A generator sizing calculator converts your electrical loads into a practical generator capacity recommendation using three connected values: kW, kVA and amps.
The real electrical power your connected equipment consumes while operating.
The total apparent power the generator must provide after power factor is considered.
The current at your selected voltage, useful for cable, breaker and distribution planning.
The calculator combines your running loads, the starting surge of the largest motor, a sizing margin and relevant site conditions such as altitude and temperature. The result is intended to give you a usable generator recommendation rather than a simple total-load estimate.
Supports single-phase and three-phase systems at both 50 Hz and 60 Hz.
Includes power factor, motor starting method, altitude derating and future expansion.
Recommendations can be matched against ShanHua generator sets from 8 kVA to 4,000 kVA.
Use the calculated requirement as the basis for a factory-direct quotation.
Choose Quick Mode for appliance-based sizing or Advanced Mode for industrial load calculations. Both methods estimate the generator capacity required to handle running load, motor starting demand and your selected operating conditions.
Add the equipment that may operate at the same time. Starting surge is automatically included.
Enter your project load and operating conditions for a more detailed sizing estimate.
Different applications tend to fall within predictable generator capacity ranges. Use these values as a quick reference to check whether your calculator result is in a realistic range before confirming the final load calculation.
| Application | Typical Range | Typical Loads |
|---|---|---|
| Home / Essential Backup | 10–30 kVA | Refrigeration, lights, pumps, furnace fan |
| Small Business / Retail | 30–100 kVA | Lighting, HVAC, refrigeration, POS |
| Small Construction Site | 20–40 kVA | Tools, site offices, dewatering pumps |
| Medium Construction Site | 80–200 kVA | Concrete mixers, compressors, workshops |
| Large Construction / Infrastructure | 200–500 kVA | Multiple pumps, heavy equipment, parallel sets |
| Warehouse | 100–400 kVA | HVAC, conveyors, loading dock equipment |
| Factory / Manufacturing | 100–2,000 kVA | Production lines, CNC, compressors, lighting |
| Data Center | 500–4,000 kVA | N+1 redundancy, UPS input, cooling systems |
| Hospital / Healthcare | 200–1,000 kVA | Life-safety loads and standby systems |
| Agriculture / Farm | 20–150 kVA | Irrigation pumps, barns, grain handling |
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.
Calculate the capacity you need, then let our engineers verify your selection. With 25+ years of experience and generator solutions from 8–4,000 kVA, ShanHua delivers reliable factory-direct power worldwide.
A: For essential backup — refrigerator, lights, well or sump pump, furnace fan — you typically need 3–5 kW. For comfort coverage that includes air conditioning, plan on 7–12 kW. A full-home setup with central AC and electric appliances can require 15–30+ kW. Run your appliance list through the calculator for a precise answer.
A: It depends on the phase of work. A small residential site with basic tools and lighting needs 20–40 kVA. A medium commercial site with concrete mixers and site offices needs 80–200 kVA. Large civil and infrastructure projects with multiple pumps and compressors start around 200 kVA and often use several paralleled units.
A: Divide kW by the power factor, typically 0.8 for industrial three-phase: kVA = kW ÷ 0.8. A 100 kW load therefore needs a 125 kVA generator. Our kW to kVA calculator does this for you instantly.
A: Running watts are what equipment consumes continuously. Starting watts are the extra burst needed for a few seconds when motors start — often 3 to 7 times the running load. The generator must handle the starting watts of its largest motor, or it will trip when that equipment switches on.
A: It will struggle or fail to start large motors, drop voltage, and can damage sensitive equipment. On critical sites, an undersized unit means downtime exactly when you need power most.
A: You pay more to buy it, and running it under light load causes wet stacking, higher fuel consumption per kWh, and faster wear. Correct sizing saves money on both purchase and operation.
A: Yes. Diesel engine output falls roughly 1% for every 100 m above 1,000 m because thinner air limits combustion. Above 1,500 m you should size up or confirm the derating with the manufacturer.
A: Yes. We offer OEM/ODM customization across our full range. If your calculated size or configuration does not match a standard model, our engineering team will build the unit to your specification — voltage, frequency, enclosure, control system, and more.
