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- Cheaper to fit them in a new roof than on top of one

How much photovoltaic is needed to run an electric car?

I like all technology, but often prefer the tried and tested stuff over the bleeding-edge stuff. I am also aware that much technology is misplaced, over used, and often, pointless.

But having said that, transport is one area that technology has been well used, Personal transport has been a huge global success and even allowing for its shortcomings, is destined to grow globally.

One of the main shortcomings is the pollution produced by combustion engines. Without going into too much detail here, when a hydrocarbon fuel is burnt, be that wood or diesel, some of the carbon atoms in the fuel join up with some of the oxygen atoms in the air and create carbon dioxide. This is a problem as carbon dioxide allows very short wavelenghts to pass though it, but absorbs longer wavelenghts, which causes the carbon dioxide molecule to spin or vibrate faster, and as we all know from physics, if you move, or accelerate a mass, you create heat in a system.

Getting back to electric vehicles, if we could run them off electricity that is created solely from a non-combustion technology, some of the problems of vehicle ownership are alleviated.

Realistically the only technology to generate electricity on a small scale is photovoltaics. These panels, and the associated kit needed to condition the power they produce are easily available, relatively cheap for renewable technology, are silent in operation and can be fitted on a house roof.

So how much photovoltaic is needed to run an electric car?

There is no perfect answer to this as it depends on many things, not least the location of the panels, the type of car used, how much sunlight there is and how many miles you need to travel in the car.

Having stated that, it is possible to calculate how much energy is needed to charge a battery and how far an EV can travel on that charge.

If I create a ‘cartoon’ car that uses 200Wh to travel 1 mile, and the charging efficiency is 90%, then a total of 220Wh/mile is needed.

Many may be wondering what a Wh is. It is a watt hour. This is really just the energy that is stored in the battery. People my be more familiar with the kWh, which is what we pay about 15p for from our electricity supplier, before taxes and daily meter rental. Without going into detail again, a watt, the capital W in Wh, is a joule per second. A joule is the force needed to move a 1 kilogram mass, 1 meter. This is why, generally speaking, the lighter a vehicles is, the less fuel it uses. A kilogram of gasoline has around 13 kWh of energy in it. At £1.20/lire is is cheaper than electricity by about 6p/kWh.

So how much photovoltaics are needed to produce 220 Wh of electricity?

The United Kingdom has an average annual solar irradiance of around 950 kWh per meter squared. So there is really no lack of sunlight in the UK if you could capture all of it, your EV could travel 4300 miles a year on that. Sadly you cannot capture all of it, we can only realistically get about 15% of it, so that means 650 miles a year for every 1 meter squared of photovoltaic installed.

This sounds pretty good, if you travel 12,000 miles a year, you would only need about 20 metres squared of panels on your roof.

Unfortunately, as the UK, and most of the developed world, is not between the Tropics of Cancer and Capricorn, we get seasons. These season give us long hours of daylight, with high sun angles, in the summer, and short hours of daylight, with low angles of sunlight in the winter. This means that we cannot produce electricity from sunlight in the winter as well as we can in the summer.

With an absolute resource difference of between 156 kWh/ metre squared in May (the best month) and 35 kWh/metre squared in the December (the worse month) of insolation, we have to design any system to cope with the winter output. So instead of just 20 metre squared to travel an average 1,000 miles a month, we would need 42 metre squared of panels. Still possible on a lot of houses as that is around a 6 kW system, though you do need special permission to install above a nominal 4 kW system in the UK.

Th one big drawback, unless you work and travel only at night time is that your car will be away from your house during the hours of daylight. To get around that, you would need a battery storage system at home. That would mean another set of losses to charge that battery system, and then charge your car. Being generous, and not having seen real world figures I would estimate that you would need another 15% of panel area. So that would be 48 metre squared, which is around 30 panels and an 8.5 kW installed capacity system. If you fitted that all on one side of a south facing house, then the roof area would need to be, 31 metre squared. Which is quite large, your house would be about 16 metres wide by about 7 metres deep. Which is almost 4 times the size of mine. I have a 20 mile commute for work, and Cornwall gets a bit more insolation than the rest of the UK, so I may be able to do it, if I had an extra £12,000 to spend on a photovoltaic and battery storage system, after purchasing a new car. Oh, and an extra £300,000 to buy a larger house.

The up side is that, for the rest of the year, you can heat your water and run your fridge for nothing.

Could work out expensive

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