Every motor-driven appliance has two power numbers: the continuous power it uses while running and the brief surge it needs at startup. Understanding the difference is the first step to sizing a generator correctly.
The steady watts an appliance uses during normal operation. Refrigerators, pumps and HVAC systems all have a rated running wattage.
The extra power a motor draws for 1–3 seconds as it starts up. This surge can be 2–7× the running wattage.
Lights, heaters and toasters have no starting surge. Their running watts and starting watts are the same.
1. List every appliance you plan to
run at the same time.
2. Add the running watts of all
loads.
3. Add the single highest
starting surge — appliances don't all start at once.
4. Apply a 15–30% safety margin for
future expansion and site conditions.
Runs at 150–800 W, but can draw 600–2,200 W when the compressor kicks in.
A 1 HP pump draws ~1,000 W running, but requires up to 2,800 W to start.
A 3‑ton unit runs at 3,500–5,400 W and can surge above 7,000 W at startup.
A 1 HP compressor runs at ~1,600 W but can draw 4,500 W or more during startup.
Typical running and starting watts for common residential and industrial equipment. Values vary by brand and model — always verify from the nameplate or manual before final sizing.
| Appliance | Running Watts | Starting Watts |
|---|---|---|
| Refrigerator (18–24 cu ft) | 150–800 | 600–2,200 |
| Freezer (upright / chest) | 700 | 1,200–2,200 |
| Microwave oven | 1,000–1,500 | 1,000–1,500 |
| Coffee maker | 600–1,500 | — |
| Toaster | 800–1,500 | — |
| Electric water heater | 4,000–4,500 | 4,000–4,500 |
| Electric stove (per element) | 2,100–5,000 | 2,100–5,000 |
| Electric clothes dryer | 5,400–5,750 | 6,750 |
| Washing machine | 300–1,500 | 600–1,500 |
| Dishwasher | 1,200–1,500 | 1,500 |
| Window AC (5,000 BTU) | 500 | 1,500 |
| Window AC (10,000 BTU) | 1,200–1,500 | 2,200–3,600 |
| Central AC (3 ton) | 3,500–5,400 | 7,200 |
| Furnace fan (⅓ hp) | 700 | 1,400 |
| Furnace fan (½ hp) | 875 | 2,350 |
| Sump pump (⅓ hp) | 800 | 1,300–2,900 |
| Sump pump (½ hp) | 1,050 | 2,150 |
| Well pump (1 hp) | 1,000 | 2,800 |
| Garage door opener (½ hp) | 875 | 2,350 |
| LED television | 80–300 | 0 |
| Desktop computer | 200–500 | 0 |
| Laptop | 50–250 | 0 |
| Wi‑Fi router & modem | 10–30 | 0 |
| LED light bulb (60 W equiv.) | 9–60 | 0 |
| Hair dryer | 1,200–1,875 | — |
| Equipment | Running Watts | Starting Watts |
|---|---|---|
| Circular saw (7¼ in.) | 1,400 | 2,300 |
| Table saw (10 in.) | 1,800 | 4,500 |
| Miter saw | 1,800 | 2,300 |
| Electric drill | 600 | 900 |
| Bench grinder | 1,400 | 2,500 |
| Air compressor (1 hp) | 1,600 | 4,500 |
| Air compressor (2 hp) | 2,800 | 7,200 |
| MIG welder (240 V) | 2,500–5,000 | 5,000+ |
| 3‑phase motor (per hp) | ~746 | 2–6× running |
| Water pump (3‑phase, 5 hp) | ~3,700 | ~11,000+ |
Different applications fall within predictable generator capacity ranges. Use this table as a quick sanity check after your load calculation — if your result falls well outside the typical range, it's worth reviewing the inputs.
| Use Case | Typical Size | Typical Loads |
|---|---|---|
| Camping / tailgating | 2,000–2,400 W | Lights, small appliances |
| RV with air conditioner | 3,500–4,500 W | AC unit, refrigerator, lights |
| Home essentials backup | 4,000–6,500 W | Refrigerator, furnace, lights, router |
| Larger home / multiple appliances | 9,000–12,000 W | Well pump, sump pump, AC, dryer |
| Whole-home standby with central AC | 20,000+ W | Central AC, water heater, oven |
| Construction / job site tools | 5,000–10,000 W | Saws, drills, compressors |
| Industrial / three‑phase facility | 20 kVA – 4,000 kVA | Motors, pumps, production lines |
If you prefer to work the numbers by hand — or want to verify the calculator result — follow this five-step method to move from your connected equipment to a practical generator rating.
Write down every appliance, motor and piece of equipment that may operate at the same time. Record the running watts from the equipment nameplate or specification sheet.
Express all loads in watts or kilowatts before adding them. For motors, 1 HP ≈ 746 W. For balanced three-phase loads use W = 1.732 × V × A × PF.
Sum the continuous power requirement of all equipment expected to operate simultaneously. This establishes the site's normal connected running demand.
Find the motor-driven load with the highest startup requirement and add its additional starting demand to the total running load so the generator can handle the transient without tripping.
Add an allowance for transient demand, operating conditions and expected growth. A planning factor of approximately 1.20–1.25 is commonly used, subject to the actual project requirements.
An 80 kW connected load corresponds to 100 kVA at a 0.8 power factor. Applying a 25% sizing allowance gives 100 × 1.25 = 125 kVA. The practical selection is therefore a 125 kVA generator, equivalent to approximately 100 kW at 0.8 PF.
A connected-load calculation is only the starting point. Engine output, alternator performance and transient capability can all change with the operating environment and the type of electrical load.
Engine output decreases as air density falls. A planning estimate of roughly 1% loss per 100 m above 1,000 m means a site at 3,000 m may have substantially less available engine output than the sea-level rating.
High ambient temperature reduces cooling effectiveness and usable output. Above approximately 40 °C, additional derating may be required for hot climates and poorly ventilated generator installations.
Direct-on-line motors can draw around 6–7× full-load current during startup. Star-delta starters, soft starters and VFDs reduce the transient demand and can sometimes allow a smaller set.
Standby generators operate intermittently during utility failures, while prime-rated sets are designed for extended operation under varying loads. The required rating should reflect the actual duty cycle.
UPS systems, VFDs and electronic loads can introduce harmonics. If non-linear loads form a large percentage of the system kVA, alternator sizing and transient performance require additional consideration.
Planned equipment additions should be considered before ordering the set. Allowing approximately 10–25% reserve can reduce the risk of replacing an otherwise suitable generator as demand grows.
Most sizing problems come from a small number of recurring assumptions. Avoiding these errors helps prevent both undersized equipment and unnecessary excess capacity.
kW is real usable power, while kVA is apparent power. The relationship depends on power factor. Failing to convert between the two can directly result in an undersized generator.
A generator may carry the continuous load normally and still fail when a compressor, pump or HVAC motor starts. The largest transient demand must be included in sizing.
Diesel generators operating for long periods at very low load can experience incomplete combustion and wet stacking. Excess capacity also increases capital cost and reduces operating efficiency.
A generator rated under standard conditions may not produce the same usable output at high altitude or in extreme heat. Apply the necessary derating before choosing the final capacity.
Facilities frequently add machines, HVAC capacity, pumps or production equipment after commissioning. Leaving a realistic expansion margin can avoid a costly generator replacement later.
Final generator selection should be checked against the complete load schedule, motor-starting sequence, power factor, site conditions, duty rating and selected engine-alternator combination before equipment is ordered.
A wattage chart is only the first step — the point is to arrive at a generator that actually fits. ShanHua Power manufactures diesel generator sets from 8 kVA to 4,000 kVA with 100% pre-delivery testing. Tell us your loads and we’ll confirm the right capacity — no obligation.
A: A typical refrigerator draws 150–800 watts while running and 600–2,200 watts to start its compressor. Size for the starting surge, not the running figure.
A: Running watts are the continuous power an appliance uses in normal operation. Starting watts are the brief surge motor-driven devices need at startup — often 2–7 times the running wattage.
A: Most homes need 4,000–6,500 W for essential backup, or 20,000 W and above for whole-home coverage including central air conditioning and an electric water heater.
A: Divide watts by volts. A 1,200‑watt appliance on a 120‑volt circuit draws 10 amps (1,200 ÷ 120). For three-phase systems, the formula also includes the power factor and √3.
A: kW is real power; kVA is apparent power. They are related by the power factor: kW = kVA × PF. For three-phase systems at 0.8 PF, kVA = kW ÷ 0.8.
A: Add the running watts of everything you’ll run simultaneously, add the single highest starting surge, then add a 15–30% safety margin. Use the wattage chart on this page as your reference.
