How to Size a Home Generator: UK Loads, kVA Maths and the Part P Limit

Two mistakes dominate. Someone spends £1,100 on a 6 kW open-frame set that drinks three litres an hour to run a fridge and a router, or buys a £280 suitcase generator that stalls the instant the freezer compressor kicks in. Both were chosen off the biggest number on the box — almost always the surge rating, not the continuous one.

British homes are far easier to back up than American ones, and most advice online ignores it. Three-quarters of us heat with mains gas, so there is no 3 kW air handler to carry: a combi boiler and pump draw about 130 W. Done honestly, the sums put a three-bed semi on a 2.5 kW inverter set.

Carbon monoxide fills the coroners' reports. A generator runs outdoors, on hard standing, four metres or more from any door, opening window, trickle vent or air brick. Not in the garage with the door up, not in a porch, not in a shed. A running set in a garage reaches lethal CO levels in under ten minutes.

Second is backfeeding — a double-ended 'suicide lead' plugged into a socket to energise the house. It pushes 230 V back onto the network, where it can kill an engineer working on a line he believes is dead. Unlawful, uninsurable, and precisely why changeover switches exist. Third, fuel.

• •Petroleum (Consolidation) Regulations 2014: up to 30 litres at home unnotified, in approved containers only — 10 litres per plastic can, 20 per metal one. Above that, notify your Petroleum Enforcement Authority.
• •Petrol goes stale in three to six months; add stabiliser or run the carburettor dry. LPG and diesel keep far longer.
• •Refuel cold only, and fit a BS EN 50291 carbon monoxide alarm in the nearest room.

Sizing is a desk job with one cheap gadget. Installation afterwards is another matter.

• •Plug-in energy monitor, £12–£20 — reads real watts, not the optimistic label figure. A clamp meter with inrush capture (£40–£70) is optional but catches starting spikes.
• •Your main fuse rating, printed on the cut-out: 60 A, 80 A or 100 A. A 100 A single-phase supply is roughly 23 kVA, the ceiling of what the house can ever draw.

Measuring takes 48 hours with the monitor on the fridge-freezer and boiler, plus twenty minutes of arithmetic. Budget hardware separately from the connection — the connection is what catches people out. Doing how to calculate your home's electrical load first makes these figures fall out almost automatically.

• •2.0–2.2 kW petrol inverter, £380–£600: boiler, fridge-freezer, lighting, router, charging.
• •3.2–3.5 kW inverter, £700–£1,100: adds a microwave or a kettle, one at a time.
• •7–10 kVA diesel or LPG standby with automatic transfer, installed: £6,000–£15,000. Read what a standby generator really costs first.
• •Changeover switch and weatherproof inlet, fitted by a registered electrician: £450–£900 with the certificate.
• •Fuel: a 3 kW inverter at half load burns 1.0–1.2 litres an hour, so £30–£36 per 24-hour outage.

Measured figures from British kit at 230 V, not badge values. Note how small the heating load is and how enormous the water-heating load is — that asymmetry is why UK sizing differs from American guidance.

• •Combi boiler, controls and pump: 120–150 W running, about 250 W on pump start. Oil boiler with burner motor: 300–500 W running, 800–1,200 W starting.
• •Fridge-freezer: 100–200 W running, 600–1,200 W surge. Chest freezer: 80–150 W, roughly 600 W surge.
• •Whole-house LED lighting 100–200 W; router, chargers, laptop and TV 150–250 W. Cellar or sump pump, 0.4 kW: 400 W running, 1,200–1,800 W starting — and if yours cycles constantly, read why a sump pump keeps running first.
• •Kettle 2,800–3,000 W; microwave about 1,300 W input. Electric shower 8,500–10,500 W and immersion heater 3,000 W belong nowhere near a domestic generator.

Work through these in order; step five is the one people skip. Headroom separates a set that hums for three days from one that browns out and cooks a boiler PCB.

• •1. List tier one: the loads that must stay live. For most UK homes that is boiler, fridge-freezer, lighting and communications. Nothing else.
• •2. Measure each with the plug-in monitor over 24–48 hours, then add the running watts. A three-bed semi lands between 450 W and 700 W.
• •3. Find the single largest starting surge, usually the fridge-freezer or a pump. Count one only — two motors starting together is vanishingly rare.
• •4. Add that surge to the running total: 600 W plus a 1,000 W surge gives 1,600 W peak demand.
• •5. Add 25% headroom for future kit and derating in heat: 1,600 W becomes 2,000 W.
• •6. Convert to kVA if the set is rated that way — divide by a power factor of 0.8, so 2,000 W is 2.5 kVA.
• •7. Compare against rated continuous output, never the 'maximum' figure. A set sold as 3.2 kW may be rated at only 2.8 kW continuous.
• •8. Check harmonic distortion. Anything with a circuit board — condensing boiler, heat pump controls, inverter fridge — needs a true sine wave under 5% THD, so an inverter set, not a cheap AVR one.
• •9. Check runtime: a 4-litre tank at 1.1 litres an hour gives under four hours.

• •Engine bogs down when the freezer starts: starting surge exceeded. Stagger loads, or step up one size.
• •Boiler fires then locks out on a fault code: harmonic distortion or unstable frequency, usually an AVR generator.
• •Generator runs but its own socket is dead: the built-in RCD has tripped, or the set has a floating neutral needing earth-neutral bonding — your electrician's call.
• •Diesel set smoking and wet at the exhaust: wet stacking from chronic under-loading. Run at 50–80% of rated output.
• •It copes, but you refuel every three hours: right set, wrong tank. Consider LPG on a 13 kg bottle, or a home battery with an EPS output.

You can size a generator yourself and run an arctic-grade extension lead to a fridge. What you cannot lawfully do in England and Wales is wire it into the fixed installation. A generator inlet, changeover switch or automatic transfer switch all involve new circuits and consumer unit work, notifiable under Part P of the Building Regulations: a registered competent person (NICEIC, NAPIT or ELECSA) issuing a certificate, or building control notified before work starts. Scotland and Northern Ireland have their own equivalents.

A second layer gets missed. Any generator that can run in parallel with the mains, even momentarily, falls under the network operator's G98 or G99 rules and must be notified. A break-before-make changeover never parallels; an automatic transfer switch almost always needs the paperwork. Landlords carry a further duty: the Private Rented Sector electrical safety regulations cover the whole installation, generator included, with an EICR every five years.

Get two quotes — typical electrician rates show whether £600 for an inlet and changeover is fair locally. Meanwhile work through a power outage checklist: a torch, filled water containers and a gas hob carry most households through a 12-hour cut with no engine.