Running watts are not enough
Portable AC and dehumidifiers can need a large start allowance. Size the generator for start behaviour as well as the steady load.
Electrical calculator · generator / kVA / compressor / start current
Generator kVA & Compressor Start Calculator
Size a generator for portable AC, dehumidifiers, fans and heaters with a start-current allowance rather than only running watts.
For event sites, emergency cooling, flood drying and temporary power jobs.
Field notes
Field notes for Generator kVA + start current
Practical checks to run before this calculator result turns into a site decision.
Stagger starts where possible
If the result is marginal, a staged start plan can be cheaper and safer than simply oversizing the generator.
Check voltage sag symptoms
Hard starts, dimming, alarms or nuisance trips point to generator/load mismatch, long leads or poor phase allocation.
FAQ
Generator kVA + start current FAQ
Short answers for UK temporary electrical and HVAC planning.
What is the Generator kVA + start current used for?
Size a generator for portable AC, dehumidifiers, fans and heaters with a start-current allowance rather than only running watts. It is mainly for temporary HVAC, drying, cooling and site-power planning, especially where a quick pre-check is needed before selecting equipment or changing a temporary setup.
Can this replace BS 7671 design, inspection or testing?
No. Treat it as a pre-check for the conversation or job file. Final decisions still need current BS 7671 requirements, manufacturer data, inspection, testing, risk assessment and the actual site conditions.
What should I verify before acting on the result?
Check supply rating, protective device rating, cable length, voltage drop, start current, phase balance and the condition of temporary leads or distribution boards. If any assumption is uncertain, use the result as a prompt to investigate rather than as clearance to switch on.
What does an amber or red result usually mean?
It normally means the margin is weak, an assumption is missing, or the load should be split, staged, moved closer to the supply, reduced or checked by a qualified electrician before use.
Next tools in this workflow
See job flows
Calculator · 110V / site transformer
110V transformer heater check
Check whether a 110V site transformer can realistically power temporary heaters, dehumidifiers, fans or drying kit without overload or unacceptable voltage drop.
Calculator · RCD / earth leakage
RCD leakage budget
Estimate cumulative leakage from compressors, dehumidifiers, portable AC, heaters, pumps and long wet leads before stacking everything behind one RCD.
Calculator · three-phase / phase balance
Three-phase load balancer
Allocate single-phase heaters, dehumidifiers, fans and portable AC units across L1/L2/L3 to reduce imbalance and nuisance trips.
Calculator · 13A / 16A
Connector selector
Select a likely temporary connection class based on load current, voltage, environment and duty cycle.