In short. Runtime equals usable fuel divided by consumption rate. Size the tank for the outage you plan to ride through, allow for unusable volume and a reserve, and plan resupply for longer events. Fuel storage also brings safety, spill and regulatory requirements that must be checked locally.
The calculation
Tank volume (L) = Fuel rate × Target hours ÷ Usable fraction
Usable fraction accounts for fuel below the pickup, tank margins and a deliberate reserve. A common allowance is 80 to 90 per cent, but check the tank design.
Worked example (illustrative)
A set burns about 77 litres per hour at its typical load. The site wants 24 hours of runtime and a usable fraction of 0.85. Tank volume = 77 × 24 ÷ 0.85 ≈ 2,174 litres. A 2,500-litre tank is a sensible standard size, giving about 27 hours of usable runtime at that load. At full load the consumption is higher, so runtime would be shorter. Check both the typical and the worst-case loads. See fuel consumption.
Tank arrangements
| Type | Use | Notes |
|---|---|---|
| Base tank (under the set) | Common for small to medium sets | Compact; runtime limited by size |
| Day tank | Small tank near the engine, fed from bulk | Needs transfer pumps and level control |
| Bulk tank | Larger storage, above or below ground | Rules on location, bunding, venting |
| Mobile or rental tank | Extra runtime for events | Planned connection and transfer |
See fuel tank sizing and rental fuel tanks.
Design points
- Fuel transfer pumps and valves should be reliable and, for critical sites, backed by generator power or a gravity or manual alternative.
- Fuel quality: diesel degrades in storage. Sample, filter and polish periodically. See fuel cleaning and polishing.
- Level instrumentation and low-level alarms.
- Spill containment and bunding.
- Venting, overfill protection and fire protection.
- Compliance with local rules for storage quantity, location and licensing.
Planning for longer events
For outages beyond the tank’s runtime, you need resupply: a contract with a fuel supplier, with response time guarantees, access to the site and a safe way to refill while running. Consider how a regional disaster may disrupt supply. A common approach is to plan tanks for a target runtime and a resupply agreement for beyond.
Tank size from runtime
Required usable fuel (L) = consumption (L/h) × required hours × reserve factor
Worked example (illustrative)
A 500 kVA set at 75 percent load burns around 85 L/h. Required autonomy 24 hours with a 10 percent reserve: 85 × 24 × 1.1 ≈ 2,244 litres usable. Allow for unusable bottom volume and expansion space, so the tank might be rated 2,500 to 2,700 litres. Check that the tank is installed and licensed under the applicable rules for storing diesel.
| Design aspect | Note |
|---|---|
| Day tank | Small tank near the engine fed by a transfer pump |
| Bulk tank | Larger tank with transfer pumps and a backup |
| Fuel quality | Water drain, filtration, polishing |
| Transfer pump | Duplicate pumps and manual option for critical sites |
| Spill containment | Bund and venting as per local rules |
Refuelling logistics
Autonomy is the tank plus the realistic delivery time during an emergency. During floods or area-wide outages, delivery may be delayed, so confirm supplier arrangements.
Fuel ageing
Stored diesel degrades over months. Rotate stock, test and polish where needed.
See fuel consumption guide and emergency and standby architecture.
Common mistakes
- Sizing the tank from catalogue full-load consumption but running at a different load.
- No allowance for unusable volume.
- Stale fuel from long storage.
- A transfer pump that stops during the blackout it is meant to serve.
What to send us
Send the target runtime, the generator model and space for tanks. Send your application details.
Questions engineers ask
How many hours of fuel should I store?
It depends on your outage risk and resupply options. Many critical sites plan for a day or more, with resupply for beyond.
How long can diesel be stored?
It depends on conditions and treatment. Sample and test it regularly, and polish or replace it as needed.
Do I need a day tank?
For larger bulk storage, usually yes, to feed the engine safely and consistently.
Are there rules for storing fuel?
Yes. Quantities, location, bunding and licensing are regulated locally. Check with the authorities.
Figures are illustrative. Fuel storage must comply with applicable regulations.