Choose a generator fuel tank by the usable fuel it provides between refills, not its nominal capacity alone. The tank must cover the planned operating hours until the next reliable refill, plus any fuel reserve you choose to retain. For propane vapor systems, adequate stored fuel does not by itself establish that the tank can supply the required gas flow. Generac’s residential standby guidance, for example, identifies fuel demand and climate as additional tank-sizing factors.
Convert the Refill Interval into a Fuel Requirement
For a constant assumed fuel-consumption rate:
[
V_{\text{required}} = q \times t + R
]
Where:
- (V_{\text{required}}) = usable fuel required at the start of the interval, in US gallons.
- (q) = generator fuel consumption at the planned operating load, in US gallons per hour.
- (t) = generator operating hours before the next reliable refill.
- (R) = usable fuel you intend to retain when that refill arrives, in US gallons.
Use liters and liters per hour instead if preferred; do not mix US gallons with Imperial gallons.
Obtain (q) from the specific generator’s manufacturer data at the relevant load, fuel type, and rating. Fuel consumption is not a universal gallons-per-kW value. For example, Cummins’ QSB5.9 specification sheet, Revision 03, November 2022, lists separate consumption figures at 75% and 100% load for its C140D5P and C160D5P prime-rated diesel sets, including radiator and fan operation, and specifies a +5% fuel-consumption tolerance. Those conditions and figures apply to the listed equipment, not generators generally.
For a changing load, calculate fuel for each operating period using an appropriate consumption rate, then add the period totals and the reserve. Do not assume consumption varies linearly with electrical load.
The refill interval should reflect when fuel can reliably reach the site, not just the supplier’s normal delivery schedule. If a generator operates intermittently, distinguish elapsed delivery time from actual engine-running hours.
Tank Size vs. Runtime Selection Table
This table converts a required refill interval into usable fuel capacity and compares illustrative tank options. Every row assumes a constant consumption of 2.0 US gal/h and a retained reserve of 6 US gal. These are calculation inputs—not manufacturer fuel-consumption data, standard tank sizes, or prescribed reserve requirements.
| Operating hours until refill | Fuel consumed at 2.0 US gal/h | Usable fuel required, including 6 US gal reserve | Smallest sufficient illustrative option | Data basis |
|---|---|---|---|---|
| 12 h | 24 US gal | 30 US gal | 30 US gal usable | Assumed inputs stated above; calculated, no source edition applicable |
| 24 h | 48 US gal | 54 US gal | 60 US gal usable | Assumed inputs stated above; calculated, no source edition applicable |
| 36 h | 72 US gal | 78 US gal | 90 US gal usable | Assumed inputs stated above; calculated, no source edition applicable |
| 48 h | 96 US gal | 102 US gal | 120 US gal usable | Assumed inputs stated above; calculated, no source edition applicable |
The available illustrative options are 30, 60, 90, and 120 US gal of usable fuel. They are not nominal tank capacities. Replace them with documented usable capacities for the tanks being considered, and replace the assumed consumption and reserve with project-specific inputs.
Worked Example: Selecting for a 24-Hour Refill Interval
Assume a diesel generator consumes 2.0 US gal/h throughout the planned operating period. The next reliable refill is 24 operating hours away, and the owner wants 6 US gal remaining at delivery.
[
V_{\text{required}} = 2.0 \times 24 + 6 = 54\text{ US gal}
]
A tank providing 30 US gal of usable fuel is insufficient. An option providing 60 US gal of usable fuel satisfies this arithmetic requirement.
The estimated time before reaching the retained reserve is:
[
t_{\text{to reserve}} = \frac{V_{\text{usable}}-R}{q}
= \frac{60-6}{2.0}
= 27\text{ h}
]
That provides three operating hours beyond the assumed 24-hour refill interval at the same consumption rate. It is not a guaranteed runtime: a higher load, a higher actual consumption rate, or a lower starting fuel level reduces the interval.
Check What the Tank Capacity Actually Represents
For this comparison, usable fuel means the amount available between the planned starting fill level and the lowest permitted operating level. The retained reserve is part of that usable amount; fuel below the permitted operating level is not a reserve you can count on consuming.
Before choosing a nominal tank size, establish:
- The permitted starting fill quantity, rather than assuming the tank can be filled to its entire geometric volume.
- The fuel remaining when the generator must stop, including applicable pickup, low-fuel shutdown, or minimum operating-level restrictions.
- Whether other equipment draws from the same supply.
- Whether the quoted capacity is nominal storage volume or documented usable fuel.
Obtain these quantities from the tank documentation, generator fuel-system requirements, and installation design. Do not apply one usable-capacity percentage to every diesel, gasoline, or propane installation.
Propane Requires a Separate Supply-Capacity Check
A propane tank can contain enough liquid fuel for the calculated duration yet still be unsuitable for the required vapor supply. Tank selection must also account for the applicable generator requirements, fuel demand, and climate.
Generac’s residential standby support guidance, dated December 29, 2025, states a minimum 250-gallon tank for the residential standby applications it describes and notes that a larger tank may be needed depending on installation, demand, or climate. This is manufacturer guidance for those applications—not a universal minimum for all propane generators.
Have the propane supplier and installer confirm tank capacity, vapor supply, pressure regulation, and piping against the specific generator documentation and applicable local requirements. Do not treat nominal tank gallons as gallons of propane available for consumption.
What the Table Does Not Decide
The table compares fuel availability with an assumed consumption rate and refill interval. It does not determine whether the generator can start the connected motors, carry the electrical load, or operate under the intended duty rating.
Before ordering, confirm the generator’s exact model and rating, the tank’s documented usable capacity, and compatibility with the fuel system. Any larger or auxiliary tank arrangement must also satisfy the equipment instructions and applicable local installation requirements, as assessed by qualified professionals. A tank with sufficient calculated runtime is only a candidate for that installation—not an approved fuel-system design.