Inputs

Tank size(gal)

Usable fraction (safe-fill cap × cutoff)

Most generators are not rated to draw the tank below ~20%. Default 0.8 is conservative.

Manufacturer burn rate at 50% load(gph)

Pulled from the generator spec sheet — typically labeled "LP fuel consumption at 50% load".

Expected load(%)

Methodology

Runtime = usable gallons ÷ burn rate at the chosen load.

Usable gallons: 250 gal × 0.8 = 200 gal. The cap reflects the safe-fill maximum (~80% liquid) and a typical low-fuel cutoff at the regulator.
Burn rate scaling: most propane standby generators publish a single GPH number at 50% load (here, 1.5 gph). At 50% load, effective burn ≈ 1.5 × (0.4 + 1.2 × 0.50) = 1.5 gph.
Runtime: 200 ÷ 1.5 = 133.3 hours (~5.6 days).

For a precise number under your real load profile, use the manufacturer's published load-vs-consumption curve.

Generator Runtime Calculator