Question 1

What size generator do I need for a commercial building?

Accepted Answer

Commercial building generator sizing depends on total electrical load, emergency load percentage, and equipment starting requirements. A typical office building needs 3-5 W/sq ft for emergency power. A 50,000 sq ft office would need approximately 150-250 kW. Use our free calculator for an exact estimate including altitude derating and code requirements.

Question 2

How do I calculate generator size for a hospital?

Accepted Answer

Hospital generator sizing uses 3-5 kW per bed for emergency loads, or 10-15 W/sq ft. A 200-bed hospital typically requires 600-1,000 kW. NFPA 99 requires three separate branches (life safety, critical, equipment), and generators must start within 10 seconds per NFPA 110 Type 10.

Question 3

Does altitude affect generator output?

Accepted Answer

Yes. Generators lose approximately 3.5% of rated power per 1,000 feet of elevation per ISO 8528-1. A 500 kW generator in Denver (5,280 ft) delivers only about 408 kW with a naturally aspirated engine, or about 470 kW with a turbocharged engine.

Question 4

What is the difference between standby and prime generator ratings?

Accepted Answer

Standby (ESP) rating allows up to 200 hours/year at up to 70% average load — used for emergency backup. Prime (PRP) rating allows unlimited hours at up to 70% average load — used for primary power or extended runtime. For the same physical generator, prime rating is approximately 90% of standby rating.