provider will review your details and contact you about next steps.