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How to measure your energy consumption

Posted by greentomatoenergy . on 6 November 2012 at 2:19 pm

Most householders are made aware of the energy efficiency of their home using energy performance certificate (EPC) ratings. While EPC ratings are relatively simple and are useful for providing high level information, they are not very transparent, not possible to measure without outside help and don’t provide enough information to make decisions on what improvements to make.

At greentomatoenergy, we’re regularly asked to measure energy consumption more accurately, so that householders can measure the effect of the measures they are taking and so that they have a better idea of orders of magnitude. 

For many of our clients intending to renovate their properties, we build a PHPP (PassivHaus Planning Package) model and use PassivHaus principles to help clients to decide what measures to adopt and how far down the low-energy route they wish to go.

The beauty of this approach is that PHPP is simple to understand, the assumptions are easy to see and clients are able to review the effect of a number of different scenarios on the energy consumption of their properties (and therefore their energy bills).

We’re often asked by clients how they can measure the energy consumption by kWh/sq.m/year themselves.

The short answer is to divide up a year's worth of space heating energy use by the measured floor area of your house…

For the detailed calculations, we assume PassivHaus conventions.

First, you need to be clear about the difference between delivered energy (useful if you're only interested in how well insulated the building is) and primary energy (which also takes into account how efficiently the heat is generated and is relevant to climate change).

PassivHaus uses delivered energy for space heating (15 kWh/sq.m/year) and also has a limit on how much primary energy can be used in total to heat and service the building (120 kWh/sq.m/year). This means if you want your number to be comparable with the PH convention you would need to multiply your gas consumption in kWh by the efficiency of your boiler (about 0.6 for non condensing and 0.9 for condensing) to find out how much heat it has actually delivered to the building.

Floor Area

Finding the floor area is easy, but boring -- Exclude the area of stairwells, internal walls, cupboards, plant rooms, doorways. -- Exclude areas where the room height is less than 1m  -- multiply areas where the room height is between 1m and 2m by 0.5

Specific Primary Energy Demand (PH = 120 kWh/sq.m/year)

The first step would be to make good use of whatever meters your building has -- there's a very good website called imeasure.org.uk which allows you to input your meter readings at irregular intervals and it will attempt to estimate your annual energy consumption, it also takes the floor area and outputs the data as primary energy consumption in kWh/sq.m/year. If you add the two numbers for electricity and gas together you can compare that to the 120 kWh/sq.m/year part of the PH standard. Normal range is 300 - 800 kWh/sq.m/year

Specific Heating Demand (PH = 15 kWh/sq.m/year)

Finding heating energy is a bit more involved, because most people don't meter their space heating separately from other end uses, and because people fairly often use wood or oil for heat which can be estimated, but lends a degree of uncertainty. Assuming you heat with gas…

Take the gas portion of your consumption and multiply it by the efficiency of your boiler to get the delivered energy (this will be less than the primary energy consumed because some of it is lost as hot exhaust gasses).

The next step is to work out what proportion of that is space heating by subtracting hot water, cooking etc. that is also supplied by the same meter that supplies your heating energy.  As a general rule of thumb, if you have gas heating and hot water then hot water accounts for about 10% of consumption. More accurately it's about 600 kWh/year plus 400kWh/year/person, where children are counted as about half a person. 

According to DECC one should allow about 3% of total energy consumption for cooking.

If you also have a stove you'll need to make an estimate of how much energy came from wood/coal and add that to the space heating.

At the end of all that you should know your delivered space heating energy in kWh/sq.m/year which you can compare with the PH 15 kWh/sq.m/year figure, bearing in mind that PH assumes you heat to 20C all the time. Answers for 'normal' buildings should be in the range 100 - 300 kWh/sq.m/year.

If you’re at the upper end of this range, it might be time to consider improvements.

Alexander Rice, senior engineer

Photo by southend in transition


About the author: greentomatoenergy specialises in cost-effective renewable technologies and low carbon building.

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