Engineers spend a lot of time during the design of a new building predicting how much energy it will consume, banging away on spreadsheets or simulation software and ending up with deceptively precise numbers. The predictions are useful as a like-for-like comparison with buildings of a similar type but the problem comes when you treat the results as a reflection of real energy use in the building.
Simulation results are almost meaningless once the occupant comes on the scene, even in a low-energy house. The Passiv Haus Institute monitored over a hundred houses built to the German Passiv Haus standard (homes so efficient they don’t need conventional heating) and found that mean levels of consumption were almost always higher than predicted, in some cases more than three times higher. And there are many examples of monitoring studies for commercial and domestic buildings with similar results.
Increasingly, UK planning authorities require you to show how you’ll reduce carbon dioxide emissions from your scheme by 10% or more using renewable energy. And the way you prove it to them is by predicting the emissions from the scheme using one of these bits of software. So it follows that the resulting renewable energy systems are usually much too small to succeed.
The only answer is either to build an allowance for inefficient behaviour into the calculations or get people to use buildings more effectively (or both). One way of doing the latter is to provide feedback to occupants on their energy consumption.
Last year, the Environmental Change Institute did a paper for DEFRA on the effectiveness of feedback on energy consumption. They found that direct feedback (e.g. direct display meters, individual cost meters on appliances, interactive feedback via a PC, etc.) typically resulted in energy savings of up to 20%. That’s about the same improvement as is required by the latest review of building regulations.
Turnover of housing stock in the UK runs at less than 2% per year, so significant medium term benefits come not from changes to building regulations but from improving the performance of existing homes. Instead of trying to persuade people to insulate their cavity walls, maybe we should be providing information that leads them to change their own behaviour.
no great surprises there then! what’s interesting of course is the balance between the sincerity of the client and the informed-ness of the planners etc. I have a project where the client is adamant that they want to achieve a zero-carbon development – 5 dwellings, all new build for private sale. They have some experience in low-energy but zero-carbon is an extra step for them.
The building fabric is to achieve passiv-haus standards, and have wood pellet stoves c/w back boiler and solar thermal. The complexity kicks off when you look at the electricity consumption. I offered them three scenarios, UK typical electricity consumption, a m2 rate derived from EcoHomes 2005, and a predicted based on a schedule of usage, including 100% CFL, all A rated appliances etc. Guess what, even the low-energy schedule, the predicted annual consumption was 50% higher than UK average, and 65% higher than the Ecohomes values. The client is sincere adds serious cost to the requirements for either wind/PV compared to the alternative guides for consumption.
The 20% reduction through metering is valuable to know though. Will go and read it during my endless round of coffee-breaks.
N
What were you using for the UK average? This week there was an email conversation going back and forth between Rob and the team about this. He asked us for electricity consumption figures for the average UK house and each of us came back with a different number and a different methodology.
Our man in Brazil came back with 3.3MWh/yr from the EST Solar PV Guide, though he says they don’t cite their source. He also pointed to the England Housing Survey 2001 which gives 5.3MWh/yr.
Jayde came back with figures from the AECB Yearbook, which gives SAP results for an 80m2 semi: 3.35MWh/yr.
I went to the DTI Energy Tables and divided the total UK domestic electrical energy consumption by the number of UK households given in the 2001 census. This results in 4.77MWh/yr.
Pretty wide variation. But an important question is what makes an “average” household? Is it the national average, in which case the DTI figures would be best, or is the national average skewed by a limited number of high consumers?
Maybe we need a separate low carbon blog.
to muddy the waters further, I would agree with RM’s 3300, but I cant recall the source for that either, I expect that I told him at some point in the past.
A report titled “carbon future for european households” by the envrionmental change institute at Oxford uni. also states that the uk average electricity consumption is 4500kwh/yr, but suggests an alternative source of 5300. (table 7.4) for 1996. Of course, there has been significant additional penetration of consumer electronics in this time which have negated any benefit from improved efficiency in white goods etc, tvs etc.
the same report then continues to suggest that of this, onyl c. 60% is for lights and appliances, excluding cooking. I think cooking is c. 850kwh/yr for a 100m2 dwelling (according to NHER pro-rata rates), so what is the remaining 1200kwh for. Maybe additional space and water heating. thats a lot of power to be unnaccounted for.
but if you start from first principles alone, the following annual figures apply for A rated appliances, 8ft3 fridge freezer – 350kwh, washing machine 1.05kwh per wash, say about 4-5 washes per week in my house, so 4.5 x 1.05 x 52 = 245kwh, dishwasher the same. So, allowing for a few tumble drys during the year, thats already 1000kwh for the kitchen appliances alone.
i hate to say it, but the thing I have always hated about using statistics is that they are just that. Often they dont add up to what my common sense or nerdy spreadsheet says… strangely, in the end I prefer my gut reaction to statistic, especially given that they never agree.
anyway, as the dude says, there’s lots of ins and outs…its very complicated man.
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