Generator sizing calculator

Use the calculator

Add each load on the site (running kW, PF, quantity, and motor LRA if applicable), pick a NEMA class, and the calculator returns the required kVA, the next standard genset size, and the diesel fuel rate at full and average load. The largest motor on the list automatically becomes the starting-surge contributor — the worst-case assumption for sizing.

The four sizing formulas

The alternator is sized by current (which scales with kVA), not by mechanical power (kW). Two installations with the same kW but different PF need different generator sizes — the lower-PF one needs a bigger alternator.

Locked-rotor amps is the current drawn during the first half-cycle of motor start, when the rotor is still and the back-EMF is zero. Typical induction motors have LRA = 5–8 × FLA, codified by NEMA Code Letters A–V on the nameplate (A = lowest, V = highest).

The "max" rule means the genset must satisfy both sizing checks. For sites with no large motors, the 1.25× margin dominates. For motor-heavy sites, the surge condition is the binding constraint and forces a much larger genset than the running load alone would suggest.

0.27 L/kWh is the mid-range modern-diesel specific fuel consumption (210–280 g/kWh BSFC × 0.85 kg/L diesel density). Idle consumption is roughly 0.07 L/kWh + 5–7 L/h baseline. Annual fuel: average load × runtime hr/yr × 0.27 + idle hours × idle rate.

How to size a generator, step by step

1. Build the load list. Catalogue every load the genset must supply: lighting, HVAC compressors, pumps, IT racks, kitchen appliances, welders, elevators. For each, record the running kW (nameplate or measured), the power factor, and the quantity. Motors get an extra entry: locked-rotor amps from the nameplate or the NEC 430.7 code letter table.
2. Apply duty cycle and continuous-load factors. For loads that cycle on and off (refrigeration compressors, intermittent welders), use the time-averaged kW. For continuous loads (≥3 hr at full draw — datacenter, electric heat, fixed lighting), include the full kW with a 1.25× margin per NEC 210.19(A).
3. Compute running kW and weighted PF. Sum (kW × qty) across all loads to get total running kW. Compute weighted PF as Σ(kW × PF) / Σ(kW). Running apparent power: kVA = kW / PF. The genset alternator must supply this kVA continuously, plus any transient surges.
4. Size the starting-surge headroom. The single largest motor on the bus drives the starting surge. Compute its starting kVA = LRA × V × √3 / 1000 (3-phase). The genset must supply this on top of the existing running load — a 20 hp motor with 290 LRA at 400 V starts as 200 kVA on top of whatever else is running. Soft-starters or VFDs cut this 3–5×.
5. Apply margin and pick the next standard size. Required kVA = max(1.25 × running_kVA, running_kVA + starting_kVA). Apply the NEMA derate factor (1.0 for standby, 0.90 for prime, 0.80 for continuous). Round up to the next preferred kVA size: 100, 125, 150, 175, 200, 230, 250, 275, 300, 350, 400 kVA, etc.
6. Verify fuel storage and ventilation. Diesel fuel rate ≈ 0.27 L/kWh at full load. A 200 kVA genset at 0.85 PF runs 170 kW × 0.27 ≈ 46 L/h. NFPA 110 Level 1 requires 1–24 hr of fuel on site depending on the application. Air for combustion: ~0.05 m³/h per kW. Cooling-air requirement: 5–10× combustion air. Always check the manufacturer spec sheet.

Reference values

NEMA preferred kVA sizes (standby ratings)

Standard sizes stocked by Cummins, Caterpillar, Generac, Kohler, MTU. Pick the smallest size whose standby rating ≥ your derated requirement.

NEMA motor code letters → kVA per HP at locked rotor

The nameplate Code Letter encodes the motor's starting kVA per HP. Multiply by motor HP to get total starting kVA for sizing the bus.

Worked example: small office building

An office of 800 m² needs a standby generator. Loads: rooftop HVAC unit 12 kW (PF 0.85, motor with 200 LRA), lighting 4 kW (PF 0.95), IT loads 8 kW (PF 0.95), fridge / kitchen 3 kW (PF 0.95). 400 V 3-phase bus.

The HVAC starting surge dominates the sizing — without it, a 50 kVA genset would suffice, but the inrush forces a 175 kVA unit. Adding a soft-starter to the HVAC compressor would cut starting kVA by ~3× to ~46 kVA, dropping the required size to ~75 kVA — saving roughly $30 000 in capital cost. This is exactly the trade-off the engineering report should make explicit.

Variants and special cases

Datacenters and N+1 redundancy

Datacenters use multiple gensets sized for the IT load + cooling + UPS rectifier overhead, typically with N+1 redundancy (one extra unit for failover). Each unit is sized so that any one can be taken offline without losing capacity. Tier IV datacenters require 2N redundancy — duplicate independent power paths each fully sized for the load.

Hospitals (NFPA 110 Type 10)

NFPA 110 §5.6 mandates that life-safety branch loads (egress lighting, fire alarms, elevator hoist motors, ICU outlets) transfer to the genset within 10 seconds of grid failure, with the alternator at full voltage and frequency. Hospital standby gensets run with very high transient performance specs and 96 hours of on-site fuel.

Cogeneration and CHP

Combined heat-and-power (CHP) installations capture engine waste heat for hot water or steam. Sizing is driven by thermal demand more than electrical demand — the genset is sized to the heat load, and the electrical output is what comes "for free." CHP economics depend on heat-to-power ratio (typically 1.5:1 for diesel, 3:1 for gas microturbines) matching the site profile.

Renewable hybrids

Solar + battery + diesel hybrid systems use the genset for backup or peak shaving. Sizing for solar/battery primary: the genset only runs when battery SOC drops below threshold or load exceeds inverter capacity — annual runtime drops from 8 760 hr (continuous) to 100–500 hr (backup-only), which permits using a standby-rated unit at much lower cost than continuous-duty.

NEMA standard kVA series

Standby ratings shall apply to a generator set used as a backup to the normal utility-supplied power source. The standby rating is the maximum power output that the generator set can deliver for the duration of an emergency outage, with no overload capability above the rating, and with limited annual operating hours not to exceed 500 hours per year.

Generator quick reference

This 3 ph generator sizing tool also doubles as a generator selection calculator, generator load calculator, generator wattage calculator, and generator wattage estimator. Common workflows: calculate generator wattage / calculate wattage for generator from a load list (Sizing mode); cross-check the watts calculator for generator output against the actual fuel rate (the calculator also returns L/h diesel and m³/h natural gas); generator watts calculator and generator calculator watts both reduce to the same kVA × PF × derate equation.

Related calculators and references

Frequently asked questions

How to calculate generator size?

How to calculate generator size: build the load list, sum running kVA, add the largest motor's starting kVA surge, apply 1.25× continuous-load factor, and pick the next standard NEMA preferred size. The calculator above automates this — enter your load list and it returns the required kVA and recommended next size up.

How to size generator calculator?

How to size generator calculator workflow: pick the calculator above (it doubles as a generator selection calculator and generator wattage estimator), enter each load with running kW and PF, mark the largest motor for starting surge, and read off the recommended kVA. The same engine works as a generator wattage calculator, generator watts calculator, calculate generator wattage, calculate wattage for generator, watts calculator for generator, wattage calculator for generator, and generator calculator watts use case.

How many kw do I need in a generator?

How many kW do I need in a generator: rule of thumb for a whole-house standby genset is roughly 1 kW per 100 sq ft for essential loads (lighting, fridge, well pump, basic HVAC), or 2 kW per 100 sq ft for whole-house comfort. A 2 000 sq ft home typically needs 14–22 kW; a 4 000 sq ft home with 5-ton AC needs 24–48 kW. The generator load calculator above gives the exact answer from your specific load list.

How many kilowatts generator do I need?

Same answer — how many kilowatts generator do I need is set by the sum of running kW × continuous-load factor (1.25) plus the largest motor's starting surge. Round up to the nearest standard size: 14, 17, 22, 24, 26 kW (residential air-cooled); 30, 36, 48, 60, 80, 100 kW (small commercial liquid-cooled); 125+ kW (industrial).

How does a generator create current?

How does a generator create current — by Faraday's law of electromagnetic induction. A rotating rotor (driven by an engine, turbine, or wind) carries an electromagnet; as it spins inside stationary armature windings, the changing flux induces a voltage in the windings. Connect a load and current flows. The output frequency = (rotor speed in RPM × pole pairs) / 60 — for a 4-pole 60 Hz generator, RPM = 1 800.

How is the speed of an AC generator rotor calculated?

How is the speed of an AC generator rotor calculated: RPM = (120 × f) / P, where f is the desired frequency in Hz and P is the number of magnetic poles. For 60 Hz with 2 poles → 3 600 RPM; 4 poles → 1 800 RPM; 6 poles → 1 200 RPM. For 50 Hz: 3 000, 1 500, 1 000 RPM. Standby gensets typically use 4-pole alternators driven at 1 800 RPM (60 Hz) or 1 500 RPM (50 Hz).

How to calculate generator load?

How to calculate generator load: total running load (kW) divided by the genset rating (kW) gives the load fraction. Most gensets run optimally at 50–80 % load — too light wastes fuel and "wet-stacks" diesel engines, too heavy reduces life. The generator load calculator above shows both running kW and the load fraction for any candidate genset size.

What size generator do I need for my house?

Add up the running kW of essential loads (HVAC, fridge, well pump, lighting, basic outlets) and double it for safety, or pick by service-entrance ampacity: 100 A service → 14–22 kW genset; 200 A service → 24–48 kW. Whole-house standby gensets typically rate 14, 17, 22, 24, 26 kW (60 Hz residential air-cooled). Add the largest AC compressor's starting surge — a 5-ton AC has ~125 LRA and adds about 30 kVA of inrush.

How do I calculate kVA for a motor?

Running kVA = kW / PF, with PF typically 0.80–0.85 for induction motors at full load. Starting (locked-rotor) kVA = LRA × V × √3 / 1000 for 3-phase. A 7.5 kW (10 hp) motor has running kVA ≈ 9 kVA but starts as ~50–60 kVA on a 400 V bus — that 6× ratio is normal. NEMA Code Letter G motors have ~6 kVA/hp locked-rotor; Code Letter A is ≤3.15 kVA/hp.

What is the difference between kW and kVA?

kW (kilowatts) is real power — the work the load actually does. kVA (kilovolt-amperes) is apparent power — what the alternator and conductor have to carry. They differ by the power factor: kW = kVA × PF. A genset is always rated in kVA because the alternator size is set by the heat in the windings, which is driven by current (and current is determined by kVA, not kW). Lower PF → more kVA for the same kW → bigger genset.

Standby vs prime vs continuous — which class do I need?

Standby — emergency only, ≤500 hr/yr, peak rating equals nameplate. This is what most residential and commercial standby gensets are. Prime — unlimited hr/yr at variable load (avg ≤70 % of rating). Used for sites without grid power. Continuous — 24/7 at constant load. Lowest derate, biggest physical genset for the same nameplate. Pick by expected runtime and load profile, not by peak.

Do I need a soft-starter for my motor?

If the motor's starting kVA exceeds 25 % of the genset rating, the bus voltage will dip below 85 % during start — modern electronics may reset and other motors may stall. Soft-starters reduce starting current 2–3× by ramping voltage; VFDs reduce it 5–10× by ramping frequency. A 50 hp motor on a 100 kVA genset normally needs a soft-starter; the same motor on a 250 kVA genset usually does not. Run the calculation with and without and check.

How much fuel does a generator consume?

Diesel rule of thumb: 0.27 L/kWh at full load, 0.07 L/h at idle. A 200 kVA genset at 100 % load consumes 46 L/h; at 50 % load (typical prime-power average) about 23 L/h. Natural-gas generators use ~10 m³/h per 100 kW. NFPA 110 Level 1 fuel-storage requirements: 96 hr for hospitals, 8–24 hr for most other emergency systems. Plan tank sizing accordingly.

What is power factor correction and does it help?

Power factor correction (PFC) capacitors at the bus reduce the genset's apparent power requirement by ~15–25 % for typical industrial loads. PFC is especially valuable when the running PF is below 0.80 (lots of induction motors). However, gensets running below ~0.7 lagging PF can experience instability (under-excitation), so check the alternator spec — most standby gensets are rated 0.8 PF lagging.

How is single-phase generator sizing different?

Single-phase: starting kVA = LRA × V / 1000 (no √3). The whole load runs through one winding rather than three balanced ones, so single-phase gensets are physically larger per kVA than 3-phase equivalents. Most commercial / industrial gensets are 3-phase, with a separate centre-tap for 240/120 V residential service if needed. Pure single-phase is mostly residential below 25 kW.

Sources and methodology

1. NEMA. NEMA MG 1 — Motors and Generators, 2021 Edition. Part 22 (synchronous generators), §22.40 standby ratings.
2. IEEE. IEEE 446 — Recommended Practice for Emergency and Standby Power Systems for Industrial and Commercial Applications (Orange Book), 1995.
3. NFPA. NFPA 110 — Standard for Emergency and Standby Power Systems, 2022 Edition.
4. Cummins Power Generation. Generator-Set Sizing Application Manual T-030. Sizing margin tables and starting kVA references.
5. NFPA. National Electrical Code (NEC) NFPA 70, 2023 Edition. Article 430 (motors), 700 (emergency systems), 701 (legally required standby), 702 (optional standby).
6. BSI. ISO 8528 — Reciprocating internal combustion engine driven alternating current generating sets, Parts 1–13.