Generator Sizing Calculator

A generator sizing calculator that determines the correct kVA rating for a standby or prime-power generator based on your actual electrical loads, installation altitude, and ambient temperature. It correctly accounts for motor starting surges — the single most common cause of undersizing — and applies the standard ISO 8528-1 / NFPA 110 derating factors so the recommended size is valid for your specific site.

Why generator sizing matters

An undersized generator does not simply run slowly — it trips on overload the moment a compressor starts, causes voltage sags that damage sensitive electronics, and runs constantly at high temperature, dramatically shortening engine life. Equally, a grossly oversized generator is expensive to buy and fuel, and runs inefficiently at low load (a diesel engine below 30% load is prone to “wet stacking” — unburned fuel deposits that foul injectors and turbochargers). Getting the size right is both a safety requirement and an operational cost issue.

How the calculation works

Generator sizing follows a four-step process codified in NEC Article 445, NFPA 110 (Emergency Power Supply Systems) and ISO 8528-1 (Rating of generating sets):

1. Running load (kVA)

Each appliance has a rated wattage (real power, W) and a power factor (PF). The apparent power it draws from the generator is:

kVA = W ÷ (1000 × PF)

Sum this across all loads and quantities to get the steady-state running kVA. A resistive load such as an electric heater has PF = 1.0; motors, compressors and fluorescent lighting typically run at PF 0.8–0.9.

2. Motor starting surge

Electric motors draw 2–6× their running current during the first half-second of startup (the “locked-rotor current”). This creates a peak kVA spike that the generator must supply without the alternator voltage collapsing. The worst case is usually the single largest motor starting while all other loads are already running. The calculator adds the incremental surge of the largest motor to the total running kVA:

Peak kVA = Running kVA + (Surge kVA − Running kVA) of largest motor

3. Site derating

A generator’s nameplate rating is measured at 1000 m altitude and 25–40 °C ambient (varies by standard). At higher altitudes or temperatures, output falls:

• Altitude derating: −3 % per 300 m above 1000 m (ISO 8528-1 Annex D)
• Temperature derating: −1 % per °C above 40 °C ambient

The derated available capacity is:

Effective kVA = Nameplate kVA × altitude_factor × temperature_factor

4. Safety margin and rounding

Standard practice is to add 20–25 % headroom above the calculated peak demand, then round up to the nearest commercially available generator rating.

Required kVA = Peak kVA × (1 + margin/100) ÷ total_derate_factor

Worked example

A small office in Nairobi (1650 m altitude, 35 °C ambient) with the following loads:

Running kVA = (3600÷850) + (2000÷950) + 0.6 + (150÷900) ≈ 4.24 + 2.11 + 0.60 + 0.17 = 7.12 kVA

Largest motor surge (each AC unit): 1800 × 3.5 ÷ 850 = 7.41 kVA running = 2.12 kVA each. Incremental surge = 7.41 − 2.12 = 5.29 kVA extra.

Peak kVA = 7.12 + 5.29 = 12.41 kVA

Altitude derate (1650 m): 1 − 0.03 × (650/300) = 0.935 Temperature derate (35 °C, below 40 °C threshold): 1.000

Required kVA = 12.41 × 1.20 ÷ 0.935 = 15.93 kVA → round up to 17.5 kVA standard size.

Without the motor surge correction a naive sizing would suggest only a 10 kVA unit — which would trip every time the air conditioning compressor kicks in.

Formula note

The derating coefficients used (3% per 300 m altitude above 1000 m; 1% per °C above 40 °C) come from ISO 8528-1 Annex D and are widely reproduced in NFPA 110 and IEC 60034-1 guidance. Some manufacturers publish their own correction curves — always cross-check the datasheet for the specific generator model. The standard surge multipliers (2.5× refrigerator compressor; 3–4× HVAC; 5–6× large pump/industrial motor) are representative starting points; the actual locked-rotor current is on the motor nameplate as “LRA” and is the most accurate source.