Microgeneration
There’s been a lot said lately about generating your own power at home and so it’s time to do an analysis. The average home uses about 3,500 kWh per year (purely electricity – add gas and you get to 4,500 to 5,000). One kWh is enough energy to run a device needing one kilowatt (such as a kettle or a microwave) for one hour or a device needing half a kilowatt for two hours and so on. The going rate is £0.15 per kWh.
Wind Turbines
The basic idea is spin a rotor in the wind via a gearbox to an electrical generator. £1,100 (including a 30% grant) gets you a 1,000 watt turbine (A Windsave WS1000) with a rotor length of just over four feet – quite imposing (and noisy) on a mast. They need 20 mph winds to even start and cannot operate with winds over 55 mph. Peak efficiency is usually at 40 mph. For ours, the manufacturer claims it will provide 500 kWh per year.
The average wind speed for most parts of Britain is merely 20 mph and to make matters worse, wind turbines operate at the cube of the wind speed, doubling speed gets you 3x the energy. As 20 is half of 40, most wind turbines in Britain, certainly residential, would operate at a third of their design efficiency over the course of an arbitrary length of time.
Now if we take the manufacturer’s 500 kWh figure (usually a very optimistic sum) and work out what we’d get if it were running constantly at peak efficiency:
8,760 hours in a year
Generation rate of 1 kW
Peak possible of 8,760 kWh.
Actual generation 500 kWh
Efficiency is then 5.7% which is about right for an urban or suburban area (winds are very poor due to buildings causing drag). An exposed hilly area will do substantially better, but shorten the life of the turbine accordingly. The manufacturer says the turbine is good for 10 years, so we’ll use that figure to see how much money we save.
Most people pay an average of £0.15 per kWh, so if we make 5,000 kWh over the life of the unit, we’ve made £750 worth of electricity. However, we paid £1,100 for the unit! The net loss is £350 over ten years, or £35 per year. Put another way, each kWh made by the wind turbine has cost £0.22, or 68% more than the going rate. To say that’s a bad deal is putting it lightly.
A wind turbine is just like adding £35 a year to your energy bill!
Solar Photovoltaic Panels
Solar panels have been around forever, usually living in desk calculators. They’ve gotten a bit better over the years, so let’s take a look at one. For £874 we can get a 150wp panel capable of 9.72 amps in full sunlight (at just over 15 V). An overcast day, but still bright enough for shadows, will net us just under half that, about 4 amps. Of course direct sunlight isn’t always possible (the sun moves!) and the sun is weaker when it is lower in the sky. On a partially cloudy day, many solar panels operate above their maximum rating, since clouds are brighter than blue sky, so light from the clouds and sunlight combines.
Crunching the numbers, however, shows a sad tale. 15 hours a day is too dark for a solar panel to work in Britain. For 40% of the time, we’re clouded out – I’ll assume the panel operates at 25% efficiency to account for light cloud and heavy cloud. The remaining 60% of the time I’ll consider the panel works at peak efficiency.
So, we have 8760 hours in a year, we have to subtract 5,475 of those as too dark, giving us 3,285 hours to work with. 40% of those hours are only 25% effective, so we can divide them by four to simplify the mathematics.
3285 – 1314 = 1971
1971 + (1314/4) = 2,299.5; Call it 2,300.
We have the equivalent of 2,300 hours of full sunlight over the course of a year. It’s a huge overestimate, since it doesn’t at all account for the sun being lower in winter or the panel not being able to track the sun, but it’ll do.
At 10 amps and 15V (15V DC is normal), we are back to our 150 W rating for the panel, which gives us a grand total over a year of 345 kWh. That’s even worse than our wind turbine managed and we used extremely optimistic assumptions. The solar panel would provide us with £51.75 of energy per year, so taking an ungodly seventeen years to pay back what it cost to buy, which isn’t even including price for an inverter (so you can actually use the thing!) or fitting, all of which could very easily bump the cost to beyond that of the wind turbine above. The only saving grace is that a solar panel could well last upwards of 20 years, so produding a profit of about £160 if it is not replaced.
Just for the fun of it, if this panel were in direct sunlight constantly (e.g. on a spacecraft) it would make 1,320 kWh per year or about a third of your total electricity demand for that year. It would also make £197 worth of electricity per year, so paying for itself in just four years and five months. Sadly we are yet to get personal space programmes.
A solar panel is a bit better in terms of price effectiveness than a wind turbine, even in Britain, but only in the very long term. Will you really be using the same out-dated solar panel in twenty years time? You won’t be. If we replace it ten years down the line, we’ve only made 3450 kWh out of it, working out to be £517. The solar panel over ten years has cost £357, representing almost exactly the same as the wind turbine: £35.70 more on your electric bill every year.
Solar Thermal
For £2,000 you can install a system which would provide half of your hot water. You use around a third of your total household energy budget each year for heating water, which works out to be around 4,500 kWh (electricity and gas combined) – hence 1,500 kWh per year is used to heat water.
An indirect solar water heater can provide 40% to 50% of all hot water you’ll need, so we’ll take the 50% figure – 750 kWh per year. In 2008, British Gas customers paid £0.073 per kWh of gas. 750 kWh costs, then, £54.75 (which jives well with the yearly energy that most people use to heat water of £120).
Our solar thermal heater saves us £55 (we’ll round up) per year. Or, put another way, it will take over thirty six years to pay itself back. For our ten year comparison, the solar thermal heater generates 7,500 kWh which certainly improves on the figures for wind and solar electrical, but then the thing cost us twice as much! It saved us £550, but cost us £2000 per year, resulting in a net cost of £1,450 or £145 per year. That’s awful!
You know what else can cut your energy use in half for hot water? A £12 jacket around your hot water tank. That saves the same £55 per year, but pays itself back in under three months! The same calculations done, we’d save 7,500 kWh (which costs £550) and spent just £12, a net SAVING of £53.8
Is There Hope For Us Yet?
The bottom line, then, is that you need something capable of at least 300 watts for at least 6 hours per day (1.8 kWh per day or 660 kWh per year) and costing no more than £500. Does such a thing exist?
No. Unless you can get huge amounts of subsidy (and you can’t, the best is a 30% grant, we’d need closer to 70%) then microgeneration is extremely bad fiscal sense. It isn’t “green” due to the materials used to make these things essentially going to waste, it’s basically setting twenty pound notes on fire.
Until we get systems paying themselves back in the ten year timeframe, microgeneration is a fad. A dangerous waste of resources fad.
In summary:
Method Cost per year (10 years) Wind £35 Photovoltaic £36 Solar Thermal £145 Tank jacket £-54