Generator Sizing Load Calculator

Construction generator load form

Project settings

System power factor

Reserve capacity (%)

Generator voltage

Phase

Starting method

Altitude derating (%)

Temperature derating (%)

Load schedule

Enter connected load per item, quantity, demand factor, and surge multiplier.

Load item	Load per unit	Quantity	Demand (%)	Surge multiplier
Site lighting (W)
Hand tools (W)
Power tools (W)
Site office cabins (W)
Cabin HVAC (W)
Water pumps (W)
Air compressors (W)
Hoists or lifts (W)
Tower crane auxiliaries (W)
Welders (VA)
Concrete mixers (W)
Miscellaneous load (W)

Example data table

Construction load	Typical input	Demand	Surge note
Site lighting	3,000 W	90%	Usually low starting surge
Cabin HVAC	5,000 W	85%	Compressor motors need allowance
Water pump	4,400 W	70%	Motor start may be three times running load
Welder	6,000 VA	40%	Duty cycle affects demand

Formula used

Running watts = load per unit × quantity × demand factor. Running kW = total running watts ÷ 1000. Running kVA = running kW ÷ power factor. Starting kVA = running kW plus selected surge allowance, divided by power factor. Required kVA = larger demand × reserve multiplier. Final generator kVA = required kVA ÷ derated capacity factor.

Three phase current = kVA × 1000 ÷ (1.732 × voltage). Single phase current = kVA × 1000 ÷ voltage.

How to use this calculator

Add every expected site load. Use nameplate watts or volt amps. Enter the quantity that may operate during the same work period. Set demand lower when not all tools run together. Use a higher surge multiplier for motors, pumps, compressors, and HVAC. Choose the largest starting load option for normal staged starts. Choose all starting loads together for conservative planning. Add reserve for future tools, poor power factor, and site changes. Review the final kVA before requesting supplier quotes.

Construction generator sizing guide

Why correct sizing matters

A construction generator must support changing site loads. It must start motors, feed tools, and keep temporary facilities working. A small unit may trip breakers or stall during pump starts. A very large unit may waste fuel and run poorly at light load. Good sizing balances reliability, safety, cost, and future site growth.

Load groups

Start with a clear load schedule. Include lighting, offices, heaters, mixers, hoists, welders, pumps, compressors, and charging areas. Use equipment nameplate data when available. Record watts for resistive loads and volt amps for some construction equipment. Motors need special care. Their starting current can be much higher than running current. Soft starters and variable drives may reduce that demand.

Demand and diversity

Not every item runs at full output all day. Demand factor allows a practical estimate for mixed work. For example, several drills may be connected, but only some may operate together. Site cabins may cycle. Welders may follow duty cycles. Pumps may start only when water level rises. Conservative demand factors are useful when schedules are uncertain.

Reserve and derating

Reserve capacity protects the project from later additions. It also helps with voltage drop, cable length, and weather effects. Generator output can fall in hot conditions or at high altitude. This is called derating. The calculator increases the recommended size when derating is entered, so the available capacity remains closer to the project need.

Final checks

Compare the result with standard generator ratings. Then check voltage, phase, sockets, earthing, protection, fuel storage, noise limits, and access for refueling. Confirm cable sizes and distribution boards with a qualified person. For critical lifts, dewatering, medical areas, or night work, consider backup capacity. This calculator gives a planning estimate. Final selection should match site drawings, local codes, and supplier data.

FAQs

What size generator do I need for a construction site?

Add all running loads, include starting surge, apply power factor, then add reserve. The final size is usually selected from the next standard generator rating above the calculated kVA.

Why is kVA important for generator sizing?

Generators are commonly rated in kVA because they supply apparent power. kW shows real power, but kVA also considers power factor, which affects current and generator loading.

What is starting surge?

Starting surge is the extra power needed when motors begin rotating. Pumps, compressors, HVAC units, mixers, and hoists often need much more power at startup than during steady running.

Should I use all starting loads together?

Use all starting loads together only when equipment may start at the same time. For many sites, the largest starting load method gives a practical staged-start estimate.

How much reserve capacity should I add?

Many temporary power plans use 10% to 25% reserve. Use more reserve when loads are uncertain, the site will grow, or equipment may be added later.

Does altitude affect generator size?

Yes. Engine output can drop at higher altitude because air density is lower. Enter a derating percentage when the supplier or site conditions require it.

Does hot weather affect generator capacity?

Yes. High ambient temperature can reduce engine and alternator performance. Add temperature derating when the generator will run in hot conditions or poor ventilation.

Can this replace an electrician or supplier check?

No. Use it for planning and comparison. Final generator selection should be checked against nameplate data, site distribution, cable lengths, protection, and local construction rules.