Why Proper Generator Sizing Matters
An undersized generator will overload and shut down — often at the worst possible moment. An oversized diesel generator presents its own serious problem: wet stacking, which occurs when a diesel engine runs at less than about 30% of rated load for extended periods. Incomplete combustion causes unburned fuel and carbon to accumulate in the exhaust system, damaging the engine and reducing reliability over time.
Correct generator sizing requires understanding the difference between running (continuous) watts and starting (surge) watts, applying NEC Article 220 demand factors, and selecting the right power rating class per ISO 8528.
Running Watts vs Starting (Inrush) Watts
Most motors, compressors, and HVAC equipment draw 3–7 times their running current for the first few cycles when they start. This inrush current is brief (under 1 second) but it must not exceed the generator's surge capacity.
- Running watts — The steady-state power a load consumes during normal operation.
- Starting watts — The peak surge required to start the load, typically 1.5x to 3x running watts for motors.
The generator must be able to supply starting watts for the largest single motor that will start while the generator is already carrying its other loads. This is the critical sizing constraint that most homeowner guides ignore.
Residential Generator Sizing
For a home generator, the first decision is whether you need essential-load backup (a subset of critical circuits) or whole-home backup (full service continuity).
Essential Load Sizing Example
Common essential loads and their approximate power requirements:
| Load | Running Watts | Starting Watts |
|---|---|---|
| Gas furnace (1/2 HP blower) | 600 | 2,400 |
| Refrigerator | 150 | 600 |
| Sump pump (1/3 HP) | 800 | 1,300 |
| Lights and outlets (6 circuits) | 1,200 | 1,200 |
| Well pump (1/2 HP) | 750 | 2,000 |
| Total | 3,500 | 7,500 peak |
This example home requires a generator with at least 3,500W running capacity and 7,500W surge capacity — a 5,000–7,500W portable or standby generator would cover it comfortably.
Whole-Home Sizing Using NEC Article 220
For a whole-home standby generator connected to a 200A service, use NEC Article 220.82 (optional method for dwelling units):
- General lighting and receptacles: 3 VA per sq ft multiplied by house square footage
- Small appliance circuits: 1,500 VA x 2 minimum (NEC 220.52)
- Laundry circuit: 1,500 VA (NEC 220.52(B))
- All large appliances at nameplate rating (HVAC, range, water heater, dryer)
- Apply demand factors from NEC Table 220.82(B): first 10 kVA at 100%, remainder at 40%
A typical 2,000 sq ft home with central AC calculates to approximately 18–22 kW demand. A 22 kW standby generator is the most common whole-home choice for this size home.
Generator Sizing Formula
Once you know the total kW demand, convert to kVA using the power factor of the connected loads:
kVA required = Total kW demand divided by Power Factor
For resistive loads (heaters, incandescent lights), power factor = 1.0. For motor loads, power factor typically ranges from 0.75 to 0.85. Using 0.8 as a conservative average:
Example: 20 kW load divided by 0.8 PF = 25 kVA generator required
Always add a 20–25% margin above your calculated demand to handle load growth and maintain the generator above 30% load for diesel units.
Transfer Switches: Manual vs Automatic (ATS)
A transfer switch is required by NEC 700.5 and 702.5 to prevent backfeed from the generator into the utility grid — which can electrocute utility workers restoring power. Transfer switches must prevent simultaneous connection of the generator and utility source.
- Manual Transfer Switch — Requires the homeowner to start the generator and manually flip the switch. Less expensive (approximately $300–$800 installed). Suitable for portable generators.
- Automatic Transfer Switch (ATS) — Monitors utility voltage, starts the generator automatically on power failure, transfers loads, and retransfers to utility when power is restored. Required for emergency systems under NEC Article 700. Typical cost approximately $1,500–$5,000 installed for residential.
ISO 8528 Power Rating Classes
Generator nameplate ratings follow ISO 8528, and the differences are critical for correct sizing:
| Rating | Description | Overload Allowance | Typical Application |
|---|---|---|---|
| Emergency Standby Power (ESP) | Maximum power available during a utility outage; limited annual hours | 10% for 1 hr/12 hrs | Backup power for buildings |
| Limited-Time Running Power (LTP) | Maximum power for defined limited hours per year | None | Peak shaving |
| Prime Power (PRP) | Maximum power available continuously with variable load, unlimited hours | 10% for 1 hr/12 hrs | Remote sites without utility |
| Continuous Power (COP) | Maximum power at constant load for unlimited hours | None | Base load generation |
A generator rated 100 kW Standby may only be rated 90 kW Prime. Always size to the correct rating class for your application.
Fuel Type Comparison
| Fuel | Energy Density | Runtime | Pros | Cons |
|---|---|---|---|---|
| Natural Gas | ~1,030 BTU/cu ft | Continuous (utility piped) | No fuel storage; low maintenance | Pressure drops during emergencies; not portable |
| Propane (LP) | ~91,500 BTU/gallon | ~2–3 hr/gallon at full load (20 kW gen) | Portable; stored on-site; long shelf life | Requires tank; pressure-sensitive in cold |
| Diesel | ~138,700 BTU/gallon | ~0.5–1 gal/hr per 10 kW at rated load | Most efficient; best for large commercial | Wet stacking risk at low load; fuel degrades; DEF needed on Tier 4 |
Commercial Generator Sizing: Emergency and Standby Systems
Commercial buildings must comply with NEC Article 700 (emergency systems) and NEC Article 701 (legally required standby systems), which carry stricter requirements than optional standby systems (Article 702):
- NEC Article 700 — Emergency systems (exit lighting, egress lighting, fire pumps) must transfer to backup within 10 seconds of utility failure (NEC 700.12). The generator must be sized to carry the full emergency load simultaneously.
- NEC Article 701 — Legally required standby (HVAC for hazardous locations, refrigeration for perishables, sewage disposal) must transfer within 60 seconds.
- Both require periodic testing: NEC 700.4 requires monthly testing of emergency systems under load.
For commercial sizing, work from the electrical single-line diagram. Identify all loads connected to the emergency bus and the legally required standby bus. Size the generator to carry both buses simultaneously, plus a growth margin, using the kVA formula above.