Variously called electric bikes, pedelecs and electric-assist bikes, e-bikes combine a fairly conventional bike with a battery and motor that helps out when you're pedalling to make it easier to get up hills, accelerate away from lights and cruise along.

E-bikes are very popular in Europe, especially among older riders who appreciate the speed boost they provide. Theyre taking off in the UK too; retailers that specialise in e-bikes report healthy sales.

An e-bike is not just an electric scooter. To be road legal in the UK, it has to have pedals that can propel the bike, hence the designation "electric-assist". As long as the motor doesn't operate if you're travelling at more than 15.5mph (25km/h) and puts out no more than 250 watts, 14-year-olds and over can ride it without a licence, tax, insurance or helmet.

You may also find "twist-and-go" e-bikes where the motor operates without the rider pedalling. If they came under the 250 watt power limit and the motor couldn't propel the bike over 15.5mph, these were legal as recently as 2015. New rules introduced in January 2016 put their status into serious doubt.

Steve Garidis of peak trade body the Bicycle Association says that even they and the Motorcycle Association aren't clear on the requirements. "It is apparent that the licensing, helmet, and insurance requirements are … not clear cut," he says. We'd advise playing safe and only buying a bike where the engine assists pedalling.

Outside those rules, any electric bike is treated as a motor vehicle for which the usual rules about licensing and insurance apply. That means some of the more outlandishly powerful e-bikes and e-bike concepts out there probably wouldn't be legal in the UK.

In Europe there's another option: S-pedelecs. These higher-speed e-bikes are limited to 45km/h (28mph) and 500 watts. In countries where they are permitted, you need a moped licence, insurance and suitable helmet to ride them on the road, and they must have rear view mirrors. Buzzing long effortlessly at almost 30mph is exhilarating, but it's a thrill that's technically not permitted in the UK. S-pedelecs available in the UK are usually marketed as 'not for use on public roads’ or ‘for off-road use only’.

Why an e-bike?

As Court Rye of US e-bike site ElectricBikeReview.com mentioned in this interview with Treehugger there are as many ways of using an e-bike as there are regular bikes. People turn to e-bikes because they want to go further or faster than their level of fitness makes them comfortable, or they want to haul loads that are difficult on a conventional bike.

For example, one rider I know does his shopping and other gear-hauling on an electric-assist cargo bike, allowing him to live car-free in hilly San Francisco.

Others choose e-bikes because they're great for getting to the office without ending up lathered in sweat or they're simply not as young and fit as they once were but want to stay active.

James Heslop of the Cambridge branch of e-bike specialist electricbikesales.co.uk agrees that the uses of e-bike vary hugely. "You might get two people buying the same bike, but their idea of what they're going to use them for will be very different," he says.

But Heslop adds that there's a big secret that's gets people on e-bikes as well as the practicalities: riding an e-bike is a lot of fun.

It starts with the power kick away from a stop. Instead of struggling to get up to speed, you have the power of a decent club cyclist giving you an extra shove. That gets you to 15mph faster than most cars. It's like having a super-power, except you don't have to wear your underpants on the outside.

Then there's climbing. Instead of toiling your way up hills, you zoom along, those extra 250 watts more than compensating for the increased weight of the bike.

As our Dave Atkinson put it when he reviewed the Koga E-Nova RT, good e-bike could be a genuine alternative to a second car — and a lot more fun to boot.

Let's take a look at your options.

E-bike styles

For the most part, e-bikes are based on either flat-bar hybrids or mountain bikes, and the hybrid style is the most common. Riding positions vary between the slightly stretched out stance of a cross-country mountain bike to sit-up-and-beg positions of bikes based on Dutch town bikes.

The larger wheels and skinnier tyres of a hybrid style e-bike make it a bit nippier, while the wider tyres and smaller wheels of an e-mountain bike are great for round-town pothole-bashing and allow you to venture on to tracks and trails.

Electric mountain bikes are probably the fastest-growing group at the moment, with retailers reporting lots of interest from riders who want help getting uphill so they can zoom down under gravity.

James Heslop says that these bikes are attracting a younger demographic to electric bikes, and are also attractive to older riders who want the versatility of a mountain bike. The boost from the motor overcomes the big disadvantage of a mountain bike on the road, the drag from knobby tyres, he says.

For the most part, though, Heslop says e-bikes are being used round town, and the hybrid style dominates. The European influence and practical intent mean these bikes often have a rack, mudguards and lights too.

We've seen a few electric road bikes at shows, but they were usually concept bikes that hid the battery and motor inside carbon fibre frames to demonstrate that an e-bike could be light and look normal. E-bikes with drop handlebars that you can actually buy are still very rare, but we do fancy a go on Haibike's XDURO Race 28.

Motors

E-bike motors can be built into either of the wheels or the frame. Wheel placement is more common on cheaper e-bikes; from about £1,700 they tend to be built into the frame and so drive the chain directly.

Motor placement is a point of debate among e-bike fans, though the folks at the Pedelecs forum say it doesn't really matter that much and people really should just go back to debating Macs v Windows.

However, one significant difference is that a motor in the frame will be coupled to the cranks. That means the control electronics can include a sensor that detects how hard you're pedalling and meter out the assistance accordingly.

That has advantages and disadvantages. Having your power assisted according to how much you put in can feel more natural, but you might want all the available assistance without having to go full-tilt yourself. James Heslop says another advantage is that the ability of the system to better react to how much power you're putting out means you'll get more range.

Various manufacturers make kits that bike makers then build into bikes. Referred to as the 'eGroup', this includes the motor, battery, control electronics, switches, and display — all the bits you don't find on a conventional bike. Names to look for include Bosch, Panasonic, Yamaha, Sony, Impulse and Shimano.

Batteries & charging

Lithium-ion batteries are everywhere, so it's no surprise to find them powering e-bikes. There are several different lithium-ion chemistries, and some are more energy-dense than others. The details are beyond the scope of this article, but the executive summary is that you get what you pay for: more expensive e-bikes have higher-tech batteries that are lighter for the charge they hold and charge faster.

Battery capacity is measured in watt-hours or amp-hours. Since most e-bikes now have 36V motors, converting the two units just means multiplying by 36: a 10 amp-hour battery is the same as a 360 watt-hour battery. Capacity is most often given in amp-hours, abbreviated Ah.

Capacity is typically around 7.5-10Ah, though some go as high as 18Ah. Many manufacturers offer a battery upgrade option.

Batteries degrade over time, holding less charge as they age. The quality of the battery makes a difference, so look for a reputable named battery manufacturer in the spec, such as Sony, Samsung or Panasonic, and make sure the warranty covers the battery for at least two years.

Lithium-ion batteries are typically rated to last 800 full charge cycles. That's about three years of weekday commuting. They survive longer with careful use; you should get at least 2,000 half-charge cycles, so topping up an e-bike's battery at every opportunity is good practice.

Those are pessimistic estimates though. In practice, a battery life of several years is quite easily attainable.

A full charge typically takes between two and a half and six hours depending on the manufacturer, battery capacity, and battery chemistry. Most people will just charge the battery overnight, though if you have a long commute and are concerned about battery longevity having a second charger at the office is a good idea.

Home on the range

The range — how far an e-bike will go on one charge of the battery — is probably the most important specification. If your commute involves a big hill, for example, you don't want to run out of juice halfway up it. Without power, an e-bike is just a heavy bike, and that's no fun on a steep climb.

A longer range usually means a bigger battery and more cost and weight, so it's not necessarily an unalloyed benefit. If you're only going to do a five mile commute, you don't need a 50-mile range. Nevertheless, you should buy a bike with a higher range than you absolutely need because the range will drop as the battery ages and loses capacity.

The range you get in practice will depend on how much you pedal, the assist level you choose (see below), the terrain you ride and how fast you go. You'll go further on a charge in the flatlands of Cambridge than the hills of Sheffield, but you might have more need for an e-bike in the hills anyway.

ElectricBikeReview's Court Rye reckons a good rule of thumb for range is to divide the battery capacity in watt-hours by 20. That gives you the minimum range if you use full assist all the time, or ride in 'twist-and-go' mode. Dial down the assistance and you'll go further; you might get as much as 40-50 miles from a charge of an 11Ah battery if you're prudent.

Assist levels

You can usually choose how much help the motor provides, typically in three or four increments. More sophisticated e-bikes also have a 'walk' mode that means you don't need to push the bike along yourself, which is especially useful if you need to get it up a ramp.

Choosing your assist level helps control the bike's range, as mentioned above. It also gives you the choice of just how much effort you want to put in. If you're riding for exercise, but just want a bit of help on the hills, or to go further, use the minimum setting. Feeling lazy? Dial it up to maximum and blow serious roadies away on the hills.

Costs

Good e-bikes are not cheap, even compared to good unassisted bikes. You can pick up a very basic machine for £500, but high-performance e-bikes, with light frames, high-capacity batteries and smart control electronics cost over two grand; bikes worth having start at about £1,000.

That's a lot of money. Or is it? A London Zone 1-3 Travelcard costs £1,508 per year, a Bristol City peak Travelpass is £1,848 and a Cambridge Megarider Plus bus ticket is £1,104. Even allowing a budget for maintenance and spares over two years even a high-end e-bike starts to look very competitive.

The low end e-bikes are not the bargains they appear to be either, according to James Heslop, who warns against buying a cheap Chinese e-bike off eBay. "You won't be able to get the parts if anything goes wrong," he says. He adds that reliability of these bikes is often poor. You can end up spending a lot if something does go wrong as mechanics work out the problem with unfamiliar electronics.

As for running costs, the electricity to power your e-bike will cost about 7p per charge. You'll eventually have to replace the battery, and that's the biggest expense of long-term e-bike ownership; they cost between £200 and £600.

Looking after the battery is key to saving money in the long run. Lithium-ion batteries like to be used, and shouldn't be stored empty or fully charged. If you want your battery to last years, treat it well.

Where to buy

Even more than conventional bikes, trying e-bikes before you buy is essential. The way the control electronics put out the power varies a lot between e-bikes, and you'll have your own preference in the resulting feel of the bike.

That means a specialist e-bike dealer is a must, especially given the size of the investment you're about to make. There are now e-bike specialists in most major cities, though they tend to be most common in places that already have lots of cyclists. It's no coincidence that electricbikesales.co.uk's shops are in London, York, Oxford, Cambridge and Bristol.

An e-bike specialist will explain the various bike options available, and impart essential knowledge such as how to care for the battery and use the controls and other options to get the most out of your e-bike.