Choosing a propane standby generator is half a purchase — the other half is the tank, and the propane tank size for whole house generator duty determines how many days your outage-proofing actually lasts. Undersize it and the most expensive machine in the yard goes dark halfway through the storm; oversize it and you've buried capital in rentable capacity. Here's the runtime math in plain numbers, the tank sizes that match house profiles, and the leasing-versus-owning decision that shapes the whole installation.== Step One: Know Your Generator's Propane Appetite ==Propane consumption scales with load and generator size. The planning figures (full-load, with realistic derating for the half-load cases):| Generator size | Full-load propane use | Half-load propane use ||---|---|---|| 14 kW standby | ~2.5–3 gal/hour | ~1.5 gal/hour || 20 kW standby | ~3–3.5 gal/hour | ~1.8–2 gal/hour || 24–26 kW standby | ~4–4.5 gal/hour | ~2.3–2.5 gal/hour |Two clarifications that prevent bad math: propane consumption is usually quoted in gallons of liquid propane (the tank's unit), and real outage loading sits closer to half-load than full-load — a standby generator running an average house with load management cycles between 40–70% as compressors and furnaces cycle. For planning, use half-load figures and add a 25% margin.== Step Two: The Runtime Table (Tank Size × Days of Coverage) ==Multiply consumption by days. The planning table for a 20kW-class unit at half-load (~2 gallons/hour, running 24-hour days):| Tank size | Usable gallons* | Days of 24/7 half-load | Realistic coverage ||---|---|---|---|| 120 gal (vertical, small yard) | ~96 | 2 days | Essentials, short outages || 250 gal | ~200 | 4 days | Essentials+, 1 AC, typical suburban || 500 gal | ~400 | 8 days | Whole-house, extended outages || 1,000 gal | ~800 | 16 days | Whole-house, rural, long-duration planning |*The usable fraction matters: propane tanks hold liquid with headspace for vaporization — the usable take is roughly 80% of nominal capacity, and freezing weather lowers vaporization rates further (large generators in cold climates need the tank sized for *vaporization capacity*, not just volume — the propane supplier does this calculation).More on this topic: generator for house guide