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How Big a solar system to get?

Discussion in 'Energy, Environment, and Policy' started by Piney999, Jul 27, 2017.

  1. Piney999

    Piney999 Member

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    Hello!
    As my family eagarly awaits our model 3, we are looking at getting a solar system for our house. We have a large south facing roof, so the question is, how big do I need to go. I know that is a complex question, but since I know next to nothing about solar, I was hoping some folks could give me some things to think about. Is it as easy as "as big as you can afford"! or is there more too it then that? Here in Washington the electric rates are tiered, with the first 800Kw a month being cheap, but then they expensive quickly after that. I would love your thoughts!
     
  2. Lloyd

    Lloyd Well-Known Member

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    #2 Lloyd, Jul 27, 2017
    Last edited: Jul 27, 2017
    I have found that it takes about 4 kw of solar generation to power the average use of 1 tesla for charging at home. The rest depends on your household usage, and your solar installer can help you determine what your needs and options are.
     
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  3. kev1n

    kev1n Member

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    I would start off with getting quotes from 3 or so different companies, they will ask for your past year of annual usage(you can download this from pge or whatever electric company you have) and they have software they can plug your numbers into/look at solar projection in your area. also would check if your area has net metering or not as that will play a big part.

    some companies that come to mind you may want quotes from such as solar city, sunpower, sunrun.
    what i have found out though is that, dont tell them you are getting an electric car, their software module they use will not calculate it correctly if net metering comes into play and I had to do my own calculations.

    i personally went with sunpower
     
  4. gabeincal

    gabeincal Enjoying Napa life the electric way

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    I'm basically in the same situation as OP. I went with rough calculations to our baseline, luxury items (like AC running when not really needed), and the rough mileage for our 2 electric cars (Tesla and Fiat). But then there is also the other end of the spectrum where you don't really want to be above what you reasonably use because (in the case of PG&E) the utility pays next to nothing to buy electricity off of you. But then, you want to generate more power during the day, so you can use 2-3x as much at night with net metering and a good TOU plan to buy that power back for free. Complex question. I'm going with about 4kWh system and see how things work out. Also with PG&E, you apparently are better off owing them about $120 at the end of the year as then your true-up bill will be $0. Yes, they make it very nice and transparent to do solar..............

    See my thread here, maybe you find some useful info:

    Solar panel sizing?
     
  5. MorrisonHiker

    MorrisonHiker S 90D 2017.42

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    Be aware that depending on where you live, there might be a limit as to the size of the solar system you are allowed to install. In some places, you are limited to a certain number of kW. In others, you are limited to a percentage of your current usage. We used to have a really small electric bill. When we first got a quote for solar, we would've only been allowed to install up to a 4 kW system. It didn't make any sense to install such a small system knowing we'd need to add to it in another year. Now that we have multiple Teslas, a larger electric bill, etc., we would probably be able to install a 16 kW system based on our past year's usage.
     
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  6. kev1n

    kev1n Member

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    here is a good link with rough estimator and also you can get quotes,

    Project Sunroof
     
  7. Sharkbait

    Sharkbait Member

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    #7 Sharkbait, Jul 27, 2017
    Last edited: Jul 27, 2017
    Congratulations on your decision to go solar.

    It’s a little more complex than going “as big as you can afford”. Most utilities will give you next to nothing for your excess power. Why spend more money on panels than you need or can afford? Also, do you plan to buy outright or lease? I purchased my system and calculated my ROI to be about six years, plus I’ll get some residual value of my purchased system when I sell the house.

    If you lease a system and want to move and sell your house with leased panels, the new buyer will have to agree to assume your lease in most cases. Could be a deal breaker, maybe.

    Unless you’re excited about doing all the research yourself, you probably need a consultant or a company that can visit your site and make all the calculations, specifically taking a look at current, annual needs and then factoring in future consumption for your Model 3. I “gas” up my Model S about once a week, so I use about 300 kWh per month. On average, my system generates a little over 1,000 mWh/month. Edison recently went from four tiers to three, and I’m almost always in the .16/kWh tier even in the summer when my “net” usage for the month never goes above 500 kWh/month. I’m grandfathered on NEM 1.0 billing, which officially closed on 7/1/17 to new customers much to SCE’s delight. My system over-generates power in the spring, fall and a little bit in the winter. I consume the excess during the summer months.

    The farther north you go, the more extreme the sun angles are in winter and summer. Instead of buying more panels that are installed at a fixed angle, maybe consider buying fewer but on a two, or even three axis mounting system. Gets more expensive but captures maximum solar winter and summer. But, how much property do you have, can you mount on the ground, etc,? If I was younger, I would certainly be looking at two or maybe three of the new Tesla Powerwalls ( Tesla Powerwall ) but I’ll probably die before the N. Koreans, Chinese, Russians take down the national grid or the apocalypse arrives.

    I have a large property and mounted my fixed angled system on the ground at about 20º elevation. It would be well over 40º elev. up where you lived if you did a fixed angle. In addition to having the extra space for ground mounting, the panels are less expensive to service and clean. Panel maintenance hasn’t been an issue at all but ease of cleaning and cost is also a factor. My cleaning company charges $70 for washing and rinsing with DI water. Panels mounted on the ground are more efficient on hot days, when temperatures start to rise because you have ambient air ventilation on the underside of the panel. On the roof, you’re lucky if they’re a few inches off the hot asphalt or concrete tiles. Roof top temperatures can be 30º-50º warmer than the ambient ground temperatures.

    Call your insurance company to talk about insuring your system or make sure it’s covered (generally under “other structures” clause). My system is covered for $50K beyond the replacement value of the house. If you mount on the roof, make sure your liability insurance would cover any worker that goes up there to service or clean them that isn’t covered with workers comp. Before signing any contracts make sure you see your chosen’s company insurance coverage documents, covering not only damage to your roof and home but also ALL workers for falls or injuries. There are lot of rogue, fly-by-night companies out there so read as many reviews (that you can believe). The market has weeded a lot of them out.

    So my system is comprised of 24 LG MonoX (monocrystalline) panels. (Know what kind of silicon you’re being sold.) The system is rated at 6.6 kW. I used a 7.6 kW inverter, which allows for a little growth, maybe another four panels. My wife and I are empty nesters, we run the 5-ton air-con 24/7 for five months of the year in a 2,600 sq. ft house. Last month’s bill was a mandatory charge of $1.09 varies between $1-$9/month. The system was $28K installed. Got back $9K from fed and state tax credits, which came off the bottom line and is real money in your pocket via refunds the year after you install. I figure we saved $10K in power bills during last three years, so break even is about 36 months out. I'm ashamed to say I don't worry about leaving a light on here or there.

    I’m sure there’s more to mention, and a lot of this doesn’t matter or apply to a system that is leased. Would I do it again? Hell yes, and if I was 30 years younger I would build my own house totally off the grid!
     
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  8. Sharkbait

    Sharkbait Member

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    Pretty accurate in my case.
     
  9. Sharkbait

    Sharkbait Member

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    I should have also mentioned that in most cases the utility will have to approve your system sizing, so it can't be overbuilt in theory. As a rule of thumb, SCE was allowing 2 kW for every sq. foot of home under heat and air-con (not the garage). If a year's worth of historical billing is available, they would let you build up from there.
     
  10. oktane

    oktane Active Member

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    I believe you can overbuild with a waiver from SCE. It seems more of an acknowledgement rather than a permission from SCE.
     
  11. Sharkbait

    Sharkbait Member

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    So initially, I did not understand the permitting process. Both the city and utility (SCE) had to approve. The city was concerned about code, setbacks, safety, etc. When you tie into SCE, they have to approve as was in my case. Edison wanted to know the output of the system and the electrical interface to the grid. Because I bought my system shortly after I bought the house, I did not have 12 months worth of bills to justify the size. Instead of two kW per sq ft under heat and air, I was allowed to fill out a myriad of forms listing everything that would be connected to power, the wattage, hours per day used, on and on. It wasn't very difficult going from 2 kW sq. ft (5.2 kW system) to a 6.6 kW system. Three years down the road I'm using everything I produce when you throw in the EV.

    Perhaps you can overbuild a system with SCE but when they give you only 3 cents a kW for your excess power at the end of your relevant, annual period, it's going to take a whole lot of time to pay off those expensive panels and associated equipment. The array is big enough as it is, about 13' X 33', ground mounted at a 20º angle.

    Couple that with the fact the no two people at SCE will give you the same answer, that's my experience. In the end, I completely happy with the system that was built.
     
  12. oktane

    oktane Active Member

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    Yes you are correct. You can overbuild but it would be silly (unless you predict adding more EVs to your household in the future) since you get $0.02 per kWh produced as overage. It would not make any financial sense to overbuild unless you anticipated the extra future need. Plus, solar panel pricing is declining rapidly.
     
  13. SageBrush

    SageBrush 2018: Drain the Sewer

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    Are you trying to build up enough PV to cover your EV and home electricity ?

    The EV will be ~ 4 miles a kWh
    Your home generation per kW is best estimated by pvwatts.org
    Your home consumption is read off your bills, but you may want to spend money on efficiency first; it is usually less expensive than PV and will also reduce NG use if you have it.
     
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  14. Sharkbait

    Sharkbait Member

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    SCE took a couple weeks to contact my installer with "Permission to Turn On". My installer called me and said, "flip the lever on". Except for the forms, I never dealt with them directly during and after the install. I gave up calling them with questions prior to the install because I kept getting conflicting information.
     
  15. MorrisonHiker

    MorrisonHiker S 90D 2017.42

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    Here's the correct link: PVWatts Calculator
     
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  16. dhanson865

    dhanson865 Active Member

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    Since you got a few serious answers can I just give my first thought without offending?

    I'd go for one at least 300 million km (186 million mi) since that is how big the orbit of the earth is (diameter not radius). Too many issues with living in a smaller solar system.

    That was literally my first thought when I read your thread title.
     
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  17. SageBrush

    SageBrush 2018: Drain the Sewer

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    There are fixed costs related to PV, and the inverter is often over-spec'd or very inexpensive to add capacity so the marginal cost of adding more can be very cheap. Certainly under $1 a watt after the tax credit for the materials, or about 2 cents a kWh over the panel's lifetime.

    That will not make you money, but it lets you add clean energy to the grid for close to free. You say 'no,' while I say no-brainer.
     
  18. Sharkbait

    Sharkbait Member

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    Well, the way I see it, one can fudge a little here when you have NG heating. There have been times when I see my total, excess power building up during the relevant period, so I use electric space heaters in some rooms to use up some of that excess power and keep the NG bills to a minimum. Each electric heater runs at about 1300 watts. Running four of them will gobble up power almost as much at the air-con unit. Not pretty but it works to keep the NG bills down to a minimum, and we get a lot of 15º-25ºF nights in the high desert for about 100 days/year. Conversely, we get many clear winter days, which are good for solar production. It's then when I wish I could ratchet up the panels to face lower on the horizon. Power production in Dec is half of the produced in June.
     
  19. SageBrush

    SageBrush 2018: Drain the Sewer

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    Improve the insulation and air leakage in your home and then use the excess kWh in heat pumps during the day for hot water and air heating. You'll get ~ 4x more heat for your PV generated energy.
     
  20. Sharkbait

    Sharkbait Member

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    In practice, my panels rarely produce their rated output, so going up in rated panel output to about 8 kW on my 7.6 kW inverter wouldn't be a problem. You might get some "clipping" but not on many days if at all. You are correct. The marginal expense of increasing the size is mouse nuts and this article seems to support over-spec'ing:

    SolarPro Magazine
     

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