Estimate diesel consumption (L/h, gal/h) at any load from kVA or kW — or from the datasheet — then size the day tank and main storage tank for the running hours you need.
Estimates are typical for modern diesel gensets; actual consumption depends on engine model, altitude, temperature and fuel. Use the manufacturer's fuel curve for final sizing. Tank installation must comply with NFPA 37/30, local civil defence and environmental rules (bunding, venting, leak detection).
| Load | kW | L/h | US gal/h | L/kWh |
|---|
A modern diesel generator burns about 0.26 litres per kWh at full load. At part load it is less efficient, because the engine's friction and auxiliary losses stay roughly constant. Published fuel curves fit closely to
F = F100 × (0.08 + 0.92 × load fraction) — F100 the full-load consumption, either from the datasheet or 0.26 L/kWh × rated kW.
So a generator at 25% load uses about 31% of its full-load fuel, not 25% — and the fuel per kWh rises sharply at light load.
| Generator | 25% | 50% | 75% | 100% |
|---|---|---|---|---|
| 20 kVA (16 kW) | 1.3 | 2.2 | 3.2 | 4.2 |
| 50 kVA (40 kW) | 3.2 | 5.6 | 8.0 | 10.4 |
| 100 kVA (80 kW) | 6.4 | 11.2 | 16.0 | 20.8 |
| 250 kVA (200 kW) | 16.1 | 28.1 | 40.0 | 52.0 |
| 500 kVA (400 kW) | 32.2 | 56.2 | 80.1 | 104.0 |
| 1,000 kVA (800 kW) | 64.5 | 112.3 | 160.2 | 208.0 |
| 2,000 kVA (1,600 kW) | 129.0 | 224.6 | 320.3 | 416.0 |
Estimated at power factor 0.8 and 0.26 L/kWh full load. Small gensets typically use more per kWh than shown.
The day tank sits next to the generator and feeds it directly; it is refilled automatically from the main storage tank, which holds the fuel for the full autonomy the authority requires. Fuel inside the generator room is usually limited — NFPA 37 caps tanks in a generator room at 660 US gal (2,500 L) unless the room meets additional fire-protection conditions — so larger storage goes in a separate tank room or an external tank.
500 kVA (400 kW) generator at 75% average load, 8 h day tank, 48 h main storage, 90% usable volume.
Roughly 0.26 L per kWh at full load. A 500 kVA (400 kW) generator uses about 104 L/h at full load and about 80 L/h at 75% load. Use the manufacturer's fuel curve for exact figures.
Multiply the fuel consumption at the expected load by the hours the day tank must run, then divide by the usable fraction of the tank (typically 90%).
Part of the fuel goes to overcoming the engine's own friction and running its pumps and fan, which hardly changes with load. At light load that fixed share is spread over fewer kWh. Long running at very light load also causes wet stacking in diesel engines.
NFPA 37 generally limits fuel tanks in a generator room to 660 US gallons (about 2,500 L), with larger quantities allowed only under specific fire-protection conditions. Local fire authorities may set their own limits.
It depends on the NFPA 110 class and the local authority. Healthcare and other critical facilities often need 48 to 96 hours; commercial standby often 8 to 24 hours.
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