In short. The number and size of generators is a trade-off between redundancy, efficiency at partial load, capability to take steps, space, cost and growth. More, smaller units give flexibility and resilience; fewer, larger units cost less and are simpler. Decide from the load profile and the redundancy target, then compare options side by side.
Inputs you need
- Load profile: minimum, average, peak, and expected growth.
- Critical load, and the redundancy target: N, N+1, N+2 or 2N.
- Largest motor or step the plant must take.
- Maintenance policy: can a unit be out while the plant runs?
- Space, fuel, noise and emissions limits.
How the arithmetic works
Units for N+1 = Units for N + 1
Usable capacity is the unit’s rating after derating and target loading. Redundancy means the remaining units must carry the critical load with one unit out.
Option comparison (illustrative)
Design load: 1,800 kW; usable capacity per unit is taken as the full kW rating for simplicity.
| Option | Installed | With one unit out | Notes |
|---|---|---|---|
| 2 × 1,000 kW (N+1 not met) | 2,000 kW | 1,000 kW | Only 55 per cent covered after a failure |
| 3 × 1,000 kW (N+1) | 3,000 kW | 2,000 kW | Covers 1,800 kW; units run at about 60 per cent |
| 4 × 600 kW (N+1) | 2,400 kW | 1,800 kW | Exactly covered; lower installed capacity |
| 5 × 500 kW (N+1) | 2,500 kW | 2,000 kW | Better part-load; more units to maintain |
The 4 × 600 kW option meets N+1 with the least installed capacity but leaves no margin for growth or derating. In practice, the usable capacity per unit would be lower than the rating, so more capacity would be needed.
Other factors
- Part-load efficiency: multiple units let you run only what the load needs. See loading and spare capacity.
- Step handling: several units share block loads; a single large step may still need staging.
- Maintenance: N+1 means one unit can be serviced while the others carry the load.
- Growth: additional units can be added if the paralleling bus and space allow.
- Complexity: more units need more paralleling and control equipment, and more spares.
Same-size or mixed
Identical units simplify load sharing, spares and maintenance. Mixed sizes can match a wide load range, such as a small unit for light load and large units for peaks, but need careful control. See operating combinations and dispatch.
How many sets, and what size
For a design load L and a unit rating P, the number of sets needed to carry the load is the roundup of L ÷ P. Redundancy adds sets.
| Option | Sets for 1,500 kW critical load | Comment |
|---|---|---|
| One 1,750 kW set | 1 | No redundancy |
| Three 500 kW, N | 3 | No spare |
| Four 500 kW, N+1 | 4 | Survives one failure |
| Three 750 kW, N+1 | 3 (2 needed) | Larger units, fewer sets |
| Five 400 kW, N+1 | 5 (4 needed) | Finer granularity, better light-load efficiency |
Trade-offs
- More, smaller sets give finer matching and lower redundancy cost per unit, but more maintenance and more paralleling.
- Fewer, larger sets are simpler but each failure removes a larger share.
- Fault level rises with the number of sets in parallel.
Light-load operation
At night load might be 400 kW. With four 500 kW sets, running one at 80 percent is healthy; running all four at 20 percent is not. Load-dependent start and stop avoids this.
Verification
Check the largest motor start with N−1 sets running, not with all of them. See N+1, multi-DG capacity studies and redundancy capacity guide.
Common mistakes
- Counting installed capacity without derating.
- Choosing N+1 by units but not by capacity after the failure.
- Ignoring shared failure points. See multi-generator plant architecture.
What to send us
Send your load profile, redundancy target and space available. Send your application details. See also multi-DG capacity and N+1 studies.
Questions engineers ask
How do I choose between three large and four small generators?
Compare total installed capacity, capacity with one unit out, part-load efficiency, space, cost and maintenance complexity for your load profile.
Does N+1 mean one extra generator?
It means capacity for the load with one unit out. With uneven sizes, that is not necessarily one extra unit of the same size.
Can I add generators later?
Yes if the bus, controls, fuel and space were designed for it.
Is paralleling necessary?
For shared load and redundancy on one bus, yes. Separate units on separate loads do not need it but give less flexibility.
Figures are illustrative. Final selection needs a load study and manufacturer data.