How to buy the best generator

A capable home generator could be a savvy investment if you regularly need access to back-up energy. The best will ensure your home has power during outages, too.

In our guide, we explain the different types of generators, including petrol, diesel and liquid petroleum gas (LPG). Plus, we give advice on how to pick the best option based on wattage and your estimated power needs.

Hopefully, you won't need to call on a generator too often, but having one tucked away for emergencies will bring peace of mind. Keep scrolling for our expert advice.

Heating and air treatment – explore our expert reviews and advice covering boilers, electric heaters and dehumidifiers.

What is a generator?

Battery, petrol, diesel and LPG-powered generators are designed to provide electricity at times when the mains supply is unavailable. All of them will be relatively heavy, whether they're weighted down by the parts of an internal engine or a large battery.

Generators come in various shapes and sizes. If you're purchasing one primarily to use as a power-cut backup, most households will be well served by a temporary portable generator that's small enough to carry around and can be neatly stored away. 

Bear in mind that these won't provide enough power to keep your whole home running as normal, but should be enough to keep you going with the essentials. 

In other use cases, you might need a larger generator with wheels.

How do generators work?

Many transportable generators are comprised of an internal combustion engine, an alternator and a fuel tank.

• Petrol or diesel generators are filled with liquid fuels, which are then used to generate power. The engine in a petrol or diesel generator turns an onboard alternator, which converts mechanical power into electrical power.
• LPG generators connect to a propane tank, which you'll need to replace once it's empty.
• Battery generators use battery packs to store electricity. These are smaller and more portable than generators that use a fuel tank, but are only really useful if you have low power demand. They can sometimes be charged with solar panels. 

The Anker Portable Power Station (below), for example, is powered by lithium iron phosphate (LiFePO4) batteries. The brand claims it can power a mini fridge for five hours, but it's only compatible with items that operate at less than 200W. It weighs 3.7kg and can be charged with a solar charger.

Certain generators are more weather-resistant than others, so pay close attention to the manufacturer's claims. If you're planning to take a generator camping, it'll need to be well protected against the elements.

Charging a generator

Most battery-powered generators need to be charged at a wall outlet, although some models can be charged using a car port or solar panels – useful for camping trips or festivals.

On the side of a generator you'll spot a power button, starter switch or pull-cord mechanism, along with various power outlets – you can plug gadgets directly into the generator, or use an extension lead. Some generators have cable inputs as well as plug sockets, so you can plug a USB or USB-C cable directly into the device.

Choosing the right size generator: wattage explained

Ultimately, the price of a generator is based on its wattage. To convert watts into kW, divide the total by 1,000.

Generators are often rated in kilovolt amps (kVA). This is the amount of power being used by the generator but, as no electrical system is 100% efficient, the kW (the usable power) will be less than this – 80% of the kVA. So, you need to take this into account when deciding whether a particular generator will suit your needs.

The higher the wattage, the more items a generator can power at the same time. So you need to consider which devices you'll be charging or using – are you looking for something that can keep a couple of smartphones charged during a power cut, or do you need to power something larger, such as a fridge for medical equipment?

Take a look at the output of the generator, then compare that with the power needed by the appliance(s) you're looking to operate. You can find these numbers on the appliance itself, inside the product manual or online on the brand website.

If you're hoping to plug in multiple products at the same time, you need to ensure that the generator can handle the combined wattage (plus 20% to be safe). Overloading can cause it to fail and potentially damage your connected appliances as well.

Bear in mind that items with an induction motor, such as fridges and drills, have a higher starting wattage of three times the running wattage, so this is the figure you should use when working out the total kW you need.

If you just want to charge your smartphone on the move, you'd be better off with a portable power bank. Check in with our guide to the best portable power bank chargers.

Types of generator

Petrol generators

• Pros Petrol is cheaper than diesel, quieter than diesel generators
• Cons Require regular maintenance

You should expect to spend at least £200 on a petrol generator. Popular petrol generator brands include Champion, Honda, Hyundai and Kohler.

Petrol generators can be a tempting purchase if you're looking for a model that won't be used very often. These machines are usually less expensive than diesel, and some models are small enough to carry with one hand if that's what you need.

They are quieter to run than diesel generators, which could be a consideration if you plan to run one for hours at a time.

Diesel generators

• Pros Generally last longer than petrol generators, long running time
• Cons Diesel is more expensive to buy than petrol, can be noisier than petrol alternatives

You'll spend at least £1,000 on a diesel generator, and prices can soar to £10,000 and beyond. Popular diesel generator brands include Hyundai, Stephill and Warrior.

Diesel generators are more efficient compared to petrol-powered ones, although the fuel will cost you more. Unless you're using a generator on a regular basis, you might not notice any savings.

These models are much larger than petrol generators, making them a popular pick among business owners – those needing to power a wedding marquee or a street-food stand, for example. If you only need to power a couple of smaller emergency appliances during a power cut, you might find that a diesel generator is overkill.

Portable generators

• Pros Small enough to carry, affordable models available  
• Cons Reduced wattage means some models won't power all of your appliances

Prices for portable generators start at around £160 but can go up to £2,000 or more. Popular portable generator brands include Anker, Draper, Impax and Kohler.

Portable generators are popular thanks to their manageable size and ease of use, with labelled inputs helping you make sense of the sockets. Some are powered by fuel, while others are basically a large rechargeable battery.

They can be transported using carry handles and stored away when not in use. You won't need to work through a complex installation, either – just switch the portable generator on and start connecting your devices.

As they're small, portable generators have a limited output capacity. If you want to power larger appliances during a power cut, a beefier generator with higher wattage might be better suited to your needs.

How much do generators cost to run?

If you're using a petrol or diesel generator, the running cost will vary depending on fuel prices and the size of the engine, as well as the running time of the machine.

Let's use Hyundai's P1 petrol generator as an example. It has a 3L fuel tank and a claimed run time of 8.5 hours. With average UK petrol prices at the time of writing near £1.45 per litre, you'd pay around £4.35 to fill the generator with petrol and run the tank empty. That's about 51p an hour.

For a battery-powered generator, you'll need to calculate how long it takes to charge the battery from flat to full, and factor in current electricity prices.

Some battery-powered generators can be hooked up to a solar panel for free off-grid charging, as shown below, but this will vary depending on the manufacturer. You'll usually need to buy the solar panel separately.

Where to buy a generator

If you're buying a relatively small generator, shopping in-store and transporting it home the same day is an option. When buying a large model in-store, you might need to arrange delivery and pay the associated fees.

Shopping online is convenient and you'll be able to compare tech specs at a glance between the generators on your shortlist.

Popular online retailers that stock generators include:

• Amazon offers a varied selection of portable and petrol generators from popular brands including Anker, Draper and Hyundai.
• B&Q has a range of petrol generators. Brands include Draper and Champion Power Equipment.
• Screwfix sells a selection of generators, the majority of which are powered by petrol. Brands include Evolution, Hyundai and Kohler. 
• Toolstation stocks generators from Draper and Festool.

Find out which retailers are rated highly by Which? members with our expert guide on the best furniture, homeware and DIY shops.

Generator hire

If you need power for a one-off event or a camping trip, hiring a generator is another option to consider.

There are plenty of brands willing to provide a generator on a temporary basis. HSS Hire, for example, provides petrol, diesel and portable generators – a 1.6kVA portable generator would cost around £76 for a day, or £110 for a week.

Building material retailer Jewson also hires out portable generators. Its cheapest option is a 2.7kVA generator, which is around £42 for the first day, or £84 for the week.