How to Calculate the Right Diesel Generator Capacity for Your Building

Undersizing a generator means it trips or stalls when you need it most; oversizing wastes fuel and capital. Getting the capacity right comes down to a straightforward calculation, done properly. Step 1 — List every load you actually need during an outage. Not your total building load — only what must run: lifts, lighting, essential AC/ventilation, pumps, servers, medical equipment, refrigeration, etc. Buildings often over-size generators by simply adding up every circuit in the building, including loads that don't need backup. Step 2 — Convert to kVA and account for power factor. Most equipment nameplates show kW; divide by the power factor (commonly ~0.8 for mixed inductive loads) to get kVA. Add these across your essential-load list to get your running load. Step 3 — Add starting/surge capacity for motors. Motors — lift drives, pumps, compressors — draw a much larger current at start-up than while running, often 2-3x their running value. If several large motors can start at once (for example, a lift and a water pump both restarting after a power cut), your generator needs to handle that combined surge, not just the steady running total. This is the single most common sizing mistake we see. Step 4 — Apply a safety margin. A generator run consistently near 100% of rated capacity ages faster and has no headroom for future load growth. Sizing to roughly 75-80% of the generator's rated capacity under normal running conditions is a reasonable working target. Step 5 — Match to a capacity tier and confirm with a site survey. As a starting point: - Small (10-50 kVA): homes, shops, small offices. - Mid (100-350 kVA): offices, clinics, small commercial buildings with continuous backup needs. - Large (500-1000 kVA): malls, factories and buildings with heavy motor/HVAC load. - Industrial (1250-2000 kVA): full plant backup for industrial and manufacturing sites. These tiers are a starting point, not a substitute for the calculation above — the right model within a tier still depends on your specific load list, altitude/derating, fuel storage and noise/emissions requirements for the site. Send us your load list through the contact page and our team will confirm sizing and recommend the right model before you commit to a purchase.