Best portable generators 2026: what size a house needs in a power cut, and how long 30 litres of petrol really lasts

In Great Britain a power cut is not an open ended event. If your supply fails in normal weather because of a fault on the distribution system, the network operator has 12 hours to restore it, and if it misses that, a payment is due to a domestic customer under Ofgem's guaranteed standards, with a further payment for each additional 12 hour period. In severe weather the clock and the amounts change with the storm category, and Ofgem re-sets the figures every April, which is why the number to look up is the current one on your own operator's page rather than one written in an article.

That changes what you are buying. A generator is not insurance against losing power: it is insurance against the first twelve hours, and against the storms where the twelve hour clock does not apply. Everything in this guide follows from that, because the honest question is not how many watts a house uses, it is how many hours you want to cover and where you can legally put the thing that covers them.

A house that wants to keep the fridge, the freezer, the lights, the router and the television running asks for around 750 running watts. That is not the generator's figure. When the fridge compressor starts it asks for several times that for about a second, and that step is added once only, because the motors in a house do not start together. Two numbers come out, and both have to be checked.

The guides in this category all solve it the same way, with a percentage: add twenty or thirty per cent to the total. The same pages then explain, a few paragraphs down and without correcting the percentage, that a motor can ask for two to three times its rated power at the instant it starts, or that fridge and air conditioning motors have starting peaks three to five times their rated draw. Between thirty per cent and three hundred there is an order of magnitude, and it is exactly the one that decides whether the generator trips.

The practical consequence is that the appliance which decides the size of the generator is almost never the one that draws the most. A 2,000 watt heater has no motor: it asks for 2,000 watts and that is that. A 200 watt fridge asks for 600 for a second. And an 800 watt water pump, which is what sits in any house on a borehole, can ask for 2,400 at the start: on its own it doubles the generator that everything else needed.

A petrol generator has two power figures because it has two limits: how much current it can give for a short while without the protection tripping, and how much it can give indefinitely without overheating. The second sits between 7 and 10 per cent below the first in the manufacturer spec sheets we read. The problem is not that the gap is large. It is that the listing does not say which of the two it is showing, and you do not have to leave one manufacturer's own catalogue to see it.

Read the eight against both figures at once, with the fuel and the sound power beside them, and the list sorts itself into three groups: the ones you can carry, the ones you install, and the one that does not burn anything.

Every listing in this category sells autonomy by the tank, and in Britain the tank is not the constraint. Under the Petroleum (Consolidation) Regulations 2014 you may keep up to 30 litres of petrol at a private home without notifying your local petroleum enforcement authority, in containers of up to 10 litres in plastic, up to 20 in metal, or a demountable fuel tank of up to 30. Above 30 and up to 275 litres you have to notify them in writing. The store must not be in living accommodation, and if it is not in the open air it needs a direct exit to outside and ventilation to it.

Apply that ceiling to the figures the manufacturers publish and the autonomy question answers itself. The 3 kW inverter here declares 5.9 hours at half load on a 4 litre tank, so the full legal store is seven and a half tanks and about 44 hours. That is comfortably past the 12 hour standard and into a serious storm. And the ceiling is a petrol ceiling: those regulations are about petrol, so the diesel set in this guide is not caught by them, which is the single biggest practical difference between the two fuels here and it appears in no listing.

Of the 33 fuel generator listings we counted on 6 September 2026, seven call themselves silent or soundproofed and none publishes a decibel. Across the 371 in the Leroy Merlin catalogue, 16 titles carry a decibel figure. The figure always exists, and not because the manufacturer is generous: because a law requires it to be on the machine, and in Great Britain that same law also caps it.

Two figures can only be compared if they are measured the same way, and if one spec sheet says 58 and another says 95, the likely explanation is not that one machine is far quieter: it is that they are giving different quantities. If it is going near the house, the one that matters is the small one, with its distance and its load written beside it. Outdoors, a sound source gains roughly six decibels every time you halve the distance, so 58 dB at seven metres is not the 58 dB you will hear from the window.

We would not buy a generator that calls itself silent and prints no figure, and the reason is not scepticism: the guaranteed sound power level has to be marked on the machine by law, so a listing without it is a listing that did not copy what is already stamped on the casing.

A conventional generator takes its current straight off the alternator, so its frequency rises and falls with the engine speed. An inverter rectifies it and rebuilds it electronically, which is why it is advertised as a pure sine wave. For a bulb or a heating element it makes no difference. For the control board of a modern boiler, a router or a laptop it does: those are the things that cope worst with a frequency that wanders.

There is a second effect, and it shows up more in the noise than in the fuel: not having to spin at a fixed 3,000 revolutions to make 50 hertz, an inverter drops the engine when the house is asking for little. That is why the K&S BASIC 33i S declares 68 dB at seven metres with 3 kW rated and its 1.8 kW sibling declares 70.

Two questions decide this, and neither is price. The first is where it will stand: anything that burns fuel produces carbon monoxide, so it goes outside, in the open air and away from doors and windows, never in a garage and never in an enclosed porch. In a flat that condition cannot be met, and the answer is a battery.

The second is what it is feeding. Running extension leads to a fridge, a lamp and a router is one thing. Feeding the fixed circuits of the house is another: it needs an isolating changeover arrangement fitted by an electrician, so that the generator and the incoming mains can never be live at the same time. Two of the eight here include that transfer gear, and it is most of what separates their price from the rest.

Then it is hours. A battery station has no awkward power limit, but it has an energy limit: 2,048 watt hours run out. A petrol machine does not run out while there is fuel in the store, which in Britain is 30 litres, and diesel is not caught by that ceiling at all. Our split would be battery for a flat and for a cut measured in hours, a petrol inverter for a house, and diesel for a large house or one on a borehole pump.

Four cases cover almost everyone who arrives on this page. Pick yours and the model appears, with the reason in one line.