[Equest-users] Actual Building Energy Cost

[Eric O'Neill]

Thanks David, that's good to hear the level of rigor there was high and expected by the client.

 

I guess my concern here, if I had to summarize it, is that being X% away from the energy bills risks conflating precision for accuracy. Now, if you're 1-2% away on month by month maybe you can start to feel like you're more accurate, but even that still seems potentially too large grained to me. Maybe that's naïve - I certainly don't have the experience you, or many of the people on this board have. I think ultimately, the energy modeler just needs to understand that the limitations of the tool and the uncertainty involved with inputs affect accuracy at least as much as being close to energy bills. Most importantly, that all needs to be communicated and understood by the client so they don't think they are receiving something that they are not.

 

Good discussion, thanks!

 

Eric

 

From: David Bastow [mailto:dbastow at mcclure-engineering.com] 
Sent: Thursday, June 17, 2010 11:20 AM
To: Eric O'Neill; aazhari at jainconsultants.com; equest-users at lists.onebuilding.org
Subject: RE: [Equest-users] Actual Building Energy Cost

 

Of course, your fee reflects the amount of accuracy the client is requiring you to have.  Some clients might only expect 5-20% accuracy, and of course you would provide them with a lower fee.  However, in over 17 years of running hourly, climate history based, energy estimating software, (not always eQUEST), I don't ever remember doing a study where we were satisfied with being more than 5% off on our final base model.  If you are happy with being 25% or more off, then why even do a model, you might as well just guess or do some kind of energy saving hand calculation.  The fun part is getting modeled gas usage and electrical usage, month to month, to follow historical monthly trends and still be close to the annual energy usage on both.  When you get both the annual energy usage and the monthly energy usage of gas and electric close to historical usage, then that's when you feel good about your base model to compare against ECM runs.

 

No one said this was easy.  That's why you earn ever dime of your fee.  Your first preliminary base model run, if you do a good job, might be 5-20% off historical usage.  To even get that close normally requires an extensive survey, documenting ever identifiable energy using item, interviews regarding usage by the occupants and maybe even data logging usage of some important items, to confirm actual usage.  It can, depending on the size and complexity of the modeled building, take several days to a week or more and a hundred or more runs to just create a good solid base model, that reflects historical usage.  That's why modeling a new building, without any historic energy usage data to match, other than approximate BTU's per square foot, is a piece of cake. 

 

David A. Bastow 

McClure Engineering, Inc.  

 

________________________________

From: Eric O'Neill [mailto:elo at MichaelsEngineering.com] 
Sent: Wednesday, June 16, 2010 3:52 PM
To: David Bastow; aazhari at jainconsultants.com; equest-users at lists.onebuilding.org
Subject: RE: [Equest-users] Actual Building Energy Cost

How much effort was put into minimizing the error of your inputs to justify that level of accuracy in your output? For example, did you convert real weather data for the two years and use an average of the simulations? Were you modeling small buildings so you could get a fairly reasonable infiltration rate empirically? Were occupancy schedules trended, and for how long? Were all the systems and controls working correctly, with all sensors calibrated regularly? 

 

1 to 2% seems to me to be fairly unreasonable. Unless you do an amazing job verifying your inputs, in my opinion that level of precision doesn't get you a better model. If you set everything up as best you can and it comes in there, great! But a correctly set up model can be off by over 2% because of a couple "El Nino" years, a facilities guy locking a humidity high limit to 50% for a summer, or any number of operational factors. On the other side of it, we've tried to match a simple DOE2.2 model to a DOE2.1e model with limited success (I don't think we got within 3%, although we didn't spend too much time with it). Who knows what the difference would be with E+.

 

I know I always squirm when I'm asked to do existing building models, and it may be irrational. Heck, if the DOE is asking for 1-2%, I probably am being irrational. But it seems to me that the error from assumptions could easily swing a model 1-2% (what would a 30% error on your infiltration do to an otherwise correct building?). What are other people's thoughts? 

 

Eric

 

From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of David Bastow
Sent: Wednesday, June 16, 2010 3:48 PM
To: aazhari at jainconsultants.com; equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Actual Building Energy Cost

 

On all the DOE modeling projects that we have done, of an existing building, they have required the model to be within 1 to 2% or less of the actual energy usage, based on an average two year history.  Is that what you are talking about?

 

David A. Bastow 

McClure Engineering, Inc.  

 

 

________________________________

From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Ahmed Azhari
Sent: Wednesday, June 16, 2010 12:10 PM
To: equest-users at lists.onebuilding.org
Subject: [Equest-users] Actual Building Energy Cost

Hi all, 

 

Does anyone have a ballpark percentage of the actual annual energy cost versus the modeled annual energy cost for a building?  

 

Thanks,

__________________________________________________________________________

Ahmed Azhari, B.Eng., LEED® AP

Energy Analyst

 

Jain Sustainability Consultants Inc.

 

2260 Argentia Road, 2nd Floor

Mississauga, Ontario L5N 6H7, CANADA

Tel:  (905) 542 7211 Ext 234

Fax: (905) 542 7622

Email: aazhari at jainconsultants.com <mailto:aazhari at jainconsultants.com> 

Web: www.jainassoc.com <http://www.jainassoc.com/> 

 

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