In short. A hybrid lets the generator run at an efficient load while the battery handles peaks, steps and low-load periods. Screening shows whether a hybrid is worth studying: how big the peaks are compared with the average, what fuel and running hours might be saved, and what BESS power and energy that implies.
Why the load shape decides
If the load is steady, a hybrid adds little. If it has large peaks over a lower base, or long light-load periods, the battery can shave peaks, allow a smaller generator and let the generator be switched off at times.
Screening steps
- Get a load profile, ideally at 1-minute to 15-minute resolution.
- Compute peak, average and the share of time above a candidate generator size.
- Choose a candidate generator size that runs in a good loading band for most of the time.
- The BESS covers the excess above that size: power equals peak minus generator size, energy equals the area above the line.
- Add allowances for losses, reserve and degradation.
Worked example (illustrative)
A site’s load averages 400 kW with a peak of 900 kW lasting about 10 minutes, several times a day. A generator sized for the peak would be 900 kW and run at about 44 per cent on average. A hybrid with a 500 kW generator and a BESS of 400 kW power can cover the peak. Energy for a 10-minute peak of 400 kW is about 67 kWh per event. With a margin for several events and reserve, a 250 kWh battery is a starting point. The generator now runs at about 80 per cent on average, using less fuel per kWh. If the load is far steadier, the saving disappears.
What to compare
| Item | Generator only | Hybrid |
|---|---|---|
| Generator size | Larger | Smaller |
| Average loading | Lower | Higher |
| Fuel use | Higher per kWh | Lower per kWh |
| Running hours | Higher | Lower, if engine can switch off |
| Capital cost | Lower | Higher |
| Complexity | Lower | Higher |
| Step response | Engine-limited | Faster with BESS |
Runtime for outage support
Runtime in outage depends on the fuel, not only the battery. The battery bridges and supports. Define the minimum reserve so it is available at the start of an outage. See DG, UPS and BESS hybrid.
What screening cannot tell you
- Control behaviour, stability and protection effects.
- Actual fuel savings, which depend on engine curves and dispatch strategy.
- Battery life under the real duty cycle.
- Commercial returns, including incentives and tariffs.
See generator and BESS hybrids and hybrid project screening.
A screening method for hybrid sizing
- Obtain a load profile (15 minute data over a week or more).
- Set the generator operating band (for example 60 to 80 percent of rating).
- Find hours where load is below the band (the battery covers) and above it (the battery assists).
- Size battery energy for the longest low-load or peak period between generator starts.
- Size battery power for the largest excursion.
Worked example (illustrative)
Night load 80 kW for 8 hours; day load 350 kW. A 500 kW generator at 80 kW load (16 percent) runs poorly. With a 400 kWh battery (usable 320 kWh), the battery can supply 80 kW for 4 hours. The generator then runs at 70 percent for about 1.5 hours to recharge the battery and carry load, then stops again. Run time falls from 8 hours at 16 percent loading to about 3 hours at 70 percent.
| Check | Result sought |
|---|---|
| Generator hours reduced | Lower fuel and maintenance |
| Starts per day | Not excessive |
| Battery cycles per day | Within warranty |
| Reserve kept for outages | Meets resilience target |
Where screening stops
Run a time-series simulation with real load and tariffs before purchase. See DG plus BESS dispatch.
Common mistakes
- Claiming fuel savings without a load profile.
- Using the battery for savings and leaving no outage reserve.
- Sizing the BESS on energy only.
What to send us
Send a load profile and the generator details. Send your application details.
Questions engineers ask
When does a hybrid make sense?
When load has large peaks, long light-load periods or frequent steps, and the economics or resilience justify it.
How much fuel can a hybrid save?
It varies widely with the profile. Estimate from your load data and engine curves, not generic percentages.
Do I still need a generator?
For long outages, yes. The battery supports but has limited energy.
Does the hybrid need special controls?
Yes, a coordinated controller for dispatch, state of charge and mode transitions.
Figures are illustrative. A hybrid study is needed before design.