Backup Generator Fuel & Runtime Calculator
Estimates diesel/propane/natural gas genset fuel burn and tank runtime from rated load
Estimates backup generator fuel consumption and expected tank runtime using the linear no-load-to-full-load fuel curve manufacturers publish on genset spec sheets, adjusted for fuel type (diesel, propane, or natural gas) and an unusable-fuel allowance for the tank.
How generator runtime is estimated
Standby and prime-power gensets consume fuel roughly in proportion to load, but with a fixed parasitic/no-load burn that manufacturers publish as 25/50/75/100%-load points on spec sheets. This calculator interpolates linearly between the no-load and full-load rates for the selected generator size, then divides usable tank volume (after an unusable-heel allowance) by that rate to estimate runtime.
Frequently asked questions
Why does load percentage matter so much for runtime?
Diesel gensets burn noticeably less fuel per hour at partial load, so running closer to 50% load rather than 100% can meaningfully extend runtime for the same tank size — though sustained very-light loading (<30%) can cause wet-stacking issues over time.
What is the "unusable fuel" allowance for?
Sub-base and above-ground tanks reserve a bottom heel (sediment, water separation, and suction pickup clearance) that isn't usable fuel — typically 5–10% of nominal capacity depending on tank design.
Why is natural gas treated as unlimited runtime?
Natural gas gensets are typically fed directly from the utility distribution main rather than an onsite tank, so runtime is not fuel-storage-limited under normal utility service — the constraint becomes upstream gas supply reliability.