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  • Questions on Solar in Flagstaff AAZ

    I am buying a 4600 SQ ft house that is under construction in Flagstaff AZ. We have the opportunity to change HVAC systems to what would be the most economical including adding PV.
    Talked to one solar contractor that estimated our electrical needs for heating and AC when needed would be handled by a 17 KW system at $55,000, before rebate. Just starting the process of accessing heating options that might utilize solar (dual fuel heat pump). I would appreciate any input that you might have. I have Solar for Dummies on order to help with my education.
    Thanks,
    SS

  • #2
    IMO, Consider the best insulation design you can afford before looking at solar. A well insulated home will have a good ROI. Work with a civil/mechanical engineer or an architect on the insulation and mechanical design.

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    • #3
      Unless you're heating with electricity, that seems to be a very extreme amount of Solar PV. That's (South facing) is going to crank out about 28,000kWh of electricity a year (quick estimate on PVWatts - which you should become familiar with, where the panels would placed, tilt, angle, etc and estimate the production for yourself.)

      I'd say do not use a Solar Contractor to give you the estimate of what your electrical needs are going to be, there job is to sell you a system, the more Watts the more money in their pocket. At least find others in the area with similar sized homes and same style HVAC are consuming.

      Also find out about how your electric company supports grid tie systems. Do the net meter, do the time of use, do they have different rates for different times of year, ....

      My knee jerk reaction though, based on weather in Flagstaff, compared to my location, and my home size, ... is that this may be as much as double what you really need.

      Also, consider living in it for a year to know your consumption, and add it on then.
      Last edited by TAZ427; 05-18-2018, 02:06 PM.

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      • #4
        Originally posted by TAZ427 View Post
        Unless you're heating with electricity, that seems to be a very extreme amount of Solar PV. That's (South facing) is going to crank out about 28,000kWh of electricity a year (quick estimate on PVWatts - which you should become familiar with, where the panels would placed, tilt, angle, etc and estimate the production for yourself.)

        I'd say do not use a Solar Contractor to give you the estimate of what your electrical needs are going to be, there job is to sell you a system, the more Watts the more money in their pocket. At least find others in the area with similar sized homes and same style HVAC are consuming.

        Also find out about how your electric company supports grid tie systems. Do the net meter, do the time of use, do they have different rates for different times of year, ....

        My knee jerk reaction though, based on weather in Flagstaff, compared to my location, and my home size, ... is that this may be as much as double what you really need.

        Also, consider living in it for a year to know your consumption, and add it on then.
        Thanks for your reply,
        I am planning on electric heating with two heat pumps if practical, would have gas furnace for the below freezing times. I am in contact with HVAC contractor that will give me the needed information on use. Good question on the POCO, its not net meter but not sure I understand how they work. The POCO is APS, if anyone has them. I want to make sure the building contractor does everything to make it easy to add if thats what we decide. He did tell me its a solar ready 400 amp panel.
        Thanks for your input!

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        • #5
          This can be a bit tricky. For starters, the good news: You will be at or close to ground zero for some of the best solar potential in the U.S., if not the world.

          The tricky part: With a well constructed, well sealed and well insulated structure that has some thermal mass, you may not need aux. heat in the winter until the exterior temp. reaches below, MAYBE 50 F. or less (that's called the balance temp. - simple but incomplete explanation: The exterior temp. below which aux. heat is required). The tricky part to that is in knowing that as the outside temp. drops, heat pumps become less effective/efficient. In the limit, a super insulated house with lots of thermal mass in a sunny, but still cold winter climate like Flagstaff may not need aux. heat until the outside temp. gets into the 20's F for, say, a day or two, meaning that a dwelling equipped with both heat pump and fossil fuel fired heating systems will, because the heat pump will be less efficient at the lower balance temp., underutilize the heat pump and instead go directly to fossil fuel system fuel if efficient operation is a consideration. Most construction is not that tight or that thermally massive. Even so, Heat pumps used in new(er)and more thermally effective construction will be operating at a disadvantage that's perhaps unintentionally introduced by having lower balance temps. from lower design heat losses.

          Things get a bit more complicated when cooling loads are considered. Since Flagstaff does also have a significant A/C or cooling DD climate, either a heat pump or simply an A/C system will probably be wanted and needed. For that, it's becoming more common, particularly for new, tight and low heat gain designs to use minisplit systems. But, whatever heat pump system is chosen, central or minisplit, for thermally efficient structures, the heat pump potion will be less utilized due to the lower winter balance temp. and the units' published SEER will be higher than the SEER actually achieved, that is, it will be less efficient in the winter due to the lower design balance temps.

          FWIW, I'd skip the heat pump for heating, go with fossil fuel for heat (and for water heating, cooking and clothes drying for that matter), and consider minisplits for summer A/C. Some added advantage w/ minisplits for A/C: You can add and/or operate them somewhat incrementally. That will help avoid oversizing an air conditioner which is never a good thing, even in dry(er)climates such as Flagstaff.

          As for PV, agree with TAZ427 in waiting until I had a year or so of usage history before sizing a potential system. 4,600 ft^2 is a big place but it'll be a real energy hog to use the 30,000 or so kWh/yr. a 17 kW system might produce. Someone is dancing with your leg on that one. Waiting a year will also give you some time to get more info and knowledge of PV.
          Last edited by J.P.M.; 05-18-2018, 04:08 PM.

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          • #6
            Something is very wrong with you numbers. At altitude not much AC load to speak of, and electric heat is not the way to go. Flagstaff is one of the best locations in the USA for Grid tied solar. You are being robbed because you have a lot of free cash to burn.
            MSEE, PE

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            • #7
              Thanks for your input, its exactly what I was looking for. Sounds like my staring point is forced air gas heating and AC (can't imagine much use for AC there) and then give it a year and see what I actually use.
              Thank you All!

              Comment


              • #8
                Originally posted by SilverSinbad View Post
                Thanks for your input, its exactly what I was looking for. Sounds like my staring point is forced air gas heating and AC (can't imagine much use for AC there) and then give it a year and see what I actually use.
                Thank you All!
                Thats the way. Pay up on insulation, doors, windows, high efficiency AC/Heat, lighting, and appliances. Those will have positive equity right from the start, and for every dollar spent there will save you $3 in solar add on later. Another thing that can save you big money is proper placement of Windows and use Sun Tubes for lighting. Have family in Flagstaff and have been there in many of Summers and never needed AC. In fact got stranded there Memorial day weekend 2008 I think, I-40 was shut down Saturday and Sunday from white out blizzard. Even snowed in Lost Wages and record snow in Denver from that storm. Imagine snow in Lost Wages NV Memorial day weekend. Phoenix got flooded. Global Warming advocates had to hide all summer from that one.

                MSEE, PE

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                • #9
                  Originally posted by SilverSinbad View Post
                  Thanks for your input, its exactly what I was looking for. Sounds like my staring point is forced air gas heating and AC (can't imagine much use for AC there) and then give it a year and see what I actually use.
                  Thank you All!
                  Probably, but your real starting point is more education about building design, including efficient techniques that reduce loads, passive solar measures that don't look like technological trashing or outer space design and finding a builder who has some experience in what works/doesn't work in Flagstaff. It can be done, and done well, but some persistence and patience is often necessary.

                  BTW, I also got snowed in in Flagstaff, but over an Easter holiday around 04/01 several years ago. Pass through there several times/yr on the 40 or if rooting around n. AZ./so. CO.

                  Interesting factoid: On average, Flagstaff gets ~ 100" of snow/yr. Buffalo, known for snow, gets about 93"/yr. If you go PV, go with a ground mount and see member Bruce Roe's posts. Among other things, Bruce is probably the snow king around here.

                  As for cooling requirements, checking my numbers, Flagstaff doesn't have one worth mentioning = ~ 150 F. cooling degree days/yr. Get a whole house fan and ventilate at night with a timer and/or thermostat.

                  For Flagstaff insolation: yr. round average is ~ 5.1 kWh/m^ per day (horizontal), and ~ 6.4 kWh/m^2 per day (direct normal). By way of and for rough comparison, same orientations for a very sunny climate (Phoenix):~ 5.7 kWh/m^2 per day (horizontal), and 6.8 kWh/^2 per day (direct normal).

                  With a low cooling load, but a healthy snow loading, as a prelim. dart throw for Flagstaff, I'd start prelim. design around a ground mount tilted at maybe latitude + 5 or +10 deg. and an az. of about due south, and expect an output per yr. of ~~ 1,700 - 1,800 kWh per installed STC kW of PV, provided you can keep most of the snow off the array.

                  Check POCO tariffs/rules to ensure best orientation with respect to such things as T.O.U. rates and times, and then iterate array size and best guess T.O.U. array revenue with (annual load divided by ~ 1,750 kWh/yr per kWh) for an initial array size in kW.

                  Given any reasonable annual usage, I bet you'll come up with something a lot smaller than 17 kW.

                  Good luck.

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