Emergency power supply – generator or BESS?
Ensuring power continuity is today not only a matter of comfort, but often a condition of safety, process continuity and meeting contractual requirements (SLA). When designing emergency power supply, two solutions are most often considered: a generator and BESS (Battery Energy Storage System).
Below we present a practical comparison and tips on when a given solution makes sense.
Basic difference between generator and BESS
Generator
Generates electricity from fuel (most often diesel). Can work as long as fuel is available and service is provided.
BESS
Stores previously accumulated energy and releases it immediately after grid failure, but only until the battery is discharged.
In practice this means:
- •Generator = generator = long operating time, but a few seconds break for startup,
- •BESS = BESS = immediate backup, but limited operating time.
When to use a generator
A generator is a good choice when:
- multi-hour or multi-day backup time is required,
- the facility has high power demand,
- a break of several seconds for startup does not cause critical consequences,
- it is possible to provide fuel, service and periodic tests.
Generator limitations:
- • noise and exhaust emissions,
- • need for regular inspections,
- • risk of startup failure with neglected operation,
- • no immediate load takeover (without additional UPS/BESS).
When BESS is a better solution
BESS works particularly well where:
- power continuity without any interruption is required,
- sensitive systems are powered (IT, automation, laboratories, control systems),
- power quality is important (no sags, short interruptions, fluctuations),
- the system is to work with a photovoltaic installation or optimize energy costs (peak shaving, reduction of ordered power).
BESS limitations:
- • operating time depends on battery capacity (kWh),
- • higher cost for long backup times,
- • fire and temperature requirements,
- • gradual battery degradation over time.
Most recommended solution: BESS + generator
In many facilities, a hybrid system is optimal:
- BESS BESS takes over power immediately after grid failure,
- Generator the generator starts in the background and takes over the load for a longer time,
- the system provides both continuity and long-term backup.
Additionally:
- • the generator works in more stable conditions,
- • load spikes and problems at low load are limited,
- • the overall system reliability increases.
How to choose the right solution – 3 key questions
What is the power of critical loads (kW)?
Not everything needs to be powered in emergency – key is load separation.
How long should power be maintained?
Minutes, hours or days – this decides the technology choice.
Is a power interruption acceptable?
If not – BESS or UPS is necessary (standalone or with generator).
Typical design errors
- system selection only for power, without considering energy (kWh),
- lack of regular generator tests under load,
- not considering fire requirements for BESS,
- lack of clear division into critical and non-critical loads.
Summary
Generator
Best for long backup and high power.
BESS
Ideal where immediate power takeover and power quality matter.
BESS + generator
Most universal and reliable solution for most facilities.
Well-designed emergency power supply is not just equipment, but an entire system, tailored to the real needs of the facility and how it is used.
Each facility has different requirements for power continuity.
Based on critical power, required backup time and load characteristics, we will help select:
- •a generator,
- •a BESS system,
- •or a hybrid solution.