measuring and benchmarking your energy use

HarryH_3

I've been reading a few threads here about taking meter readings and trying to estimate what your energy use for a year might be from that.

The best tools for measuring energy use are

http://www.imeasure.org.uk/
and
http://www.thecarbonaccount.com/

but in my opinion imeasure has better feedback.

If you want to be *really* geeky then you should plot your gas energy use (or electric if using electric heating) against how cold it was. This is measured in degree days (http://en.wikipedia.org/wiki/Heating_degree_day) and the data can be found here http://www.eci.ox.ac.uk/research/energy/degreedays.php. Just download your nearest weather station and plot degree days on the x axis and heating units on the y axis. You should get a straight line which hits the y axis at your base gas load (ie non heating, cooking and hot water).

To estimate your yearly energy use, just multiply the gradient of the line by the total degree days in a year and add 52 multiplied by the base energy use (y crossing point). Bob's your uncle!

Harry

mech_2

Yeah. I've plotted that graph. You won't get a straight line though, you'll get a cluster of points around a line. Or if you take measurements often enough, you'll get a "scribble" effect where it looks like a toddler has drawn all over the graph with crayon. This is the decoupling of swings in outside temperature from the gas consumption of the heating system, caused by things like a building's insulation and the buffering effect of solid building materials storing heat.

I you have no south-facing windows you can plot a trend line to get some kind of estimate, but this doesn't work for me without a full year of figures, as my south facing windows provide a significant amount of heat at the beginning and end of the heating season. Ie: In April I use less gas per degree of outdoor temperature than I would in January because there are more hours of daylight in April. I need to do a seasonal adjustment to get the figures to line up. It's not for the faint hearted.

Also you need to keep the heating timer and thermostat at the same settings at all times, otherwise this will also skew the results.

HarryH_3

I've done this for quite a few buildings. Sometimes I get something that looks like a line, sometimes something that looks nothing like it. These are in commercial buildings so it is easier and the demands are more constant, however. I imagine there will be a +ve correlation between degree days and temperature use in houses but it may not be so clear cut. In a commercial building, deviation from a straight line is often taken as a poorly behaving control regime, whereas in a house, it might be because someone had a bath!

using weekly data rather than daily should reduce the thermal inertia/buffering effect though.

as for your windows, this change in internal gains affects the base temperature at which your degree days should be calculated, but you're right, not for the fainthearted!