In short. Each source does what it is best at: the UPS conditions and bridges the most sensitive loads, the BESS handles power-heavy, short-duration duties and steps, and the generator supplies sustained energy. The combination can reduce generator size and running hours, provided the controls and protection are coordinated.
Who does what
| Source | Strength | Limit |
|---|---|---|
| Generator | Long energy supply from stored fuel | Seconds to start; limited step and light-load capability |
| UPS | Instant, clean supply for sensitive loads | Short autonomy |
| BESS | Fast, high power, flexible duty | Finite energy; limited fault current |
Hybrid operating modes
- Normal: utility supplies the load; BESS and UPS stay charged.
- Outage start: UPS and BESS carry the load while the generator starts.
- Generator support: the generator takes the base load, the BESS handles steps and peaks.
- Recovery: the generator recharges storage, then the system returns to utility.
Define a minimum BESS reserve so it is always able to bridge the next outage.
Sizing logic
- Size the UPS to the critical load that cannot tolerate any break.
- Size the BESS power for the largest step the generator should not carry alone, and energy for the bridge and support time.
- Size the generator for the base load plus recharge power, with fewer margins for steps if the BESS takes them.
See DG and BESS hybrid screening.
Interactions to check
- UPS and generator: input current distortion and step loading.
- BESS and generator: power sharing, frequency and voltage control, and avoiding reverse power into the generator.
- Protection: low inverter fault current, and selectivity on the generator supply.
- Control priority: who sets frequency in each mode; avoid two sources fighting.
Fuel and emissions effect
By holding the generator at an efficient load and letting storage cover peaks, a hybrid can reduce fuel burn and engine wear and cut running hours. Savings depend on the load profile, so assess them with real data rather than assuming a percentage.
Worked illustration
A site load swings between 400 kW and 900 kW with short peaks. A generator sized for 900 kW runs at low loading most of the time. A hybrid design uses a 600 kW generator and a BESS that supplies the peaks above 600 kW for short periods. The generator runs at higher average load and uses less fuel per kWh. The design is only valid if the BESS state of charge is managed so the peaks are always covered.
Testing
- Utility loss with all sources in service.
- Generator start failure, with BESS and UPS carrying load.
- BESS trip during generator operation.
- Load steps from low to high and back, observing frequency and voltage.
Roles in the hybrid
| Source | Role | Time scale |
|---|---|---|
| UPS | Protect the sensitive load instantly | 0 to minutes |
| BESS | Bridge, shave peaks, support transients | Seconds to hours |
| Generator | Long-duration energy | Minutes to days |
| Utility | Normal supply | Continuous |
The UPS protects the instant; the generator protects the duration. The BESS adds flexibility between them.
Operating scenarios to design and test
- Mains failure: UPS and BESS carry; generator starts; transfer; UPS walk-in.
- Generator fails to start: BESS and UPS run to the end of reserve; shed non-critical load; call rental.
- Generator trips on load: BESS and UPS take over; restart.
- Mains return: synchronise, transfer back, recharge in a controlled manner.
Control priorities
- Recharge limited so it does not overload the generator.
- Defined reserve of battery energy that is never used for trading.
- Clear authority: one controller decides the mode.
Hazards at the interfaces
Voltage and frequency windows, harmonic currents, fault level and protection settings in each source combination. See hybrid system studies.
Common mistakes
- Letting the BESS discharge for savings below the resilience reserve.
- No coordinated control, so sources respond independently.
- Assuming the generator can be downsized without checking all operating modes.
What to send us
Send your load profile, critical loads and outage target. Send your application details. See also generator and BESS hybrids.
Questions engineers ask
Can hybrid design reduce generator size?
It can, where the BESS covers steps and peaks. Savings depend on the profile and the control strategy.
Is a UPS still needed with a BESS?
Often yes, for load conditioning and a defined transfer. A BESS can sometimes replace a UPS, but this needs detailed design.
Who controls frequency in the hybrid?
Define it per mode: the generator or a grid-forming BESS. Two sources must not both try to set the frequency without a defined scheme.
Does hybrid improve resilience?
It can add a bridge and reduce engine stress, but it also adds controls that must be tested. Resilience comes from the whole design.
This guide gives general principles. Hybrid systems require an engineering study.