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Sizing solar PV for new construction

Bruce Davis | Posted in General Questions on
New construction, cold edge of Zone 4.  Planning on PV solar system.  Problem is sizing the system.  Had three proposals from contractors recommending 10k, 12.5k & 15k systems respectively.  I’ve found a couple of rough sizing calculators online recommending 10.2k & 11.5k.  Obviously, sizing an existing house based on electric bills is easy.
My wife & I (two total residents) currently live in two 100 year old homes next to each other with poor energy efficiency.  The main house is 2000 sq ft, has a 9k PV system and uses an average of 910 kWh per month (sized about right).  The second house, used as an office & guest house is about 1000 sq ft, has a 5k PV system and uses an average of 348 kWh per month (we knew it would be oversized upfront).  We’re minimal energy (lights, heat, cool) users.
The new house will be 4300 finished sq ft.  2300 sq ft on main level where we’ll spend 90% of our time.  The lower level walkout (reverse 1.5 story) is about 2000 sq ft.  Each level will have its own HVAC.  The LL HVAC can be set at minimums most of the time as is our current guest house.  Gas heat, probably gas water.  Heated electric bathroom floor and maybe one car width of driveway.  Maybe an EV car one day.  Insulation will be average to above average–R20 walls, R-49 ceiling.  Zip sheathing.  Electricity cost about $0.135 kWh.  Solar system cost after tax & rebate incentives for 10-12k system is $1.74 per watt.  For 12-14k systems, about $1.67 per watt.
Downside to starting small and add more later is tax incentives go down after 2019 and power company rebate goes away after installation.  Like to get it right the first time.  I’m sensing 10-12k is where I want to be, which is about a $2600 spread.  Any advice based on your personal experience or suggestions for online calculators would be much appreciated.  Thanks.

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Replies

  1. T. Barker | | #1

    There is no way anyone can properly calculate the size of solar system you need with the information provided.

    My recommendation is that you build the 15 KW system.

  2. Jon R | | #2

    The answer is highly dependent on your net metering policies (current and future). And your goals (best payback, least environmental damage, etc).

  3. Bruce Davis | | #3

    Net metering is monthly, a poor alternative to annual. I pay retail, $0.135 but the electric company only pays a penny or two wholesale for overproduction. The worst of both worlds.

    Primary goal would be financial (nice when that goal coincides with green goals).

  4. GBA Editor
    Martin Holladay | | #4

    Bruce,
    If you have a lousy net-metering contract, with the meter set to zero every month and miserly reimbursement for overproduction, you should choose a very small PV system. Otherwise, you'll be tearing your hair out in frustration from March to October, as you see that how many extra kWh your PV system is producing -- extra kWh that you won't be getting much credit for.

    Or you could buy an electric car.

    1. PierreMarteau | | #45

      yes, and heat pumps, including water heater.

      1. TrackerBob | | #48

        When you building new panels, I would also add street lighting powered by solar. Maybe this https://websolarguide.com/solar-lights-for-gate-entrance/ information will useful for you

  5. Norman Bunn | | #5

    You could run BeOpt on the new home and/or get a HERS rater to analyze your new home. I did both and they more or less corroborated each other at around 16,000 KWH annually. Then I took my shoulder season low KWH month from my current home, thinking this would give me a no HVAC load that would be similar in the new home; there are some variables (HPWH, DWHR, gas dryer, more efficient appliances, etc.), but I am just looking for an estimate. Then from the HERS and BeOpt output I got the expected heating and cooling KWH and added in the annuallized no-HVAC load to get an estimate of what my annual usage might be for my PV array sizing.

    I have access to a great net metering plan (full retail offset, calculated annually), so I can oversize without significant impact. With yours, I agree with Martin, undersize.

  6. Stephen Sheehy | | #6

    Since heat and hot water are probably your biggest energy users, all of your proposed options are oversized. I'd think about an electric heat pump water heater and minisplits instead of gas if you really plan on 10-15kw of pv. As Martin suggested, an EV could use up your summer surplus.

    1. PierreMarteau | | #46

      👍

  7. T. Barker | | #7

    I disagree that his options are oversized.

    If his goal is financial, for the relatively incremental cost to build 15 KW instead of 10KW, it is much more likely he can avoid most electricity purchases, to some degree even in the worst solar season. Total financial benefit will be optimized, although over a longer period. Excess generated during the best solar seasons - who cares. It's not "wasted" any more than it's being wasted right now without any PV panels.

    Right off the bat, under normal assumptions that size house would require at least a 12 KW system. Combined with the fact he said they may buy an electric car, and they want to heat 1/2 their driveway!, is why I said to build the biggest system possible. Never the less, there is no way to have a rational discussion until we're provided with significantly more details.

  8. Expert Member
    Dana Dorsett | | #8

    What Norman Bunn said- simulate the energy use of the house with a tool such as BeOpt.

    On the cool edge of zone 4 it's often marginally cheaper (and usually greener) to heat with air source heat pumps than gas. With 13-14 cent electricity that's probably going to be the case unless you have exceptionally low natural gas rates. Going all-electric also saves you the up front cost of connecting to the gas grid.

    Even if you don't already own the EV, install the wiring for a car charger, which will be cheaper now than doing it later, possibly more than enough to compensate for the difference in up-front cost for the heat pump relative to a cheap gas furnace.

    I suspect T. Barker is right- go with 15kw and all electric on the mechanicals would likely be the best long term bet, even if net metering rules change. Smart car chargers and battery systems are getting cheaper every day- plan for a future that includes them.

  9. Brad | | #9

    Without knowing where Bruce lives (did I miss it?) , these recommendations for 10-15 kw panels, for a home using gas heat leaves me baffled. It should be easy to get under 400 kwh/mo for electric including hot water. I'm in zone 5, all electric, net zero, with 4.4 kw. 2200 sqft house, sunny CO.
    Heating a driveway???

    1. T. Barker | | #10

      He's building a 4,300 sq.ft. home. Two HVAC systems (at least two fan motors), average insulation, no talk of minisplits, etc.. Probably 4-6 tons of A/C unit(s), drawing roughly 3-4 KW of electrical (if he buys an expensive one). This gets better the more I think about it. Maybe even two HRV's (two more fans)? Not to mention the driveway, lol.

      Bruce currently uses 1,258 kwhr./mo. If you have several computers, big screen TV's, all the modern amenities, I defy MOST people to get under 1,000 kwhr./mo. even in a house 1/2 the size Bruce is going to build and no electric car and no heated driveway. It's great that you did it, but you also built a Net Zero house and absolutely minimized every single energy source. It's just not practical for most people in my opinion.

      1. T. Barker | | #11

        In fact if I'm not mistaken, the Passive Haus standard for a 2,200 sq.ft. house would be 1,000 kwhr./mo. total electrical, including heating and cooling.

        Can you give us some more details about how you got down to 400 kwhr./mo.?!

        1. Calum Wilde | | #12

          I'm in zone 5, have a 2200ish sqft house and my usage for four people averaged 432 kWh this summer. It'll likely be in the 1000+ kWh range come winter though. 4400 ish (I'm not sure how much 4.4kW produces where he is) seems fantastic and a little absurd to be as well.

    2. Calum Wilde | | #13

      Yeah that has me wondering if this is a troll. He says "We’re minimal energy users." then says he's planning to heat his driveway. I don't get it

      Then again, I also don't get how your energy usage is that low.

      1. Expert Member
        Dana Dorsett | | #17

        If the driveway heat is only applied for snow melting right after a storm, in a zone 4 location that's barely going to move the needle on annual electricity use, unless it's an exceptionally LONG driveway.

        The annual use of that load could be better estimated if we knew the total square feet, the actual location (to estimate average snow density- it's usually lighter in zone 4B than in 4A or 4C) , and the annual snowfall numbers.

  10. Brad | | #14

    My average consumption for 8 non heating months is 299 kwh (including 2 shoulder months with a small amount of htg), average for 4 heating months is 439 kwh. Note that my non heating usage is comparable to Calum's except he has 4 people, only 2 here (Bruce has 2 also). We didn't do anything very expensive to get there, LED lights, induction cooktop, heat pump water htr, 3 ton heat pump, new appliances (but nothing expensive). The home is built to 2016 energy code plus a little extra R in the attic. Passive solar probably provides about half of our heat.

    T Barker, we have several computers, a big screen TV, all the modern amenities including flush toilets!

    We use night time ventilation to cool the house down, and didn't need any AC this summer. Cooling would add some consumption for someone in a hotter climate with warmer nights. We have most days in the 80's and a few in the 90's, but cool nights. Proper overhangs keep the sun out of our south windows.

    The PV watts prediction was 7969 kwh/yr, actual for the last 12 mo was 8206.

    1. Calum Wilde | | #15

      "The PV watts prediction was 7969 kwh/yr, actual for the last 12 mo was 8206."

      There it is. 4.4kW of solar here would make about 4.4kWh/year, give or take a small margin. If your 4.4kW of solar is making almost double that in kWh than of course 15kWh of solar is going to seem off.

      I'm just finishing some things on my house - installed the DWHR this weekend, still need to balance my HRV and I'm looking for a deal on rigid foam for my uninsulated slab - and we used the AC pretty heavily this year as 80C is too hot for us. If it wasn't for the AC use this year during the hottest summer on record we likely would have been in a similar range.

      I'm expecting our annual energy usage to be in the 8000kWh range when I'm done.

      1. T. Barker | | #18

        8,000 kwhr./yr is still aggressive, but I believe that can be done and live like a modern human being.

        But 4,148 kwhr./yr can't be achieved by most people. I mean, 299 kwhr./mo. in cooling season - that's less than 10 kwhr./day! Obviously no A/C, maybe only 1 fridge and 1 hot cooked meal a day and no toast for the kids lol, no wood working saws in the garage, and no charging the electric trolling motor for the fishing boat.

  11. GBA Editor
    Martin Holladay | | #16

    Calum,
    80°C = 176°F.
    That's too hot for me, too.

    1. Deleted | | #25

      Deleted

  12. T. Barker | | #19

    I need to go do some homework and run average consumption numbers with all new appliance ratings and new mechanicals. Maybe I'm out of date lol.

    I still can't believe it though. Minisplits, HRVs, these things all use a lot of power in average to harsh climates. The lights just in my kitchen use 0.5 kwhr./day (7 pots, 15W LED each, 5 hr./day with meals, kids homework, etc.). And that's just the lights.

  13. GBA Editor
    Martin Holladay | | #20

    Monitored small houses in Massachusetts ("pretty good houses"), all electric, used an average of 755 kWh per month. Read about it here: "It’s Not About Space Heating."

    1. Expert Member
      Dana Dorsett | | #21

      Note, that if the domestic hot water were being served up by a heat pump water heater testing at EF 3 or better rather than that standard electric tank it would again become at least kind-of "...About Space Heating", looking at Marc's pie chart:

      https://beta.greenbuildingadvisor.com/app/uploads/sites/default/files/images/Rosenbaum_-_Graph_of_Eliakims_Way_data_copy.preview.jpg

      The plug loads lights would still dominate, but the water heating would shrink by 2/3. The Heating/Cooling would edge up a bit in a heating dominated climate, since the additional heat load imposed by the water heater is over a longer period and larger than the cooling savings reaped during the shorter cooling season.

      The variations in energy use of the "Lights, plugs, appliance" slice is highly variable, much of it dependent on occupant behavior.

      1. GBA Editor
        Martin Holladay | | #22

        Dana,
        Needless to say, Marc Rosenbaum would agree with you -- which is why the article notes, "To reduce energy use in these houses, the next step would probably be to install a heat-pump water heater."

        1. Expert Member
          Dana Dorsett | | #24

          ...and you'll recall that in fact Marc did retrofit his existing water heater with a heat pump:

          https://www.greenbuildingadvisor.com/article/getting-into-hot-water-part-2

    2. T. Barker | | #23

      Yes, 755 kwh per month I can get my head around. 350-400 kwh per month I still question.

      1. Deleted | | #26

        Deleted

        1. T. Barker | | #29

          Maybe I'm missing your point, but your total consumption was approx. 10,000 kwh per year (excluding the last bill). That's exactly what I've been saying.

    3. T. Barker | | #28

      Good article. It's interesting I had just put hot water heaters on the top of my pile to look into. My next biggest energy consumer.

      1. Calum Wilde | | #34

        For very little money compared to HPWHs you can put in low flow shower heads that work well (Niagara Earth is what I'm using). That'll cut your showering hot water in half. Adding a drain water heat recovery, if it's possible, could cut your showering hot water usage in half again , or to a quarter if you use both methods. You can add the same levels of insulation to a cheap water heater as you'd get with a Marathon (pay particular attention to insulating the t&p valve while allowing it to operate), that will cut your standby loses by ~ half. Use the energy saver setting on your dishwasher and cold water for laundry. If you're using hot water for laundry with an old style washer that could make a huge difference. If you're patent enough to wait for hot water when washing hands use low flow aerators, if not just use cold water. Try to be aware of how often you allow water to cool in the pipes and reduce it where reasonable. For example if I'm loading and turning on my dishwasher I try to do any hand washed dishes at the same time so I don't get water to cooling in the pipes on separate occasions.

        Do that and it'll cost you less than a HPWH, and if you're already not doing those it'll save more and last longer. They'll also lower your water bill. At that point I doubt a HPWH would make any sense. I don't think it would for us.

        1. Norman Bunn | | #39

          There is the dehumidification factor to consider if the unit is placed in a conditioned crawlspace (mine is) or basement, where it will also help cool in the summer.

          1. Calum Wilde | | #43

            Due to my uninsulated slab my basement has never gotten above ~22°C. I've actively been trying to 8ncreasing the cooling load down there to allow my heat pump to dehumidify the area. If I were to put in a HPWH I suspect I'd have to heat that area all year long.

            I'm now finished all of the low hanging fruit type of home energy improvements, I'm about $17-17.5k into this and I've reduced my power consumption from 24-26MWh/year, down to (I'm expecting at least) somewhere between 8-9MWh/year now. Now it's time to save up for less easy things, the first on that list is insulating the slab with likely R10 rigid foam. Once that's done I'm hoping the cooling load down there will be high enough for my heat pump to work well to dehumidify, and I might entertain the idea of a HPWH.

            I'm also thinking of adding R20 rigid foam to the outside walls at some point, and some kind of high R-value windows way down the road.

            But yeah, back from my tangent, it's so cold in my basement that actively cooling it even in the middle of summer would be pointless.

  14. Deleted | | #27

    Deleted

  15. T. Barker | | #30

    So when people say they have a "heat pump hot water heater", a "heat pump dryer", heat pump this and that, does each device have it's own heat pump outside the house, or can they all tap into one main heat pump?

    1. Calum Wilde | | #31

      A heat pump dryer has the heat pump fully contained in the dryer. Heat pump water heaters often have it contained, but there are some split systems.

  16. Bruce Davis | | #32

    Followup to my original post. I appreciate all the input and opinions seem to vary wildly. I live in Kansas City. By minimal power users I mean we keep the AC at 77 daytime and heat at 67 daytime when home, adjusting when we're gone. Turn out unused lights (all LED), each of us showers every other day, run the dishwasher once every week or so, wash clothes once every week or two, compost food & yard waste, no grass, only turn on the drip irrigation system when absolutely necessary. On the downside, I run a portable dehumidifier all summer long. I do have a power sucking 65" plasma TV but it does have a picture unmatched by LCD/LED.

    Below, is my monthly usage in my 2000 sq ft very loose 100+ year old home prior to installation of solar system.

    Month kWh used
    01/04/12 786
    02/02/12 728
    03/05/12 670
    04/02/12 373
    05/02/12 393
    06/01/12 620
    07/02/12 1,116
    08/01/12 1,403
    08/30/12 968
    10/01/12 505
    10/30/12 459
    12/03/12 577
    01/03/13 656
    02/01/13 724
    03/04/13 694
    04/01/13 578
    05/01/13 383
    05/31/13 506
    07/01/13 580
    07/30/13 1,010
    08/27/13 835
    09/30/13 807
    10/30/13 522
    12/03/13 562

    avg monthly 910.33 kWh

    With a 8.925 k solar system, I annually overproduced as follows:

    2014 - 370 kWh

    2015 - 1049 kWh

    2016 - 321 kWh

    2017 - under-produced 167

    As I previously mentioned, with monthly net-metering and 2 cents buyback, I get no value from overproducing. I have started working with the BeOpt app. Thanks.

    1. T. Barker | | #35

      You're building a 4300 sq.ft. home, potentially an electric car, heated driveway, etc., and as you get older you may find you can't tolerate 67F or 77F all day long during the peak seasons. My recommendation is still 15 KW.

  17. GBA Editor
    Martin Holladay | | #33

    T. Barker,
    Q. "So when people say they have a "heat pump hot water heater", a "heat pump dryer", heat pump this and that, does each device have it's own heat pump outside the house, or can they all tap into one main heat pump?"

    A. In the U.S. and Canada, a heat-pump water heater has its own heat pump, and (with very few exceptions) the heat pump is located on top of the water heater. A heat-pump clothes dryer also includes its own heat pump.

    For more information on heat-pump water heaters, see "Heat-Pump Water Heaters Come of Age."

    For more information on heat-pump clothes dryers, see these two articles:

    "Heat-Pump Clothes Dryers"

    "Alternatives to Clothes Dryers"

  18. Bruce Davis | | #36

    I ran BEopt, see attached. Not certain what I'm looking at. Presume the end use graph in the lower left indicates the 10 kWh system I entered would supply 60% of my electrical needs. I'm using the black PV line (135) as the solar power supplied out of total use of 225. Is my interpretation correct? Thanks.

  19. GBA Editor
    Martin Holladay | | #37

    Bruce,
    Here's what I noticed:

    1. The graph shows that your estimated annual energy consumption will be 225 MMBTU/year. This seems high, since the average U.S. residential energy consumption is about 90 MMBTU/year. (Source: RECS data.)

    2. The specs you have selected show that you aren't interested in building a high-performance house. Your walls will have R-19 fiberglass; your attic will have R-49 fiberglass; your windows will be double-pane low-e argon (pretty much code minimum). You are planning to have an air leakage rate of 5 ach50, which is unusually leaky.

    So, if you want better numbers, it's time to design a better house.

    1. Expert Member
      Dana Dorsett | | #38

      Note, 2x6/R19 walls don't even meet IRC 2015 code minimum for zone 4, and R19 batts only perform at R18 when compressed to 5.5" in a 2x6 cavity. There's definitely room for improvement here.

      R23 rock wool would improve the whole-wall R by a few percent, and combined with an inch of exterior polyiso would cut the wall losses/gains by ~25%. That's still not the limits of financial rationality. At 1.5" continuous polyiso with a rainscreen gap between a foil facer and the siding a 2x6/R23 wall would come in at about R25-R27 whole-wall depending on framing fraction and materials selection for the structural sheathing & siding. Any thicker than that PV might be the better lifecycle investment (but maybe not.)

      See the "whole assembly R" in the zone 4 row of Table 2, p10 of this document, which is a rough cut at where the tipping point of financial rationality might be. When trading off against PV array size vs. grid-energy the tipping points will move a bit from those marks based on the subsidized costs, the local net-metering structures, local gas & electrical rates, etc.

      https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf

  20. Bruce Davis | | #40

    Martin, your points are well-taken. I was interested in building a high-performance house, just turned out my builder wasn't. Lesson learned if there's a next time, but for now I have to play the cards in hand. My city uses the 2012 code and has even amended that down to R-13 walls and 5ACH50. I'm shooting for higher, i.e., R-21 walls & 3ACH50. The highly regarded custom builder thinks code is fine. He's more focused on making sure the structure is sound, long-lasting & looks good. Not to mention this house is going to cost way more than planned without the high-performance energy standards.

    On some of the BEopt inputs, I did go conservative in spots, i.e., 5ACH50 when I think we'll be 3ACH50 or better. My builder says he builds a tight house, but hasn't tested. Guess I'll be the blower door guinea pig. I did get Pella Impervia windows, which seem to have above average energy ratings.

    Going back to using my current house as a frame of reference for sizing the new PV system. I found a blower test done in 2006. The numbers don't seem to compare to the standards of today. They state 5535 cfm at 50 pascels, resulting in 1.09 ACH, which they categorize as 2-3 times more leaky than the average home in 2006.

    We have 2,000 sq ft with an 8 ft ceiling. 5535 cfm and 16,000 cu ft results in 21ACH50 by my calculations. Add poor insulation to the leaky envelope. I use an average of 14,400 kWh/yr (49 mmBTU) and 392 ccf of NG (40 mmBTU). That 89 mmBTU puts me right at the national average.

    The new house will have double the square ft (4,300) and triple the cubic ft (47,000). My current 9k PV system overproduces on an annual basis. The only future usage difference I can think of will be HVAC. I'm leery of going to 15k and skipping the gas hookup without more definitive data. Changing to a double heat pump will also add $4300.

    Guess I'm still stuck between 10k and 12.1k for the new house. The cost difference after tax credit is $4200. Maybe it's time to throw darts. Thanks again for everybody's input.

    1. Calum Wilde | | #44

      21ach50?!! Did you test it with the windows and doors open?

  21. GBA Editor
    Martin Holladay | | #41

    Bruce,
    Your blower door numbers don't make much sense. If your blower-door operator calculated 1.09 ach50, and you calculated 21 ach50, someone either has the wrong numbers or is guilty of a calculation error.

    One thing's for sure: You have chosen to build a big house with unimpressive envelope specs instead of a normal-sized house with above-average specs. If it's too late to make adjustments, you'll end up with exactly the house you're planning to build.

    1. Expert Member
      Dana Dorsett | | #42

      I think that's probably 1.09 ACH-natural, not 1.09 ACH50.

  22. Deleted | | #47

    Deleted

  23. Tim R | | #49

    So, Building a 4,300 sqft house and worried about $2,600 ??

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