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Home electricity, home solar, home truths

Chewing over my electricity bill and other energy-related topics led me to do a bit of research into energy usage in the home. Here are some of the nuggets I uncovered and some estimates I calculated on the feasibility of powering my home by solar panels today. This is based on a range of sources and includes plenty of approximations, orthopedist so don’t take it as very accurate. But I was interested in some of my findings. (I’m relating a lot of the figures below to lightbulbs – because personally I can form a clear mental picture of how hot and bright they burn.) (I also give most prices in UK pounds and pence – at the time of writing, UK£1 = US$2, approx)

Energy and power? W and kWh?

Before we get into the details, an initial word about the terms used and the units of measurement.

ENERGY is a measure of an AMOUNT of, er, energy. “The capacity for doing work.” You pay the electricity company for the amount of electrical energy you use, the same way you pay for the amount/volume of any other ‘fuel’ you use. The official scientific (SI) unit of energy is the joule (J) but it’s not a very practical unit when talking about home electricity supply – 1 joule will only light a 100W lightbulb for one hundredth of a second. The unit of energy used in practice is the kilowatt-hour (kWh) – the amount of energy needed to light 10 x 100W lightbulbs for 1 hour.

POWER is a measure of HOW FAST energy is being used. It is usually measured in watts (W, 1W = 1 joule used per second) or kilowatts (kW = 1000W). Example: a 100W lightbulb uses, er, 100W (or 0.1kW).

An illustration of the difference between ENERGY and POWER: A 2400W kettle uses 24 times more POWER than a 100W lightbulb, i.e. uses energy 24 times FASTER. But if you run the kettle for 1 hour and the ligtbulb for 24 hours they will use the same amount of ENERGY.

How much power do electric devices use?

Some rule-of-thumb examples, very approx:

  • 1W – large LCD TV on standby
  • 2W – mobile phone charger
  • 18W – compact fluorescent lightbulb (CFL)
  • 35W – fridge
  • 100W – large LCD TV during use
  • 100W – incandescent 100W lightbulb
  • 200W – desktop PC
  • 500W – washing machine
  • 1000W – iron
  • 1000W – toaster
  • 1000W – microwave
  • 1250W – hairdryer
  • 1250W – dishwasher
  • 2400W – kettle
  • 4400W – tumble dryer

How much does electricity cost?

On UK electricity bills, kilowatt-hours are often referred to simply as “units” of electricity, and tariffs may be given in “price per unit” or “price per kWh” – they’re the same thing. The average UK price per kWh was for the last few years around 10p (20c). (My electricity has just gone up to almost 15p per kWh which makes my sums below less nice, so I’ll use the 10p per kWh figure as a go-by here.)

From this price of 10p per kWh, we get a nice round figure – the cost to light a 100w lightbulb is around 1p per hour! (0.1 x 10p)

If you left the 100W bulb running continuously for 1 year, it would cost you 1p x 24 x 365 = £87.60

How much electrical energy does my home use?

First I googled this and found a page [reference no 1 below] on the Power Consumption of a US Home, with an average power consumption estimate of around 1000W – that’s the equivalent of 10 x 100W lightbulbs burning all day, every day. However, I’m in the UK and hopefully my usage will be less!

I dug out a recent electricity bill estimating my average daily energy consumption as 10kWh, which gives an average power usage of 10000/24 = approx 416W. That’s just 4-and-a-bit 100W bulbs running non-stop! So on a tariff of 10p per kWh, I should be paying around £1 a day. (I seem to be paying rather more, so I’d better chat to the electricity company!)

Could I run my home on solar energy?

So couldn’t I just put some solar panels in my garden and get all the electricity I need for my home? Well, the answer is yes – sort of. (Note we’re talking about ‘photovoltaic’ solar electricity here, rather than using direct sunlight to heat your hot water.)

I found a good page [3] on the web by a guy trying out a home solar energy kit in London. His aim was to generate enough electricity to power ONE energy-efficient 20W CFL lightbulb in his home office. He bought a solar power kit (including a “20W” photovoltaic solar panel, 40Ah lead-acid storage battery and charge controller) for approx £220. The battery is used to store the energy generated during daylight hours and release it as required.) Apparently the “20W” in the panel name is its peak rating at maximum sunlight levels, i.e. not on a grey British December day.

The guy asked the solar panel supplier how much time his 20W lamp might realistically be powered each day given the 20Wp (20 Watts peak) panel in their kit. They replied: “If used every day for the same amount of time [and with] the panel mounted optimally (i.e. due south at 35-40 degrees without shading), you could expect a 20 Watt light to be powered up to about 2.5–3 hours in mid-summer, and up to 1 hour in mid-winter.”

Given this UK example, it seems a fair rule of thumb is that a 20Wp panel exposed to a day of UK winter sun might power a 20W bulb for one hour. This means that to power the bulb continuously for 24 hours you would need 24 x 20Wp panels (plus battery storage). Scaling up, to provide the average 416W that my home uses would require a 9984Wp equivalent solar panel array.

Larger solar kits from the same supplier are built up from 85Wp panels which (including batteries and supporting equipment) work out to about £700 each. Therefore my house needs 9984/85 = 118 panels, costing 118×700 = £82,600 !!!

These panels are 1m x 0.6m, but to allow for tilting, support etc, I’m assuming each panel needs 1 square metre of space. I would therefore need 118 square metres to put them in – filling my entire back garden!

Imagining a 10 year lifespan for the panels, this puts the cost per year at £8,260. The cost per year of equivalent grid electricity is around £365, so solar is certainly not competitive for my home from an immediate economic perspective. There is a push among governments and solar suppliers to achieve “grid parity” by means such as increasing solar panel efficiency – but it hasn’t been achieved in the UK in 2008!

(I’ve just also found a similar estimating process for powering a (sunny) US home by solar panels [4] where they come up with a figure of $32,000/£16,000 and a solar array of 52 square metres – so I reckon my (grey and wintry) UK estimate isn’t too wild.)


I now have a clearer picture of how much electrical energy my home is using. And I don’t think I’ll be trying to run it entirely on solar panels quite yet.


[1] Power Consumption of a US Home

[2] Power Ratings (typical) for Common Appliances

[3] Earth Notes: London Home Solar Power PV Pilot Project

[4] How many solar cells would I need in order to provide all of the electricity that my house needs?

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