Heating & energy
Portable generators
Introduction
Being without electricity can be inconvenient - and costly. A portable generator might be the insurance you need.
Maybe you live in a rural area where the power lines are at risk from weather damage; or maybe you run a catering business and have a freezer full of food. Either way, you wouldn't want to be without power for more than a few hours.
In this buying guide, we look at the main types of generators available, and the key issues you need to consider before you buy.
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.
Running costs & maintenance
Running costs
The national grid's electricity costs you around 20c per kWh. Running a small generator is never going to be as cost effective as that. For a start, a petrol-powered generator uses a heavily taxed fuel. And small generators aren't as efficient as industrial-scale plant.
- Running costs for diesel models are around 35c per kWh.
- Four-stroke models are more than double that - about 85c per kWh.
- And to run a two-stroke, you can expect to pay up to $1.20 per kWh.
Fuel consumption figures are not particularly well stated in manufacturers' specification sheets (and often they're non-existent), so for these figures we made some assumptions.
Maintenance
Generator maintenance can be required much more frequently than you might think.
It doesn't take that long to clock up engine-running hours. Let's say you get snow-bound, without power, like many Canterbury farmers last winter. If you run a generator for 6 hours per day for a week, you'll clock up 42 hours run time. At that point the generator may require servicing - or even be overdue.
Oil-change intervals for four-stroke petrol models vary, depending on the model: they're usually from every 25 to every 100 hours of operation. Filters, spark plugs, and carbon brushes (for non-brushless models) also require attention at regular intervals.
Oil, filter and spark plug changes should be within the ability of anyone who's moderately handy. Alternatively, you could take the generator to an outdoor-powertool shop.
If you ignore servicing, you'll pay with reduced engine reliability and lifespan.
Did you know?
We have George Westinghouse (picture, right) to thank for our alternating current distribution system.
If his great rival, Thomas Edison (inventor of the light bulb) had his way, we would have a direct current electricity system.
Alternating current has an overwhelming advantage over direct current for a distribution system: transformers can change its voltage. This allows high-voltage transmission lines to efficiently transport electricity over long distances. In its various guises, alternating current is used throughout the world.
So what's the difference?
In New Zealand's alternating current, for example, the supply voltage varies like a wave in a regular fashion 50 times every second. This is called the frequency.
The varying voltage means that, at any particular instant in time, the voltage could be anything from zero to about 325 volts (at its peaks). The 230 volts our system is rated at is a mathematical calculation, similar to an average, based on that 325-volt peak voltage.
The mains 230 volt supply varies very little. But non-inverter portable generators have a tough job keeping those factors constant: when the electrical load changes, the frequency and voltage can get temporarily out of shape. Inverter models (see Key decisions) are much better at keeping them stable.
Our advice
- If you live in a remote area where the power supply is vulnerable to extreme weather, a generator could be good insurance. It's also useful for powering tools and equipment in remote places.
- If you want a generator mainly for powering small portable equipment for a few hours, a two-stroke model could do the job at modest cost.
- A four-stroke model makes more sense if you think you may need to run the generator for a day or so.
- For longer periods (several days on end), there's nothing cheaper to run than a diesel model. But it may be difficult to find one that's easily portable.
Earthing
We strongly recommend using a residual current device (RCD) in the power line from the generator.
Let's say you have a freezer you want to keep cold during a power cut. When the freezer is plugged into the mains, the cabinet of the freezer is earthed via the household wiring system, and so is safe. But plug the freezer into a generator and that earthing is lost. An RCD would protect you if a fault occurs.
Report by Bill Whitley.