"How big a tank?" is the question that arrives with almost every generator enquiry, and it cannot be answered from the kVA rating alone. Two numbers decide it, and both come from the project, not from the tank maker.
Consumption at the real load
The engine data sheet gives the fuel consumption at several loads. A standby set rarely runs at its rating; the load it actually carries - the facility's load after the transfer switch has done its work - is what the tank has to feed. Sizing on full load is the safe assumption and the usual one, but an operator who knows the real load can size closer to it. What should never be used is the consumption at a low test load, which flatters the autonomy.
Hours without a delivery
The second number is a decision, not a measurement. For a factory it may be the time until the grid is normally back; for a hospital or a data center it is set by the design standard the owner follows, and it can be a day or several days at full load. During a wide outage the fuel tankers are busy everywhere, so the autonomy has to cover the time until a delivery is realistic, not the time until one is ordered.
Nominal and usable
A tank is not filled to the brim and is not emptied to the last litre. The filling stops below the top to leave room for expansion and to let the high-level switch do its job; the suction stops above the bottom so that water and sediment stay out of the engine. The usable volume between those two points is what the autonomy is calculated on. A belly tank quoted at its nominal volume delivers fewer hours than the figure suggests, and the difference shows at the worst moment.
Where the autonomy lives
Short autonomy fits under the set. Long autonomy lives in bulk storage, and the belly tank or a day tank becomes the working reserve that the transfer system keeps full. The split matters for the site: bulk tanks need their own foundation, bunding, filling point and often a permit; the tank at the set needs level control and an overflow route back to storage.
Fuel that waits
A standby tank is filled once and then waits, sometimes for years. Diesel does not wait well: condensed water collects at the bottom, microbes grow where water meets fuel, and the sludge they leave blocks filters when the set finally runs for hours instead of minutes. Autonomy therefore has a second meaning - not only enough fuel, but fuel that still burns. Periodic sampling, a drain at the low point and, for long-term storage, a fuel polishing system that circulates and filters the tank contents are part of the fuel system, not an extra.
AXON Technic sizes and supplies belly tanks, daily fuel tanks, transfer systems and fuel polishing for generator installations. Send the engine data sheet and the required autonomy through the contact page, or read the fuel systems guide first.