Need basic PV information

First time posting here. I am a real estate project manager for a large company. Someone several pay grades higher than me has decided that we are going to install solar panels on the roof of one of our buildings, and it has become my project. There's no point in discussing the cost vs payback issue, the die is cast.

The building is 5 floors with a flat roof. Location is c entral Virginia, and the building is sited such that the roof does not fall in the shadow of the adjacent buildings. Gross roof area is about 25,000 sf. The same person who approved the project has engaged a consultant who supposedly has experience in this area, but reading his proposal I think he is more of an agent for selling excess electricity and brokering deals. I don't know squat about solar power, but I have found some glaring errors in his proposal. I have not met with him yet.

So I would like to know what kind of questions to ask, what pitfalls to look for, and what kind of rules of thumb I can use to estimate how much power we can get out of the system. Is there a PV primer on-line somewhere?

One of the pages in his proposal was an aerial view of the building with some kind of color coding like a thermal image. It is supposed to show average annual insolation per sq meter. Is there a way to convert that to kW output of the solar array?

Comments

  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Need basic PV information
    Bulldog wrote: »
    .....

    One of the pages in his proposal was an aerial view of the building with some kind of color coding like a thermal image. It is supposed to show average annual insolation per sq meter. Is there a way to convert that to kW output of the solar array?


    ???? If there are no shadows on your roof, it all better be the same color, or else you have a Thermal image map, where it shows heat leaking out spots that have insulation gaps.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    Re: Need basic PV information

    The PV Watts website is pretty easy to use. Use the defaults and set the angles to what the vendor is suggesting. So far, it has been pretty accurate from what I have seen--But year to year average sun can vary by 10-20% per year.

    Many of these folks (solar leasing and such) are making their money on tax credits and other green credits/subsidies.

    Check very closely with your utility company about how they will bill you with solar power.

    It is not impossible to actually see your electric bill go up when solar is added if you have Reservation/Demand based pricing (if you have a lot of solar, it can actually push more power out to the utility than you normally use in a 15 minute period--At least in California for many commercial power users).

    School Bit by Demand Charges

    In California, from what little I have seen, roughly 1/2 the bill is Demand Charges (top 15 minute power usage in the last month or year) and the other 1/2 is actual meter powered usage.

    If your also paying for poor power factor (lots of motors/inductive loads)--Solar GT will not usually "fix that". Getting a power engineer over there to evaluate your power usage and profile can probably help you avoid most of the major pitfalls.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
    Re: Need basic PV information

    Good point. There is a mechanical room penthouse in the middle of the roof, so that creates a shadow, and there is a short parapet around the perimeter.

    Is there a formula to calculate the potential Kw based on the area of the roof and the average insolation?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    Re: Need basic PV information

    Also, there are different types of solar panels:
    • Poly Crystalline: Pretty efficient, on the higher end of price
    • Mono Crystalline: Slightly more efficient and very pricey
    • Thin Film: In general, 1/2 as efficient and 1/2 to 1/3 the the cost per watt.
    Thin film are very popular panels, but they take up about 2x the roof area and require 2x the mounting framework and wiring. Many times, these "cheaper panels" are not worth the cost savings when you take into account that you need 2x the area for the same power output.

    Also, see who is on the hook if the system does not perform or the panels fail in 5+ years. Most installers will warranty their work for 5 years, but after that, you will have to foot the bill for any repairs (hopefully, your solar panel vendor will still be capable of honoring their 20 year warranty).

    Also, plan on replacing the inverters every 10 years +/-.

    Look at the condition of your roof and work with a good roofing contractor to review how the racks will be mounted and how the roof penetrations (mounting/wiring) will be done. If the panels are supposed to last 25 years, will your roof?

    A local roofer here makes fairly good money repairing roofs after the solar installers are through with their work. In fact, he is doing so many solar roof repairs, he is probably going to go into the solar PV business and cutout the middle man.

    Do you have a quote of how many watts (or kWatts) of solar panel the folks want to install on your roof? Are they doing to a plan (how much power to offset) or are they just trying to jam as many panels on the roof as they can?

    From the PV Watts help page, you can guestimate the coverage factor for a 40 degree array to be around 0.5 (for every 1 sq.ft of panel, you need 2 sq.ft of array for about a 5% loss of output due to shading (40 degrees).

    So, 25,000 sq.ft. of roof would support around 12,500 worth of solar panels.

    For Poly, call it 12 watts per sq.ft. (Mono, ~13 watts/sqft; thin film 6 watts/sqft):
    • 12,500 sqft * 12 watts/sqft = 150,000 watts = 150kW of solar panels
    I am not a solar installer or estimator--this is just a back of the envelope estimate.

    For Lynchburg Virginia, a 150 kW worth of solar panels would look like this using PV Watts:
    "Station Identification"
    "City:","Lynchburg"
    "State:","Virginia"
    "Lat (deg N):", 37.33
    "Long (deg W):", 79.20
    "Elev (m): ", 279
    "PV System Specifications"
    "DC Rating:"," 150.0 kW"
    "DC to AC Derate Factor:"," 0.770"
    "AC Rating:"," 115.5 kW"
    "Array Type: Fixed Tilt"
    "Array Tilt:"," 37.3"
    "Array Azimuth:","180.0"

    "Energy Specifications"
    "Cost of Electricity:"," 8.0 cents/kWh"

    "Results"
    "Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
    1, 3.84, 13986, 1118.88
    2, 4.73, 15157, 1212.56
    3, 5.57, 19233, 1538.64
    4, 5.69, 18535, 1482.80
    5, 5.69, 18440, 1475.20
    6, 5.87, 17969, 1437.52
    7, 5.87, 18594, 1487.52
    8, 5.67, 17987, 1438.96
    9, 5.39, 16930, 1354.40
    10, 4.99, 16893, 1351.44
    11, 4.15, 14017, 1121.36
    12, 3.56, 12650, 1012.00
    "Year", 5.09, 200389, 16031.12
    Or 200,389 kWH per year...

    Somewhere around $1,200,000 +/- turnkey installed cost?

    Does this help? Is this anywhere close to what you are being told?

    -Bill

    PS: Should also ask--What is the chances of ground strike lighting in your area? Has anyone addressed Lightning issues and who will pay for repairs if there is a hit/damage?

    Not saying that it will take a strike and bring down part of the system--But owning your own power system does come with expenses for ongoing maintenance and unexpected repair costs.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
    Re: Need basic PV information

    Thanks for the detailed response. I sdaw that on-line calculator but it didn't make any sense to me. Still doesn't. The upper half is all input data, right? in th lower half, first line, what does the value 13986 represent? Looks like kWh produced by the array. So how do I relate that to useable power?

    You mentioned 150 kW on my roof at a cost of $1,200,000 or $8,000/kW. Is that a reasonable rule of thumb for estimating the cost? My budget isn't that large and I want to have an idea of what is reasonable before I meet with the consultant.

    Interesting you mention the roof structure, as that's one of the first things I picked up on as being absent from the proposal. The consultant is totally silent about the building infrastructure. I'm convinced his role is a deal maker, and the details are someone else's problem ... that would be me.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    Re: Need basic PV information
    Bulldog wrote: »
    Thanks for the detailed response. I sdaw that on-line calculator but it didn't make any sense to me. Still doesn't. The upper half is all input data, right? in th lower half, first line, what does the value 13986 represent? Looks like kWh produced by the array. So how do I relate that to useable power?
    Yes, the first 1/2 is "input data".
    "Results"
    "Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
    1, 3.84, 13986, 1118.88
    1 = January
    3.84 = 3.84 average hours of full sun (tilt,, tracking, & 20 year average weather included) for the month of January
    13,986 kWatt*Hours for January power output or ~451 kWH per day production
    $1,118.88 = value of kWH for January based on $0.08 per kWH power costs for your region.

    The bottom line is the 1 year results:
    "Year", 5.09, 200389, 16031.12
    5.09 = Hours of sun average over 1 year
    200,389 = kWH per year (assume +/- 10-20% year over year average based just on weather conditions alone).
    $16,031.12 = worth of power at $0.08 per kWH for 1 year

    For a typical American home, they average probably around 300 to 1,500+ kWH per month... Depending on weather, Air Conditioning or not, electric/gas heating/hot water, well pump, etc.

    In your area, if you have a ~$100 per month electric bill, that is around 1,000 kWH to 1,250 kWH per month.

    The typical off grid home is probably closer to 100 kWH per month.
    You mentioned 150 kW on my roof at a cost of $1,200,000 or $8,000/kW. Is that a reasonable rule of thumb for estimating the cost? My budget isn't that large and I want to have an idea of what is reasonable before I meet with the consultant.

    I just took a stab in the dark to give you an rough target... Right know, home installations are around $6,000 to $10,000 per kWatt of solar panels--when many people probably getting the $6,000 / kW pricing.

    I don't do solar installs--I just gave $8,000 as a mid-range price estimate (before any tax breaks, incentives, etc.).

    There can be other "gotchas"... In many areas, solar equipment is free from local property taxes (and sometimes sales tax exempt). In other regions, there are both property taxes and sales taxes... That can be a fairly big hit on return.
    Interesting you mention the roof structure, as that's one of the first things I picked up on as being absent from the proposal. The consultant is totally silent about the building infrastructure. I'm convinced his role is a deal maker, and the details are someone else's problem ... that would be me.

    Solar panels are not that heavy--If you have hurricane level winds in your region--it could be an issue.

    In any case, they should have a Civil Engineer with a PE license (professional engineer) review the building structure and provide a "wet stamp" on drawing when applying for a building permit.

    They should also have good knowledge of how the local power company will accept a large installation at that address (over ~10,000 watt systems--the approvals are usually far from automatic by the power companies--they have to verify their infrastructure can handle the generation capacity).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset