Friday, 14 October 2011

Electric Cars? I’ll stick with my bike.

My son may soon need a car for commuting to work, and the question is, is electric viable?

I recently looked up the Nissan Leaf’s statistics, and see that it’s rated at the equivalent of over 99mpg by the epa.  Of course the real question is how many miles per kWh?

The answer appears to be around 3.4 miles / kWh.

My bicycle is 62.5 miles per kWh.

Tuesday, 11 October 2011

Programmable Thermostatic Radiator Valves

Just programming up the new valves before installation next week, and thought I’d do the calculations.  The valves shown on the right aren’t the same, but the price is about the same, and the calculations still apply.

I managed to get two valves with bodies for £53.44, and our gas prices at the moment are £0.03182/kWh, so I need to make savings of 1700 kWh to reach break even point.

The plan is to put them in the lounge on the radiators there, which are 600mmx1400mm (1500 W), and 600mm x 1000mm (1000 W).

Currently the radiators have no thermostat whatsoever, and our heating is on from 6:00 until 8:00 every morning and from 15:00 until 22:00 every evening for the 180 heating days of the year.

That equates to (1.5+ 1.0 kW) * (2 + 7 hours) * 180 or 4050 kWh/year to heat that single room, but our total usage doesn’t seem to be anything near that high when scaled up.

Last year our annual usage was 15200 kWh of gas, of which we averaged 15 kWh/ day for heating water and cooking (or 5500 kWh/year), leaving 9700 for heating.

I’d guess that based on the sizes of the other radiators in the house that the heat used in the lounge is about 1/5 of the total, or 1940 kWh a year.

I propose to limit heating in that room to be only between the hours of 17:00 and 21:30, representing a reduction of 50% on the amount of time heated, generating a maximum savings of  970 kWh/year.

Being very pessimistic (by assuming we get only half that savings), it looks like the devices should pay back in under 4 years.

For comparison, last year our gas consumption in the heating season was 60.73 kWh/degree day. Our target this year for kWh of gas per degree day is now: 55.55 representing a 9.5% reduction in consumption if we are to achieve payback in two years.

Energy Saving Halogen Replacements

When we first moved into our house 15 years ago, we installed halogen lighting in our hall.  Back then, electricity wasn’t quite so expensive, and we didn’t use the room that much anyway, and there weren’t that many options for recessed lighting.

We opted for a mains voltage system that could go on the existing dimmer switch, and therefore didn’t require expensive transformers for each light of the eight downlights.


Yesterday, I changed two of the lights over the table for the new Philips MASTER LEDspot PAR 20 MV dimmable LED lights.  I have to admit, I was ready to be disappointed, however the lights are very good.  Nearly as warm as the old lights, dimmable like the old lights, and fit in the sockets neatly as well.

I bought them from in Cambridge.  I did get a quote from someone else for a discount of about £1.60 per lamp online, but I’d have had to buy 6 to get the discount.

Also on Amazon


The new lights have a slightly wider angle (40 degrees) but that was intentional, as the lighting was a bit too focused before anyway.

The downside?  These are really expensive (£26.34 each including VAT).

So, the break even calculations are:

£26.34 / £0.09 kWh = 293 kWh (amount of energy to break even at current prices)

293kWh / (50W – 7W) = 6813 hours (amount of time lights must be switched on to break even)

The old lamp has a life of 2500 hours, and they currently cost about £5.00 each, so it looks like we’d be replacing two of them during the payback period, so we put that back into the calculation:

(£26.34 - £10.00) / £0.09 kWh = 181 kWh

181 kWh / (50W – 7W) = 3693 hours

Now, we would like to leave these lights on in the evening from about 5pm until 10pm in the winter time, which probably averages out at 2 hours per night over the year.

3693 hours / 2  hours/day / 365 days/year  = 5 years.

That’s the payback for two of  the lights, but the 0ther 6 are more difficult, as they won’t be on nearly as often, probably averaging 20 minutes a day at best.  That makes it a 30 year payback, using the current levels of  lighting.

So the plan at the moment is to use up the stock of existing bulbs as they blow in the existing fixtures, and keep our fingers crossed that the prices come down over the next few years.