PlantmarginBattery or generator for backup power: the honest comparison
When the grid fails, a plant needs its critical loads back within seconds: control systems, refrigeration, lighting, pumps, sometimes a whole process line. The two common answers are a diesel generator and a battery. Each is good at different things.
What each does well
| Diesel generator | Battery (LFP) | |
|---|---|---|
| Changeover | Seconds to start and transfer | Instant, with a hybrid inverter |
| Long outages | As long as there is fuel | Limited by stored energy |
| Running cost | Fuel and maintenance per hour | Very low |
| Daily use | Not economic | Can shift solar to the evening every day |
| Investment per kW of backup | Lower | Higher |
When a battery pays
A battery used only for backup sits idle most of the time, which makes it an expensive insurance policy. It pays when it works every day:
- storing midday solar surplus for the evening;
- trimming the evening peak that sets the demand charge;
- carrying short outages so the generator does not start for every dip.
When the generator remains the answer
- Outages that last many hours or days.
- Large motor loads that the battery inverter cannot start.
- Sites where daily cycling has little value (no solar, flat tariff, no peak to trim).
Why many sites use both
A common design lets the battery carry the first hour or two of an outage and the generator take over for longer ones, charging the battery at an efficient load. The generator runs fewer hours, at better efficiency, and starts less often.
Example: a site with 12 hours of outages a month and a 60 kW critical load. Without a battery, the generator runs every outage. With a battery sized for 4 hours, most short outages never start it, and on long ones it runs near its efficient load. The fuel saved depends on how the outages are spread, which is why measured outage data beats an estimate.
Plantmargin logs every outage, generator hour and litre, so the case for a battery can be made from the plant's own history.