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Off-grid and generatorsExplainer

Solar or a Diesel Generator: Which Costs Less to Run?

Diesel power typically costs three to four times grid power in fuel alone. How solar compares for sites off the grid, and when to keep a generator for backup.

By Mujtaba Ali, Co-Founder & Director

4 min readLast reviewed

A farm running on solar and a battery by day and night, with a generator held back for backup.Illustration

The short answer

Diesel power typically costs three to four times what grid power costs, in fuel alone, before rental and maintenance. Solar has no fuel bill. For a site with no grid connection, the usual answer is both: solar and a battery run the site, with an optional generator behind them where the site needs one.

On this page5 sections

Which costs less to run, solar or diesel?

Solar costs less to run, because it has no fuel bill. Diesel power typically costs three to four times what grid power costs, in fuel alone. That is before generator rental, servicing and the trips to refuel.

For a site far from the grid, or waiting on a connection, the question is rarely solar or diesel. It is how many hours of diesel you can switch off. See how that works on our off-grid solar with generator backup page.

Why does a lightly loaded generator waste fuel?

A generator should run above roughly a third of its rated load; below that it wastes fuel and wears faster.

A generator sized for the busiest hour of the day runs at a light load most of the time. It burns fuel to idle, and it wears faster doing so. A site that runs only on diesel pays for this every hour.

Rated load
The most a generator is built to carry. Fuel use per kWh is lowest near it.

What does a hybrid controller do?

The controller reads the load, the sun and the battery, and picks the source that holds the load for the least fuel. Solar and the battery come first. The generator is called only when solar and the battery cannot hold the load, and it stops as soon as they can, so it does not idle at a light load.

The same controller is the DG-PV synchronizer (DG is the diesel generator, PV the solar). While the generator runs, it keeps solar and the generator in step: it trims the solar so the generator stays above its safe load and no power is pushed back into it, and it moves the load between solar, the battery and the generator smoothly. That is what lets the whole system run as one.

Hybrid controller
Also called a DG-PV synchronizer: the unit that keeps solar and the generator running in step, and decides, second by second, whether the load runs on solar, the battery or the generator.

Why does a battery help?

Solar makes power only while the sun is up. A battery stores the surplus and carries the evening and the night, so the generator does not have to. Those hours are where a generator would otherwise be running. The longer the battery can carry the site, the fewer hours the generator runs.

A commercial battery cabinet that carries the site through the evening and the night. Illustration

When should you keep a generator?

Keep one as backup when the cost of an outage is high, or when your load has short peaks above what the battery can supply. It is an optional generator behind the system, not the main supply.

Our price covers the solar, the controller and the battery. If you already run a generator, the design can work with it. If you want us to supply one, we quote it separately, so you see our price and your total side by side.

Off-grid isn't offered for DEWA-connected sites in Dubai; it can be, for a site that isn't connected to DEWA, such as a remote site.

Solar does the work by day, the battery by night, and the generator waits for the rare day it is needed.

If you are waiting on a supply, read how long an electricity connection takes. To see the system itself, start at off-grid solar with generator backup.

Generator FAQs

All 48 questions →

Mostly, yes. Solar and a battery can run a site by day and through the night, and an optional generator stays as backup for the longest cloudy spells and the biggest loads.

Diesel power typically costs three to four times what grid power costs, in fuel alone, before rental and maintenance. Solar has no fuel bill, so the saving grows with every hour a generator stops running.

It depends on the load. A generator should run above roughly a third of its rated load; below that it wastes fuel and wears faster. Lightly loaded sets waste more, which is why a controller that keeps them off at light load saves fuel.

A battery is optional, and we recommend it. It carries the evening and the night, so the generator is not left running at a light load while the sun is down.

Keep reading

Running on diesel today? Start with your fuel receipts.

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