Generator sizing for a shop, clinic or workshop: 3 worked examples

Using our finder's method, a small shop without air conditioning needs about 3.7 kVA; a clinic with 14 kW of equipment about 19.5 kVA; and a 20 kW workshop with a 7.5 kW motor between 31 and 48 kVA depending on how the motor starts. For a house, see our guide to generator size for a house.

What size generator for a small shop?

A small shop in Kinshasa without air conditioning needs about 3.7 kVA. Its motors are two freezers and a refrigerator. The start-up of the largest one, a 400 W freezer, sets the generator size.

The calculation follows the lines our finder displays, with the typical simultaneous use for a shop (0.7):

In our range, the first single-phase step above 3.7 kVA is 5 kVA. Three sets are suitable:

• the Bison BS7500DSE, a 5 kVA silent diesel;
• the Bison BS6250iE, a 5 kW petrol inverter;
• the Bison BS6500H, a 5 kW petrol set.

See all our silent diesel generators. These air-cooled sets are built for intermittent use, during outages of a few hours. If the shop runs on its generator for several hours every day, a water-cooled diesel is the better fit: our water-cooled range starts at 13 kVA. Our guide air-cooled or water-cooled compares the two.

What size generator for a clinic?

A clinic with 14 kW of equipment needs about 19.5 kVA. Here the finder applies a simultaneous-use factor of 0.8, higher than for a shop. The start-up of one of the air conditioners sets the generator size.

This need is beyond our air-cooled range, which tops out at 15 kVA. The right set is a 20 kVA generator with a Perkins 404A-22G1 engine: 20 kVA prime power, three-phase, water-cooled. On three-phase, each phase supplies about a third of the rating, about 6.7 kVA here, so an electrician must spread the four air conditioners across the phases.

A clinic is a critical site. For this kind of site, our finder recommends:

• N+1 redundancy, or two sets in parallel;
• an external fuel tank, to extend runtime during long outages.

It also suggests an automatic transfer switch (ATS), which transfers the load to the generator during an outage, when the operating mode is "Standby (outages)" or "Not sure".

A clinic that runs on its generator for several hours a day should be sized on prime power. Our guide to prime vs standby power explains why.

What size generator for a workshop with a 7.5 kW motor?

A 20 kW workshop whose largest motor is 7.5 kW needs between 31.3 and 48.1 kVA. The whole difference comes from how that motor starts.

This example uses the finder's Method B: you enter the total load (20 kW) and the largest motor (7.5 kW) directly. No simultaneous-use factor is applied. The first lines are the same for every start method:

• apparent power: 20 ÷ 0.8 = 25.0 kVA;
• with the 25% reserve: 31.25 kVA;
• the 7.5 kW motor running: 7.5 ÷ 0.8 = 9.38 kVA.

The step-load requirement is worked out as (25.0 − 9.38 + motor surge) × 1.1. With a VFD it stays below the 25% reserve (31.25 kVA), so the reserve sets the size. Our 30 kVA set falls just short of 31.3 kVA, which is why the 45 kVA set is offered. To compare start methods with your own motor, use the finder to work out your generator size.

All these sets are in our diesel generator range. They are three-phase: see our guide single-phase or three-phase.

Examples worked out with the finder's method; final sizing is confirmed by our technical team.

What the three examples show

Motors and the way they start set the generator size more than the total load does.

• The same workshop needs 48.1 kVA with a direct start and 31.3 kVA with a VFD.
• A soft starter or a VFD on the largest motor takes this workshop from a 60 kVA set down to a 45 kVA set.
• A critical site such as a clinic must stay powered through load-shedding in Kinshasa: our finder recommends N+1 redundancy and an external fuel tank.
• The finder also suggests an automatic transfer switch (ATS) when the operating mode is not "Prime / Main".
• Above 15 kVA, you need a water-cooled set. Our Perkins-engine range runs from 13 to 2,000 kVA.

For a temporary need, see our generator rental. For a church, a school or a meeting hall, our guide What size generator for a church, a school or a meeting hall? applies the same method.

Size the generator for your premises

The finder offers 17 types of use, including shop, clinic and workshop, each with its own simultaneous-use factor. Enter your equipment or your total load, then the start method of your largest motor. You can then work out your generator size and get a proposal from our team.

Frequently asked questions

Why does a workshop need a generator much bigger than its load?

Because of the start-up surge of its largest motor. In our example, a 7.5 kW motor started direct-on-line draws about 28 kVA as it starts. The generator must supply that surge while the other machines keep running.

Does a clinic need an automatic transfer switch?

For a clinic supplied by the grid, an automatic transfer switch handles the changeover without anyone having to step in. It moves the supply to the generator during an outage and back to the grid when it returns. Our finder suggests one when the operating mode is not "Prime / Main". Ask our team about it when you send your request.

Sources: manufacturers' datasheets (Perkins engines, Bison generators), MK Energies & Services finder calculation method.