What size generator do I need?

Add the kW of everything that runs at the same time, divide by the power factor (0.8) to get running kVA, then make sure the set can also absorb the largest motor's starting surge and a 15–25% future margin. Round up to the next standard rating. For example, a 40 kW load with a 15 HP DOL motor needs about an 82.5 kVA generator because the motor's starting surge governs the size.

What is the difference between kVA and kW in a generator?

kVA (apparent power) is what the alternator is rated for; kW (real power) is the useful power delivered to the load. They are linked by power factor: kW = kVA × power factor. Diesel gensets are rated in kVA at 0.8 PF, so a 125 kVA set is a 100 kW set.

Why does the largest motor decide the generator size?

A motor draws several times its running current at the instant it starts (around 6× for a direct-on-line start). That brief surge can be larger than the entire steady load, so the generator must be big enough to ride through it without the voltage dipping and tripping the other equipment. A soft starter, star-delta or VFD reduces this surge and lets you use a smaller set.

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Generator Sizing Calculator — What kVA Generator Do I Need?

Enter your running load, the biggest motor and how it starts. Get the recommended standard DG set size in kVA — sized for steady load, motor starting surge and future expansion.

⚠️ Get the size wrong and it costs either way. Undersize it and the genset trips every time a big motor starts. Oversize it and you waste lakhs in capital, burn extra diesel, and risk wet stacking from running the engine too lightly. The right size balances all three: steady load, the biggest motor's starting surge, and room to grow.

Total Running Load (kW)

Add up everything that runs at the same time. See the load reference table below if unsure.

Power Factor

Generators are rated at 0.8

Future Margin (%)

Headroom for growth (15–25%)

Largest Single Motor (HP)

0 if there are no motors

Motor Starting Method

How the biggest motor starts

Sizing Logic

Running kVA = Running kW ÷ PF Surge kVA = (other load ÷ PF) + biggest motor start kVA Required kVA = the larger of: • Running kVA × (1 + margin) • Surge kVA → round up to a standard set

Standard Genset Sizes

kVA	≈ kW	Typical use
5–10	4–8	Home, shop
15–25	12–20	Showroom, clinic
30–62.5	24–50	Small factory
75–125	60–100	Medium factory
160–250	128–200	Large factory
320+	256+	Industrial, hospital

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Load Reference — Typical Power Draw

Not sure of your running load? Add up the items below that run at the same time. These are typical figures — use your equipment nameplate where you can.

Home & Commercial

Load	Typical kW
LED lights + fans (per 10)	0.5
Refrigerator	0.2
1.5 ton air conditioner	1.5
Computer + monitor (each)	0.3
Water pump (1 HP)	0.75
Photocopier / printer	0.5–1.5
Microwave / oven	1.2–2.0

Industrial / Workshop

Load	Typical kW
Motor per HP (× efficiency)	0.75 / HP
Welding machine	5–12
Lathe / milling (small)	3–11
Air compressor (10 HP)	7.5
Injection moulding (small)	15–40
CNC machine	10–30
Hydraulic power pack	5–22

Quick conversion: 1 HP ≈ 0.746 kW. A motor also draws a little more than its rated kW because of efficiency — for sizing, using rated HP × 0.746 is a safe starting point.

Why Motor Starting Decides the Size

A running motor is well-behaved. The problem is the instant it starts — it pulls a big inrush of current, several times its normal draw, for a second or two. The generator has to supply that surge on top of everything else already running, without the voltage sagging enough to trip your equipment. On many factory sets it's this surge — not the steady load — that decides the kVA you need.

How you start the motor changes the surge dramatically, and that directly changes the genset size:

Starting Method	Approx. Starting Surge	Effect on genset size
Direct-on-line (DOL)	~6× running current	Largest set needed — surge often governs
Star-Delta	~2–3× running current	Can drop one or two sizes
Soft starter / VFD	~1.5–2× running current	Smallest set — surge rarely governs

If this calculator recommends a set that's far bigger than your steady load needs, the motor surge is the reason. Switching the biggest motor to a soft starter or VFD can let you buy a smaller, cheaper genset that runs at a healthier load.

Worked Example — Small Factory

A workshop with a 40 kW total running load, the biggest single machine being a 15 HP motor on a direct-on-line start, and 20% future margin:

Running kVA = 40 ÷ 0.8 = 50.0 kVA Biggest motor = 15 HP = 11.2 kW Other load = 40 − 11.2 = 28.8 kW Motor start = (11.2 ÷ 0.8) × 3.0 ≈ 42.0 kVA Surge kVA = (28.8 ÷ 0.8) + 42.0 = 78.0 kVA Steady + 20% = 50.0 × 1.20 = 60.0 kVA Required = max(60.0, 78.0) = 78.0 kVA → Next standard size = 82.5 kVA (steady loading ≈ 61% — healthy)

Here the motor starting surge (78 kVA) governs, not the steady load (50 kVA). A soft starter on that 15 HP motor would pull the surge down near 53 kVA, letting a 62.5 kVA set do the job — a big saving in capital and fuel.

kVA vs kW — Don't Mix Them Up

Generators are rated in kVA (apparent power) but your load is usually quoted in kW (real power). They're linked by power factor:

kW = kVA × power factor kVA = kW ÷ power factor (PF = 0.8 for gensets)

Genset rating (kVA)	Delivers (kW at 0.8 PF)
10 kVA	8 kW
25 kVA	20 kW
62.5 kVA	50 kW
100 kVA	80 kW
250 kVA	200 kW

Frequently Asked Questions

Add the kW of everything that runs at the same time, divide by power factor (0.8) to get running kVA, then make sure the set can also absorb the biggest motor's starting surge plus a 15–25% future margin. Round up to the next standard rating. A 40 kW load with a 15 HP DOL motor, for example, needs about an 82.5 kVA generator because the motor's starting surge governs the size.

kVA = kW ÷ power factor. Generators are rated at 0.8 power factor, so a 100 kVA generator delivers about 80 kW. Going the other way, kW = kVA × 0.8.

A motor draws several times its running current the instant it starts — around 6× for a direct-on-line start. That brief surge can be bigger than your entire steady load, so the generator must ride through it without the voltage dipping and tripping other equipment. A soft starter, star-delta or VFD reduces the surge and lets you use a smaller set.

A small margin (15–25%) is healthy for future load and motor starting. But heavily oversizing is harmful — a diesel genset run below about 30% of its rating suffers wet stacking, where unburnt fuel and carbon build up and foul the engine. Aim for 50–80% continuous loading on the prime rating.

As a rough single-motor guide with a direct-on-line start: a 5 HP motor needs about a 15 kVA set, a 10 HP motor about 25–30 kVA, and a 15 HP motor about 40–50 kVA — more if other loads run at the same time. A soft starter or star-delta start lets you drop a size or two. Use the calculator above with your actual combined load for a proper figure.

Use single-phase for homes, shops and small offices with only single-phase loads — typically up to about 10–15 kVA. Use three-phase where you have three-phase motors or a three-phase utility connection, which covers most factories and larger commercial buildings. Three-phase is more efficient for motor loads.

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