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BEOpt output

jcofvienna | Posted in General Questions on

Any BEOpt practitioners out there?  


I’m having a difficult time understanding the output of an optimization run I ran recently for a design I’ve been working on.  The optimization run mostly evaluated insulation package options (ceiling, wall cavity, continuous exterior, sub slab, exposed floor, etc.) and air change rates.  The HVAC and water heating were fixed to one selection for each.  The design is for all electric utilities.  My run has two output points of interest that I’ll use for discussion purposes.


Point A: The First Selected option, which is a minimal insulation option.  In the Cost/Energy graph this point has Energy Related Costs Annualized equal to $1628.  In the End Use graph this point has Site Electricity Use of 10,800 kWh/yr.


Point B: Is an increased insulation, tighter envelope option.  In the Cost/Energy graph this point has Energy Related Costs Annualized equal to $1375 and Site Energy Savings per year equal to 8.09%.  In the End Use graph this point has Site Electricity Use of 9927 kWh/yr.


I’ve read the BEOpt help file for the Outputs and some other blog postings I could find online.  My general understanding is the annualized energy related costs are the utility costs plus the differential cash flows from the reference point or first selected option.


I think I understand the output of Point A.  Since there are no differential cash flows—the point compared to itself is the same, so the net is zero—the energy related costs would be the cost of the electricity used.  This seems to check out.  In the inputs I defined my electricity costs as $11.70 fixed cost per month, and a marginal rate of $0.1381 per kWh.  That gives me:

Cost =( 12)*(11.70) + (0.1381)*(10800) = $1632, which I assume is within rounding error of the Cost/Energy output value of $1628.


I’m not sure what to make of Point B, though.  The cost of the annual electricity use (9927 kWh/yr) is $1511 per the same equation above.  To get to annualized costs indicated by the Cost/Energy graph, there would need to be some kind of differential cash flow great enough to close the gap between 1511 and 1375 and also offset the differential cash flows associated with costs for the added insulation and air sealing.  


What would that be?  The only thing I can think is the possibility of replacement costs for the HVAC system that are higher with the minimal insulation package than they would be with the higher performance insulation and air sealing package.  Does that make sense?  Does anybody have a better idea?


Perhaps I fundamentally misunderstand the output values.  Any clarifications would be much appreciated.  

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  1. walta100 | | #1

    I have to ask did you watch all the training videos. I was very confused but it all came together for me at the end of the third video.

    I attached a BEopt graph the best options are close to the bottom.

  2. walta100 | | #2

    When you look at the output I posted each dot is a variation on a model of a house. The dots on the line drawn on the graph are all optimal models in some way. The one on the left side of the line have lower capital cost and higher energy usage costs. The ones on the right side of the line have higher capital cost and lower energy usage costs. The ones closest to the bottom are the modes with the best return on investment given you inputs.


    1. jcofvienna | | #3

      Thanks for the replies. I actually had not watched the tutorial video, so I did that last night. While I did gain a number of nuggets, the video didn't fundamentally change my understanding of the outputs--the way they explained it is basically the way I understood it from reading the help file. My math problem above is still a head scratcher. Appreciate the suggestion to watch the video.

  3. walta100 | | #4

    I am not sure I fully understand you question.

    The “Annualized Energy related cost per year” includes a lot of factors that will not be obvious at first glance.

    The first one that comes to mind is your mortgage interest rate BEopt is financing the cost differential between the insulation installed cost over the length of the mortgage. Note the mortgage rate is one of your input numbers.

    Another thing BEopt does it has an inflation assumption so over time fuel cost are modeled to go up over time. Note the inflation rate is one of your input numbers.

    Most of the inputs have life span in years and their replacement costs are automatically enter and added to Annualized Energy related cost per year number. The best example is a heat pump has a shorter life span than an AC with a furnace. Note you can adjust this number if needed.

    No I do not think BEopt changes the life span of HVAC equipment based its BTU output number but it will change if compare a furnace to a heat pump.


  4. Jon_R | | #5

    I think the math would be clearer if you used the net present value for each option. But this isn't very useful for someone trying to put together a monthly budget.

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