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Solar & battery

Solar Generator Fuel Calculator

Fuel a backup generator burns covering the gap solar and storage cannot, and what it costs.

Your numbers

$

Result

Fuel used 5.3 Gallons or therms
Fuel cost $16.90
Consumption per hour 0.4
Cost per kWh generated $0.35
Load factor 53.3%
About 5.3 units over 12 hours, costing $16.90. That works out at $0.35 per kWh.

Generators are least efficient at light load, which is why a large unit running a small load wastes fuel and wet-stacks on diesel. Consumption figures here are broad averages; the manufacturer publishes a curve for the specific unit. Sizing a generator to the surge rather than the average is the usual reason load factors end up so low.

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Questions

About this calculator

Why is generator power so expensive per kWh?

Fuel, inefficiency at part load, and maintenance. It is typically several times grid electricity, which is what makes battery storage attractive for short frequent outages and a generator sensible for rare long ones.

Does a bigger generator use more fuel?

At the same load, yes, noticeably. An oversized generator spends its life at a low load factor where efficiency is poor, and on diesel it also causes unburnt fuel to accumulate in the exhaust.

The formula

fuel = kWh generated x consumption per kWh x part-load penalty

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