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Community and Q&A

Sanity check and suggestions for BEopt: PV and minisplit for 1200 sq. ft. two-bedroom new construction

Trevor Chadwick | Posted in Energy Efficiency and Durability on

This is probably going to ramble as I try to organize it..  I’ve been a member on here for several years, and have done a lot of reading, and thinking, now its time to use the info gained.
Having a couple issues wrapping my head around interpreting what BEopt is trying to tell me.
I’m in central NY, (zone 6) planning stages for a 2 story 30×40, first floor FPSF workshop with 10′ ceilings, second floor clear span floor trusses, living quarters 8′ ceilings, 2 br, 1 or 1.5 bath, open kitchen/living area (guess you could call it a great room, but its nothing fancy), raised heel trusses for the roof, 2′ overhang on all 4 sides.

starting plan is:
EPS under/around slab
2×6 walls, zip sheathing taped as the air barrier, reclaimed polyiso, more osb (instead of furring strips), wrb (which one TBD), and vinyl siding (I hate vinyl siding fwiw, but don’t think there are many alternatives in the budget right now)

Was planning on using a mini split to heat/cool the upstairs, the workshop area is probably going to get one too, but may end up with radiant heat (plan on putting pex in the slab as a backup either way)

There is no natural gas available, electricity supplier is NYSEG, basic monthly charge is $17.40 electricity is roughly $0.08 daytime, and 0.07 night.  (last bill was $48.13 for 373 kwh)
PVwats shows the annual solar radiation for my location is 4.4  kWh / m2 / day

BEopt always shows that the biggest PV array I choose as an option gives the lowest costs, and that the more insulation I choose gives the lowest costs.

I’m looking for advice on my ideas so far, and any suggestions as to what I’m doing wrong in BEopt, as somehow I’ve got it ignoring initial cost and just going for the lowest energy use

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  1. GBA Editor
    Martin Holladay | | #1

    Whether or not an investment in PV makes sense depends entirely on whether your local utility offers you a favorable net-metering agreement, as well as on your political assessment of the durability of any utility promises about net metering in the future.

    If you are using energy modeling software that considers the contribution of a PV system, then the lowest purchased energy costs will be achieved (of course) by installing a PV system that supplies 100% of your annual energy needs. Whether or not you can afford to install such a PV system is another question entirely.

  2. Trevor Chadwick | | #2

    I was hoping somone with a little more experience with BEopt than I, would see this and try to let me know what I'm doing wrong with the optimization. As it doesn't appear to be actually optimizing the design, but instead recommending the most insulation, and largest pv array.

    There is no longer a dedicated support forum, and the few videos I've found haven't been helpfull.

    Tried attaching beopt file, but even zipped, it exceeds the 3mb limit on here

    1. Expert Member
      Michael Maines | | #7

      Trevor, I use BeOpt regularly and sometimes answer questions about it here on GBA, but I don't use it in Optimization mode because I don't trust that their cost data will match mine, and I want to compare the effects of different specific upgrades, so I run different Cases for a given design instead. If you want to send me your file, though, I will take a quick look to see if I can see anything obvious. Michael at michaelmaines dot com.

  3. GBA Editor
    Martin Holladay | | #3

    My understanding is that BEopt is a tool for designing a net-zero-energy house. As such, BEopt assumes that every house it designs will have a PV array sized to meet the home's annual electricity needs.

    What BEopt does is inform the designer when further insulation improvements and better windows become more expensive than adding more PV modules. BEopt informs designers when it is time to stop making envelope improvements.

  4. Expert Member
    Dana Dorsett | | #4

    At 7-8 cents retail it's hard to make a financial rationality case for PV until either your rates go up or the cost of PV falls. You can try playing around with online levelized cost calculators or make your own present-value spreadsheet to figure out at what PV price the lifecycle cost beats the utility, and at what price (including subsidies) it might break-even at at term of your choosing shorter that the full lifecyle of a PV system. eg:

    With carefully calculated load numbers in hand you may want to find out if a small ground source heat pump using a company such as Dandelion would be financially rational over mini-splits, or a 2 ton LG Multi V S + Hydro Kit for heating with the radiant slab. To see if Dandelion is working in your location see:

    1. Trevor Chadwick | | #5

      Thanks for the reply. I've been looking for more info on the multi vs, (someone posted a link to it on in a this old house vid on here) but haven't had much luck finding pricing (even msrp) or a much in the way of info (even the lg website is pretty sparse on info).

      1. Expert Member
        Dana Dorsett | | #9

        >"...been looking for more info on the multi vs..."

        Did you see the link to the engineering manual?

        >"...but haven't had much luck finding pricing (even msrp) or a much in the way of info (even the lg website is pretty sparse on info)..."

        This is a fairly new unit, and isn't likely to be sold through internet third parties any time soon (if ever) the way much simpler mini/multi-split systems are. For pricing you'll have to go to the local LG distributor, and possibly a mechanical systems company primarily serving the commercial HVAC sector.

        1. Trevor Chadwick | | #10

          Thanks for the link!!

  5. Trevor Chadwick | | #6

    Any tricks for using beopt ? Or a site that may have some?

    1. GBA Editor
      Martin Holladay | | #8

      My article on the topic is 8 years old, but in case GBA readers haven't seen it, here is the link: "BEopt Software Has Been Released to the Public."

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