Solar & battery
Solar Generator Fuel Calculator
Works generator fuel for a hybrid system, where run hours drive consumption.
Result
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.
Get a Free Business AuditHow the calculation works
Generator consumption at the load it runs multiplied by the hours required, which depends on how much energy the solar and battery cannot supply. In a well-designed hybrid system the generator runs for the worst periods rather than routinely, and the fuel figure reflects how often those occur.
What moves the number
- Hours the generator must run.
- Consumption rate at the load it carries.
- How much of the deficit solar and battery cover.
- Fuel storage and resupply logistics.
Where this one goes wrong
Generators are inefficient at light load, so running a large generator to carry a small load burns disproportionate fuel, which is why charging the battery hard while the generator runs and then shutting it down uses less fuel than running it continuously at low output. Fuel storage is the other practical matter, since the quantity needed for an extended outage is substantial and fuel degrades in storage, which means rotation rather than accumulation.
Before you quote
- Run the generator at meaningful load, not lightly loaded.
- Calculate fuel for the worst expected outage, not the average.
- Rotate stored fuel rather than letting it sit.
Questions
About this calculator
How should a generator be run in a hybrid system?
At meaningful load, charging the battery hard and then shutting down, rather than running continuously at light output. Generators are inefficient at low load, so the intermittent pattern uses considerably less fuel for the same energy.
How much fuel should be stored?
Enough for the worst expected outage rather than the average one, which is a substantial quantity. The complication is that fuel degrades in storage, so the stock has to be rotated and treated rather than simply accumulated.
Does a generator make a solar system a failure?
No, it is usually the cheaper design. Sizing solar and storage for the worst weeks of the year costs far more than occasional generator use, which is why most well-designed off-grid and hybrid systems include one.
The formula
fuel = kWh generated x consumption per kWh x part-load penalty
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How to use this calculator
- Load the generator carries (kW). Enter your own figure; the page starts at 4.
- Generator rating (kW). Enter your own figure; the page starts at 7.5.
- Hours it runs. Enter your own figure; the page starts at 12.
- Fuel. Enter your own figure; the page starts at Propane (gallons).
- Fuel price per unit ($). Enter your own figure; the page starts at $3.2.
The result updates as you type, so you can try a figure, see what it does to fuel used, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Load the generator carries (kW) | 4 |
| Generator rating (kW) | 7.5 |
| Hours it runs | 12 |
| Fuel | Propane (gallons) |
| Fuel price per unit | $3.2 |
What comes out
| Result | Value |
|---|---|
| Fuel used main result | 5.3 |
| 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 $0.35 per kWh.
How sensitive the answer is
Holding everything else steady and changing only fuel price per unit, this is what happens to fuel used. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Fuel price per unit ($) | Fuel used |
|---|---|
| $1.6 | 5.3 |
| $2.4 | 5.3 |
| $3.2 example above | 5.3 |
| $4.8 | 5.3 |
| $6.4 | 5.3 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Load the generator carries (kW)
- A number, between 0 and 500, starting at 4.
- Generator rating (kW)
- A number, between 0 and 2000, starting at 7.5.
- Hours it runs
- A number, between 0 and 720, starting at 12.
- Fuel
- Choose from Propane (gallons), Petrol (gallons), Diesel (gallons), Natural gas (therms).
- Fuel price per unit ($)
- A money amount, starting at $3.2.
What the results tell you
| Result | What it is |
|---|---|
| Fuel used | The headline result. A decimal figure. |
| Fuel cost | A money figure. |
| Consumption per hour | A decimal figure. |
| Cost per kWh generated | A money figure. |
| Load factor | A percentage. |
A design still needs a site visit
Output and sizing figures use the assumptions you enter, not measured irradiance, shading or roof orientation at your address. Real production depends on all three, plus inverter behaviour and local interconnection rules.
Estimating is the easy part
The jobs you never hear about cost more than the ones you mis-measure. BDEVY builds the marketing, systems and automation that bring them in.