How do I compare energy usage between building in different locations?
January 6, 2014 9:58 AM   Subscribe

How do I compare energy usage between building in different locations?

I am involved in a project where I am trying to compare energy usage between similar buildings in different locations. I am using a measure called a degree day, which is equal to the difference between the average daily outside temperature and a 'baseline' temperature of 70 F. The idea is that the more degree days a building has, the more energy required to maintain the internal temperature at 70 F. So the plan is to add up the daily degree days for a building for a month, then divide energy use by that number, to yield a 'Energy use per degree day' variable. This will allow me to make comparisons between buildings in different environments.

There are two issues that I don't know enough about energy use (or physics!) to answer.

First, is the amount of energy required to cool/heat a building constant as temperatures get more extreme? In other words, is the amount of energy required to maintain a building at 70 F at an external temperature of 80 F equal to half the energy required to maintain a building at 70 F with an external temperature of 90 F? Or does the amount of energy required per degree of cooling/heating increase/decrease as you move farther from the 'base' temperature? Obviously if this value is not constant I will not be able to simply sum the degree days per month for a building.

Second, is there any way to account for differences in energy required to heat versus cool a building? I assume (though I am not certain) that different amounts of energy are required to heat and cool (eg 65 to 70 is not the same as 75 to 70). Is there any way to quantify this difference, if it exists, or is this going to vary based on the efficiency of each building's heating and cooling systems?

Many thanks!
posted by btkuhn to Science & Nature (7 answers total) 1 user marked this as a favorite
Can you call the local power company and ask to see historical records? It's certainly worth a shot if your goal here is due diligence.
posted by oceanjesse at 10:05 AM on January 6, 2014

1) Yes. The amount of energy lost to the outside is a linear function of the difference between the exterior and the interior, so assuming the temperature inside stays a steady 70°, 90° exterior should take twice as much energy as 80°. That's actually where we get insulation R-values from (which are in units of ft2·°F·hr/Btu).

2) Heaters and air conditioners are functionally different. I would assume that heaters are very efficient, and most gas fired would be rated in BTUs. For air conditioners, they use SEER ratings to compare their efficiency, and some information on what that means is here. Besides taking more energy, another obvious difference is most places are heated with gas and cooled with electricity. Depending on what you're trying to do, you should take that into account. Are you comparing price? Emissions?

Another thing that you should consider is what we call equipment load and light load. An office building has lots of equipment and lights, which produce heat. So in cold climates you might be heating less due to that factor, or in warm climates, you would be cooling more.

Professional mechanical engineers use software to do all this stuff. Depending on how in depth you want to get, you might talk to one, or ask your power company.
posted by Llamadog-dad at 10:23 AM on January 6, 2014

The heating is proportional to the difference in temperature between the inside and outside (or close enough). Note that heating degree days are normally calculated (but not always) using a baseline of 65F, not 70F, using the assumption that appliances, lighting, and people will provide the additional heat. Depending on the energy sources, you may or may not easily be able to separate out the non-heating energy use.

There are lots of resources out there that deal with this area of building science. Heating degree days is a good search term.

Cooling is more efficient than heating (cooling uses a heat pump, which moves heat from inside to out, while most heating is just done by adding heat directly). This will vary based on the type of equipment and the differences in temperature (energy used for cooling is not proportional to the temperature difference). Cooling will likely be around 2-3 times more efficient than heating. Bear in mind that cooling also needs to remove all the heat from appliances, lighting, people, etc.

Finally, I don't know what your project is, but building should be designed relative to local conditions, so it sometimes makes more sense to compare actual energy usage, rather than adjusting for local conditions.
posted by ssg at 10:30 AM on January 6, 2014

Have you looked at tools like the EPA/Energy Smart's Portfolio Manager, which facilitates comparisons of multiple buildings?
posted by JackBurden at 12:36 PM on January 6, 2014

[ssg:]Cooling will likely be around 2-3 times more efficient than heating.
That would be true only if you're looking at electric resistance heating versus electric air conditioning. If your heating is an electric heat pump, it would be roughly as efficient as an air conditioner. In fact it is probably the same unit running in reverse. (Except that at very cold temperatures the heat pump becomes less efficient and switches to resistance heat.) If you want to compare electric air conditioning versus other kinds of heating like gas or oil, you need to define what you mean by efficiency. Dollars per heating degree day versus dollars per cooling degree day, for example.

[from the OP:] The idea is that the more degree days a building has, the more energy required to maintain the internal temperature at 70 F.
Just to nitpick your wording, buildings don't "have" degree days, they experience them. Degree days are determined by the weather, and every building in a given location is going to experience the same number of degree days. You probably understood that, and your method of measuring energy units per degree day is a valid one. Regarding your second question, I would just do separate analyses of heating and cooling. Comparing the efficiencies of heating versus cooling is kind of pointless.
posted by beagle at 2:04 PM on January 6, 2014

And if one of the buildings is located in a hot-humid or mixed-humid climate, a good bit of the load on the HVAC system will actually come from the need to remove humidity from the air, and not from cooling or heating.

Comparing building performance is a rather large can of worms, and whole books have been written on the subject. To the library!
posted by Monday, stony Monday at 4:24 PM on January 6, 2014

Thanks everyone - this has been very helpful.
posted by btkuhn at 5:27 PM on January 6, 2014

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