Generator Sizing & Runtime Calculator

How the generator sizing & runtime works

Runtime is simply the tank capacity divided by the hourly fuel consumption — the units cancel, so it works in litres or gallons. Fuel cost per hour is consumption times your fuel price, and the full-tank cost follows from the runtime. The recommended generator size is the load grossed up by the margin, rounded to a standard rating.

What you enter

Enter the load, tank size, fuel consumption and fuel price; the calculator returns the generator size, runtime per tank and fuel cost per hour. Every field is editable, and the result recalculates the moment you change a value.

Reading the result

Runtime tells you how often you refuel; cost per hour tells you what backup power really costs. Size the generator to cover the surge of the largest motor, not just the steady load.

Safe by design

This calculator does the arithmetic of sizing — quantities, geometry and cost. It deliberately stops at the line a certified installer must sign off. It will never tell you a cable or fuse size, an earthing arrangement, a battery charge voltage, or a roof structural load: those are regulated calculations where a wrong number causes fire, electrocution or collapse. All electrical work must be carried out by a qualified, certified installer, and this page is not an installation guide.

Assumptions and limits

Fuel prices move, so you enter your own. Real consumption depends on load — a lightly loaded generator burns proportionally more fuel per kWh. Generators also need ventilation and exhaust routing that are safety matters outside this arithmetic.

Using it with the rest of BuildMetricLab

This calculator is one step in planning a system. Pair it with the other solar sizing, geometry and cost tools to go from an energy figure to a panel count, a layout and a costed bill of materials. Every BuildMetricLab tool runs entirely in your browser — no sign-up, nothing sent to a server, and the formula is shown on the page so the maths can be audited.