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Free calculator

Use this calculator to

  • Size a standby generator in kVA from up to four separate loads
  • Allow for motor starting surge with per-load multipliers
  • Add a safety margin and round up to a standard set size
  • Compare a site's running kVA against common standby sizes from 10 to 500 kVA

Generator Size Calculator

Find the right generator size for residential or commercial loads.

Add each load below. Motors require 3–6× rated current at start-up.

W
W
W
W
PF
%
Generator Recommendation

Formulas

  • kVA running = Total watts × surge factor / (PF × 1000)
  • kVA sized = kVA running × (1 + margin%/100)
  • Standard generator sizes: 3.5, 5, 7.5, 10, 14, 20, 25, 30, 40, 60, 80, 100 kVA

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Common scenarios

Select one to run it in the calculator above.

For business

Why this matters for businesses

Generator sizing is one of the line items in a business continuity plan that gets written once and then sits in a drawer for ten years, by which point the connected load has crept up, a second chiller has been installed, and the diesel set on the roof is no longer big enough to start the largest motor on the site. A hospital, a chilled distribution centre, or a tier-three data hall that loses primary supply during a storm finds out the hard way that the sizing margin was eaten by load growth nobody flagged.

The numbers move quickly with motor starting. A 75 kW chiller compressor with a direct-on-line starter draws six to eight times running current for a few seconds, and a generator sized only for steady-state kW will trip on the first restart attempt. Soft starters, variable-speed drives, and ride-through UPS systems all change the calculation, and a 500 kVA set that looked comfortable on paper can be marginal once the surge profile of the actual plant is layered in. The downstream cost of getting it wrong is a continuity event that the BCP was meant to prevent.

Purely Energy works with sites where standby generation interacts with the half-hourly demand profile, including Triad avoidance schemes, capacity market participation, and DSR contracts where the genset earns revenue while it waits. Generator size sets the upper limit on all of that, and a properly specified set is an asset, not just a depreciation entry. Pairing the engineering sizing with the right commercial wrapper is where the value gets unlocked.

Common questions

How do I convert a load in kW to a generator size in kVA?

Divide the total kW by the power factor to get kVA, then add a margin. Generators are conventionally rated at a power factor of 0.8, so a 64 kW load becomes 80 kVA running, and a 25 percent margin takes it to 100 kVA, which is itself a standard set size. The calculator does this in one pass and rounds up to the next size in the standard range.

Why do motors need a bigger generator than their nameplate suggests?

A direct-on-line motor draws three to six times its rated current for the second or two it takes to run up to speed. A generator sized only for the running load will see its voltage and frequency dip, or its breaker trip, every time the motor starts. Apply a surge multiplier to each motor load in the Type column, or specify soft starters or variable speed drives to cut the inrush at source.

What is the difference between kVA and kW on a generator plate?

kW is the real power the load consumes; kVA is the apparent power, which includes the reactive component drawn by motors and transformers. The two are linked by power factor: kW = kVA × PF. Generator sets are usually quoted in kVA at a power factor of 0.8, so a 100 kVA set delivers 80 kW of real power. Size against kVA, then check the engine can supply the kW.

How much spare capacity should a standby generator have?

A 20 to 25 percent margin above the calculated running kVA is normal. It covers load growth, measurement error and the fact that diesel sets should not sit at full output for long periods. Avoid going too far the other way: a diesel engine that runs persistently below about 30 percent load can suffer wet stacking, where unburnt fuel fouls the exhaust system and shortens engine life.

What are the common standby generator sizes for UK businesses?

Packaged sets cluster around standard ratings: 10, 20, 60, 100, 200 and 500 kVA cover most SME and mid-market sites, with larger sites pairing sets in parallel. Before specifying one, check your actual maximum demand from half-hourly metering data rather than adding up nameplates, because diversity means the real peak usually sits well below the connected load. Your Agreed Supply Capacity (ASC) is a useful cross-check.

Related calculators

The conversions that procurement and engineering use alongside this one.

Generator Size Calculator (kVA) | Purely Energy