Passivhaus total energy usage standard
WildBunchFarm
| Posted in PassivHaus on
One of the Passivhaus standards is that total energy usage including appliances cannot exceed 11.1 kwh per sq. ft. per year. If I have a 2800 square foot house, then if my total kwh usage is under 31,080 (11.1 x 2800) that is pretty good right?
Seems like that’s a pretty low bar. Or am I misinterpreting?
Thanks,
JImmy
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Replies
Jimmy,
The Passivhaus target you cite is a primary energy (source energy) target.
The usual factor for converting site energy (electricity) into source energy is 3.34. (In other words, 1 kwh of site energy = 3.34 kWh of source energy.)
In your case, 30,080 kWh of site energy = 100,467 kWh of source energy.
For more information on the difference between site energy and source energy, see Understanding Energy Units.
Yikes. So I need to get it down to 9305 kwh/year to reach Passivhaus standard. Now I see why that's so tough.
The Passive House energy modeling program, the PHPP, uses a primary energy factor of 2.6 for grid-derived electricity, and a factor of 1.1 for fossil fuels, likely reflecting the more efficient distribution systems in Europe, compared to North America. The PHPP also measures square footage differently than we typically do in the US. Between the two, you may not be as far off as you think, but it is definitely a difficult (but often worthwhile) target to reach.
Thanks Martin and Michael.
How do people usually factor in an unfinished basement? I have a 1800 square foot unfinished basement that can still be conditioned via two ductless minisplits. I am predicting that after everything is done, I will have a total primary energy usage (all grid electricity) of 18,840 kwh.
If you only count my current living area (2800 sq. ft.) then I would be very short of Passivhaus standards. (2800 sq ft. x 11.1 / 2.6) = 11954 kwh.
However, if you add in the quasi-living area that is my basement then I would have reached the target. (4600 sq ft. x 11.1 / 2.6) = 19639 kwh.
Jimmy,
For purposes of calculating the treated floor area (a PHPP input), use 60% of the basement floor area rather than 100% of the basement floor area.
For more information, see In Defense of the Passive House Standard.
Jimmy, to calculate the TFA, in addition to Martin's advice, you also don't count stairways with more than three treads, and you measure to the interior surface of all walls, interior and exterior--sometimes called the "carpeted area." Utility areas other than basements can be reduced in some cases as well, if they're not used much.
Thanks Michael Maines. Does the standard account for the number of people living in the home?
Martin,
To follow the theme of this thread, am I calculating my total source energy use correctly?
I designed/built my house to be all electric (ie mini split HVAC, tankless hot water, well, septic, plug in electric mower, etc) so that should encompass my total energy usage.
Based on my electric provider I used 7,704 KwHs in site energy in the last 12 months, multiply that by the 3.34 (source energy) you mentioned gives us 25,731.36 KwH of source energy used.
The house is 2 floors 30x42 exterior (with ICF walls of approx 13.5 total thickness drywall to stucco) for approx 28x40 interior which is 1176 *2 or 2352 sq ft total. 2352 * 11.1 KwH = 26,107.2 of total source energy use.
Did I really meet that part of the passivhaus standard? Or is my math wrong?
Either way, I'd like to thank you and all the other contributors for the tremendous amount of knowledge here, it certainly shaped how I built for the better and saved me money. Its difficult to calculate the total cost since I provided some labor and material, at around $270k including the land seems to put the final cost at a pretty decent $115 per sq ft.
Passivhaus measured "treated floor area" of the house quite differen from the way real estate is normally measured, and it will be a smaller number than stated. That will result in a higher energy/square foot ratio (both peak and average.)
In the US all area inside the exterior paint counts. Passivhaus only counts interior floor space, not the thickness of any walls (exterior or partition walls) and not ALL interior floor space- stairwells don't count as floor area, rooms between 1m & 2m in height only count 50%, basements only count 60% unless the window area is at least 10% of the floor area- there's a whole list of stipulations and fractional factors to come up with for the floor area number, most of which I don't remember.
Peak heating has to be less than 10 watts per square meter (square meters measured per Passivehaus "treated floor area" methods), which is harder to meet in cooler US climates than in more temperate western Europe. There are also minimum & maximum room temperatures that need to be met, etc.
The site's source energy factor is also not a single number such as 3.34 x kwh == source energy. The hourly & seasonal load profile of the house and the regional grid source supplies are now factored in:
https://passipedia.org/certification/passive_house_categories/per
You might still make it on total source energy use or peak energy use, but the math isn't super simple.
Dana,
Your reply echoes many of the points I made about an hour ago -- but you managed to post your response above mine in the queue!
Andrew,
Your math looks good to me. The squishy parts of the equation of the site-to-source factor and the calculation for treated floor area.
If you use the PHPP formula for converting site energy to source energy, you get 2.6 * 7704 = 20,030 kWh of source energy used at your house for the year -- well under the Passivhaus target. Of course, the actual site-to-source factor depends on the energy sources used by your local electric utility -- you could do more research into the matter if you cared to be precise.
The treated floor area calculation used by Passivhaus excludes stairways and partitions, so the total number of square feet in your house (according to TFA) might be less than you think. If you reduce the number of square feet in your house -- for example, from 2352 to 2200 -- that reduces the Passivhaus target you're aiming to reach from 26,107 to 24,420. Depending on your site-to-source factor, you're probably very close to this target -- on one side or the other. So, congratulations. Well done.
If you want to write a guest blog about your project and your accomplishment, feel free to contact me by email: martin [at] greenbuildingadvisor [dot] com.
Martin, Dana,
I really appreciate the responses, and I don't think my college math teachers would boast much confidence in me performing those more complex calculations. When building the house I didn't think it was worth the expense to pursue any certifications or testing, so I've just been curious if the house made it into the "pretty good" zone or not.
I'll email you in a bit about what you'd like to see in an article, but my goal would be to give other average folks the confidence that they can achieve some of what we see on the site.
Andrew,
I'm looking forward to your email! Sounds good.