What power factor are generator ratings based on?

That single convention explains most of the confusion around generator sizing. Two generators of the same physical size can carry different labels depending on whether the maker quotes kVA or kW, and a household adding up appliance wattages in kW is working in a different unit from the one on the sales sheet. The conversion is not difficult, but it has to be done before a set is chosen, not after.

This page sets out what power factor means for a rating, why 0.8 is the reference, how to move between kVA and kW, and where single-phase household supplies differ from three-phase. It also covers what happens when the load's power factor is lower than the rating assumes, which is the case that catches buyers out.

What power factor means for a generator rating

Power factor is the ratio between real power, measured in kW, and apparent power, measured in kVA. Real power does the work: it turns a motor, heats an element, lights a room. Apparent power is the product of voltage and current flowing in the circuit, and it is always at least as large as the real power because alternating current in inductive loads such as motors and transformers is out of step with the voltage.

A generator has two limits that matter. Its alternator and windings are limited by current, which is an apparent power limit expressed in kVA. Its engine is limited by the mechanical power it can deliver, which is a real power limit expressed in kW. The rating label ties the two together at a stated power factor. When a maker quotes a single figure in kVA, the kW available depends on the power factor of whatever is plugged in.

For a household, this matters because the loads are mixed. A resistive heater draws current in step with voltage and sits close to unity power factor. A fridge compressor, a circulating pump or a heat pump does not. The Energy Saving Trust notes that for domestic wind generation "the stronger the wind, the more energy produced", and Home Energy Scotland makes the same point for hydro: "the greater the height and the more water there is flowing through the turbine, the more electricity generated"⁵. Those are statements about output, but they underline that generation is rated against conditions, and power factor is one of the conditions a generator rating assumes.

The practical consequence is that a generator's kVA figure is not a promise of kW. It is a promise of current capacity at a stated power factor, and the kW follows from what the load does with it.

Why ratings are quoted at 0.8 power factor

The 0.8 figure is a convention, not a measurement of any particular house. It represents a mixed commercial or industrial load where motors, transformers and electronics pull the power factor below unity, and it has been used for so long that it functions as a standard reference point across the generator market.

Rating at 0.8 does two things. It gives a conservative kW figure that most real loads will not exceed, and it gives a common basis for comparing sets from different makers. A generator rated 10 kVA at 0.8 is understood to deliver 8 kW, and a buyer comparing it with a set rated 8 kW can see they are broadly the same machine.

The convention is not universal across all equipment. A Bluegen unit measured 0.9992 at full output, close to unity⁷. These are inverters and generators of a different kind, but they show that 0.8 is a rating convention rather than a physical constant.

For a household buying a petrol or diesel set, the 0.8 convention is the one that applies. It is worth checking whether a maker quotes a kW figure derived at 0.8 or a standalone kW rating, because the two are not the same claim.

Apparent power (kVA) versus real power (kW)

The distinction between kVA and kW is the whole subject in one line. Apparent power is what the alternator must be able to carry. Real power is what the engine must be able to drive and what the appliances actually consume.

The two are linked by the power factor: kW = kVA × power factor. At 0.8, kW is 80% of kVA. At unity, they are equal. Nothing in between is unusual in a house.

The Energy Saving Trust's jargon buster defines a gigawatt as "a unit of power equal to 1 billion watts, typically used to describe the output of large power plants or the total electricity"⁸. That is the scale of the grid. A household generator sits at the other end, and the same unit logic applies: watts, kilowatts and kilovolt-amperes are all measures of power, but they are not interchangeable without the power factor.

For a household, the kVA figure is the one that determines whether the set can carry the current without tripping or overheating. The kW figure is the one that determines whether the engine can turn the load. Both have to be satisfied.

How to convert a generator rating to the power your appliances need

The conversion runs in two directions, and it is worth being clear which one applies.

To go from a generator's kVA rating to the kW it will deliver at 0.8 power factor, take 80% of the kVA. A 10 kVA set gives about 8 kW. To go from a known load in kW to the kVA a connection application needs, NIE Networks gives the factor: "If you know your load information in kW, you can convert it to kVA by multiplying by the factor 1.05: kVA = kW × 1.05"².

To add up a household load, the Consumer Council's appliance cost checker gives the basic unit conversion: "1000W = 1kW"³. Sum the running watts of everything to be powered, convert to kW, then divide by 0.8 to find the kVA the generator must be rated at.

For reference, Which? lists a Dewalt dual voltage generator at "Power output: 3500W" and an Impax suitcase inverter generator at "Power output: 1900W"¹. Those are real examples of the size range a household is choosing between, and both are quoted in watts rather than kVA, which is common for smaller portable sets.

Where single-phase household ratings differ from three-phase

Most UK homes have a single-phase supply. Three-phase is uncommon: Nesta refers to "the 1% of households requiring three-phase connections"¹⁰. That single fact shapes the whole comparison, because the rating limits that apply to generation connections differ by phase count.

NIE Networks classes small scale renewable generation as "greater than 3.68kW and up to 17kW single phase and greater than 11.04kW and up to 5MW three phase"¹¹. SP Electricity North West refers to "11.04kW of generation on a three-phase supply, as part of permitted rights"¹². SSEN notes that most homes and small businesses can upgrade from single-phase to three-phase, "giving you up to 69kVA"¹³.

The phase count also affects how a generator is connected. SSEN requires a schematic drawing showing "the generator itself; name and power rating (kW) of the inverter being used; live phases, neutral, earth; lockable isolators; clear demarcation between our and your equipment"¹⁴. The power rating on the inverter type test certificate must match the rating on the schematic¹⁴.

For a household on a single-phase supply, the practical ceiling on what can be connected without a full application is low. SSEN states that an application is still required "even if the total generation across all properties or generators is below 3.68kW" in certain circumstances, including multiple properties or multiple generators at one property¹⁵. The rules are set out in more detail on the page covering generator grid connection notification.

What happens if your load power factor is lower than the rating

A load with a power factor below 0.8 draws more current for the same real power than the rating assumes. The generator's alternator reaches its current limit before the engine reaches its kW limit, so the set is effectively derated: less real power is available than the kW figure on the label suggests.

This is the case that catches buyers out. The engine has capacity to spare while the alternator is at its ceiling.

The maker's datasheet states the permitted power factor range. The Sungrow inverter datasheet gives "0.8 leading to 0.8 lagging"¹⁶, and that range is typical of the specification a generator will carry. Operating outside it can affect voltage regulation and cause overheating.

Fuel consumption tracks real power, not apparent power. Tesla Powerwall guidance states that "a 5kW petrol generator burns 1 to 2 litres per hour, with fuel expenses increasing during outages"⁴. The same guidance notes that generators "burn petrol, diesel, or LPG, releasing carbon emissions and contributing to air pollution"⁴. A low power factor load does not increase the fuel burn directly, but it does mean the generator is working closer to its current limit for the same useful output, which raises losses.

For a household, the independence question is straightforward. A generator rated at 0.8 power factor gives a predictable kW figure for a mixed load, and that is what the rating is for. The dependence that remains is on fuel: petrol, diesel or LPG, stored and replenished, with the safety and storage rules that apply. The page on generator fuel and running costs covers that side, and generator safety covers carbon monoxide and earthing.