Key decisions

The right size

Generators are rated by the electrical power they can deliver. That can be upwards of 650W to lots of kilowatts. For portability, however, the upper limit is about 6kW - and even one of that capacity would be over 100 kg, and it needs two people to carry it.

Most appliances have a label stating their power consumption. A generator should have a small margin over the power consumption of the appliance it drives. This is to allow for overload conditions.

The right type

Portable generators come in three types.

Two-stroke petrol

These are the lightest and simplest, so they're the most portable. They're also the cheapest, with prices starting at just under $100. But they're also the least powerful: their power output is typically less than 1kW. Other disadvantages are high fuel consumption (compared with four-stroke models), high noise, and the need to mix oil with the petrol. Two-stroke generators are suited to intermittent use (a few hours), rather than long hours as an electricity back-up.

Four-stroke petrol

These make up most of the portable generators on the market, although they're heavier than the two-strokes. Their prices range from $300 to around $3000, and their power outputs range from under 1kW to over 6kW. Four-stroke engines start easier, are quieter, and use less fuel than two-strokes. They're ideal as a portable back-up power source - but they're not so good for extended use (more than a few days).

Diesel

These have the lowest fuel consumption and wear out the slowest. But it might be difficult to find a truly portable diesel generator: they're the heaviest of the three engine types. They're also the most expensive.

The lowest power output we could find was 2.2kW, priced at $4300. Diesel engines are ideal for applications where you won't have to move the generator much and are likely to be using it for many hours at a time. Long hours of running are needed to make the purchase cost worthwhile.

Inverters

Newer-model generators at the "lighter" end of the market are likely to feature an inverter. This is an electronic device placed between the output of the generator and the outlet socket. Its job is to keep the output voltage and waveform as ideal as possible.

Not only can inverter generators be smaller and lighter than non-inverter models; they're also more suited to powering sensitive electronic equipment (such as computers).

Despite the odd glitch, the national grid provides a very consistent electricity supply - which means you can plug sensitive equipment like computers into the mains with little fear of damage caused by power spikes or fluctuations.

A portable generator is a different beast. It's technically difficult to make a small generator produce a stable supply. If all you want to power is an electric drill, this probably doesn't matter. But if you want to power a television, a computer, or other electronic gear, you should go for an inverter model and use a surge protector between the generator and the appliance.

Connecting the generator to the load
Most portable generators have an outlet socket that you connect to the load via an extension cord. This usually means one appliance at a time (unless you have a bigger-capacity generator).

The environment

Two-stroke engines are not only the least fuel-efficient they also pollute the most. That's another reason why they're best suited to intermittent use.

Otherwise, take your pick of two evils. Four-stroke engines produce more carbon dioxide (a greenhouse gas) than diesel ones. Diesel produces more ultra-fine soot particles (which are known to damage people's respiratory systems).

All exhaust gases are toxic. Whatever type of generator you choose, you must operate it at a safe distance from people or animals. Consult the operator's manual for distance guidelines.