A complete fuel system usually has two tanks, not one, and they serve different purposes. Understanding the difference between a day tank and a bulk storage tank is the first step to specifying the right fuel system for your generator.
A small local tank positioned close to the engine, providing an immediate gravity-fed fuel supply. Typically sized for 1–8 hours of runtime.
Holds the reserve fuel and refills the day tank automatically. Sized for the full required autonomy — often 24, 48, 72, or 96 hours.
For emergency and standby systems, NFPA 110 defines the runtime requirements and fuel system design criteria your installation should satisfy.
The day tank ensures the generator always has an immediate fuel supply. Because the engine circulates more fuel than it burns (the excess returns to the tank), a day tank must be sized to handle roughly twice the full-load flow rate, not just the burn rate.
The bulk storage tank holds the reserve fuel and refills the day tank automatically. It is sized for the full required autonomy — often 24, 48, 72, or 96 hours — and is where NFPA 110 requirements apply. The calculator's advanced mode returns both figures, making it the single most useful output for a facility engineer.
Positioned close to the engine. Sized for 1–8 hours of runtime. Fire-pump standby applications are commonly sized for 8 hours.
Holds the main fuel reserve. Sized for total runtime requirements — typically 24, 48, 72, or 96 hours depending on application.
The bulk tank refills the day tank automatically. Fuel transfer pumps and controls are part of a complete system design.
For emergency and standby systems, NFPA 110 defines acceptance-testing and runtime requirements your fuel system must satisfy.
The calculator uses a single, well-established equation: Tank volume = fuel consumption (L/h) × runtime (h) × (1 + safety margin). Choose Quick Mode for direct fuel consumption input, or Advanced Mode for deriving consumption from generator power and load.
If you know your generator's specific fuel consumption in liters per hour, enter it here. The calculator applies the runtime and safety margin to size the tank.
Enter your generator's rated power and expected load. The calculator uses a specific fuel consumption of 200–215 g/kWh and diesel density of 0.84–0.85 kg/L to estimate fuel usage.
Before you can size the tank, you need an accurate fuel burn figure. This is the most common point of error, so it is worth getting right. Modern diesel generator sets consume roughly 0.24–0.26 liters per kilowatt-hour (L/kWh) at full load.
| Generator Size | Full Load (100%) | Typical Application |
|---|---|---|
| 50 kW | 12.5 L/h | Small business / farm backup |
| 100 kW | 25.0 L/h | Retail / light industrial |
| 200 kW | 50.0 L/h | Warehouse / medium construction |
| 500 kW | 125.0 L/h | Factory / data center |
| 1000 kW | 250.0 L/h | Large industrial / critical facilities |
For emergency and standby power systems in critical facilities, tank sizing is not a preference — it is a code requirement. NFPA 110 requires on-site fuel storage equal to 133% of anticipated fuel consumption for the required runtime class.
Many commercial standby systems require this minimum runtime class. Typical for retail, small offices, and light commercial applications where extended outages are less critical.
Medium-duty standby applications. Often specified for industrial facilities, manufacturing plants, and larger commercial buildings with extended outage risk.
Extended runtime class for facilities requiring multi-day autonomy. Common in remote sites, critical infrastructure, and applications with infrequent refueling.
96 hours is commonly required for hospitals and Level 1 critical facilities. A generator consuming 25 gallons per hour at expected load would need 2,400 gallons for 96 hours — but with the 133% buffer, the requirement rises to about 3,192 gallons. Enable the calculator's compliance mode to apply the 133% factor and report the minimum on-site storage for your class.
The extra 33% covers fuel-quality variation, load changes, generator efficiency, and calibration tolerance. If your facility falls under NFPA 110, enable the calculator's compliance mode — it applies the 133% factor so your calculation matches what an inspector will expect.
A generator consuming 76 L/h at 75% load, required to run for 96 hours under NFPA 110 Class 2. Base fuel = 76 × 96 = 7,296 L. With the 133% factor: 7,296 × 1.33 = 9,704 L. Select a 10,000 L bulk tank, with a day tank sized for 8 hours at the engine. The calculator performs these steps instantly.
Here are two worked examples using the exact method the calculator follows. The formula is straightforward: Tank volume = consumption (L/h) × runtime (h) × (1 + safety margin).
Generator: 100 kW, running at full load · Consumption: ≈ 25 L/h · Required runtime: 12 hours · Safety margin: 15%
Base fuel = 25 × 12 = 300 L
Safety volume = 300 × 15% = 45 L
Recommended tank = 300 + 45 = 345 L
Select a 400 L tank (about 106 US gallons)
Generator: 500 kVA (400 kW), running at 75% load · Consumption: ≈ 76 L/h · Required runtime: 48-hour Class · Safety margin: 20%
Base fuel = 76 × 48 = 3,648 L
Safety volume = 3,648 × 20% = 730 L
Recommended tank = 3,648 + 730 = 4,378 L
Select a 5,000 L bulk tank with an 8-hour day tank
The calculator performs these steps instantly and lets you adjust the load, runtime, and margin to see how each input changes the result. Use the compliance mode to automatically apply the NFPA 110 133% factor and report the minimum on-site storage for your class.
Usable volume: Allow 80–90% of nominal capacity for thermal expansion, sediment, and fuel pickup above the tank floor. A "1,000 L" tank may deliver only 800–900 L of usable fuel. Consumption: Use ~0.25 L/kWh as a planning estimate — actual varies by engine and load.
Two practical details often surprise first-time specifiers, and the calculator accounts for both. The right design is a small indoor day tank fed by a larger, compliant bulk tank located outdoors or in a rated enclosure.
You never draw 100% of a tank's nominal capacity. Allow 80–90% usable volume to account for thermal expansion, sediment, and the fuel pickup that sits above the tank floor. A "1,000 L" tank may deliver only 800–900 L of usable fuel. Always size for the usable volume, not the nominal capacity.
Many jurisdictions cap tank size and placement, especially indoors: 240 gallons total per room and 60 gallons per individual tank is a common building-code restriction. An unenclosed day tank or supply tank in a basement or lowest story is typically limited to 660 gallons; larger tanks must be enclosed behind fire-rated construction.
An indoor day tank is limited to 1,320 gallons (5,000 L) in many jurisdictions unless the tank is in a dedicated fire-rated room. Outdoor bulk tanks have fewer restrictions but require spill containment, weather protection, and access for refueling. Plan the tank location early in the project.
These limits often mean you cannot simply install one giant tank next to a basement generator. The right design is a small indoor day tank (typically 4–8 hours of runtime) fed by a larger, compliant bulk tank located outdoors or in a rated enclosure. The calculator's advanced mode returns both figures.
Before finalizing your fuel system:
✓ Confirm applicable NFPA 110 runtime class
✓ Apply the 133% factor for code compliance
✓ Check local tank size and placement limits
✓ Size for usable volume, not nominal capacity
✓ Verify day tank flow rate is double the burn rate
Final fuel system design should be checked against the complete load schedule, engine specification, applicable codes (NFPA 110, local fire codes), and site-specific conditions before equipment is ordered. The calculator's compliance mode applies the 133% factor and reports the minimum on-site storage for your class.
We manufacture diesel generator sets from 8 kVA to 4000 kVA — with Cummins, Perkins, Weichai, and Yuchai engines and Stamford alternators — and we supply the base-frame tanks, day tanks, and bulk storage systems to match.
A: Multiply your generator’s fuel consumption (liters per hour) by your required runtime in hours, then add 10–25% safety margin. A 100 kW generator at full load burns about 25 L/h, so a 12-hour run needs roughly 345 L — select a 400 L tank.
A: Use the formula: tank volume = fuel consumption × runtime × (1 + safety margin). Estimate consumption at about 0.25 L/kWh at full load, or enter a known L/h figure, then factor in 80–90% usable volume.
A: A day tank is a small local fuel tank near the engine that provides an immediate, gravity-fed supply. Day tanks are typically sized for 1–8 hours of runtime, with 8 hours standard for fire-pump standby applications.
A: NFPA 110 defines runtime classes of 24, 48, 72, and 96 hours depending on application, and requires on-site fuel equal to 133% of the anticipated consumption for that class. Hospitals and critical facilities commonly require the 96-hour class.
A: It is the NFPA 110 Class 96 requirement for emergency power systems in critical facilities: enough on-site fuel for 96 hours at the expected load, calculated at 133% of anticipated consumption to account for variation and degradation.
A: A modern diesel generator set uses roughly 0.24–0.26 L/kWh at full load, or about 0.247 liters per hour per kilowatt. A 100 kW unit burns about 25 L/h, and a 400 kW unit roughly 100 L/h at full load.
A: Divide the usable tank volume by the fuel consumption rate. A 1,000 L tank with 900 L usable feeding a 25 L/h generator runs about 36 hours. Use the [generator fuel runtime calculator](#) to reverse the equation.
A: Base-frame (integrated) tanks usually hold only 6–10 hours of full-load fuel. For longer autonomy, or to meet NFPA 110 storage classes, you will need an external bulk tank feeding the generator’s day tank.
