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Solar PV

Page history last edited by Ulrich Bonne 14 years, 3 months ago

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Tips for Homeowners with New Photovoltaic Systems,

by U. Bonne, ulrichbonne@msn.com October 2009; rev. 1 Feb. 2010

Today, many residential and commercial building owners are finding that installing solar water heating and photovoltaic systems really pays off. Completely installed residential 2 kW(peak) systems with inverters and interconnections for net metering now are quoted from 9.5 down to 7.5 $/W(peak)[1], and for larger commercial systems down to 6.5 $/kW. Even if a 2-kW(peak) solar PV system costs $15,000 and produces only the equivalent of 3 full sun hours each day, the system will pay for itself in 10 to 18 years, at zero cost of capital. If you can get 4 full sun hours/day, the payback time reduces to 7.5 to 13.5 years, depending on whether or not you take advantage of the tax credits (State 35% and Federal 30%). respectively. If you have to borrow money at 4% to pay for the solar PV, the above payback period of 7.5 years stretches to 9.1 years. Actual data now predict a shorter payback.

In case you wondered about which forms to use for claiming rebates and tax credits for your home renewable energy investments, state tax technical support at 808-587-1577 is very helpful. I found out that individuals can use the 2009 form N-334 (to be available in Jan. 2010) to claim both renewable energy tax credits (35% but capped at $5000) and refunds (24%), as attachment to the regular tax return form N-11. Downloading of the forms is easy from the alphabetical list of forms at http://www6.hawaii.gov/tax/a1_1alphalist.htm#nLabel


What should a homeowner embarquing on installing a PV system look for? Here is a short list:

  1. Total installed cost of system, after taxes, monitor (see #10 below) and fees for engineering drawing (latter ~$ 200; I did my own, and it was a fun thing to do). Total cost should be under 7.5 $/W in 2010 for the total system, or under $30,000 for a 4kW system, before rebates or tax credits.
  2. Check that the salesman/marketer/consultant payment is included in the above
  3. Because the 35% State tax credit for a 4 kW system will be over the limit of $5000 in Hawaii, consider installing in at lest 2 steps, falling in different years
  4. Roof space: Do you have enough S-facing roof space for 24 SolarWorld 175 W panels or 18 of their 220 W panels (they are larger)
  5. Individual microinverters (recommended because 1) then the 1st and 2nd set of PV panels are considered to be “systems” qualifying for the State Tax Credit; 2) Lower cost according to the data I have and 3) shading or failure of one panel does not pull down the whole system output) or one big inverter. If you also need to put panesl on E or W-facing roofs I just learned that E-facing is better (assuming no shading), because the atmospheric haze generally increases in the afternoon.  Another way to add “roof space” is to add trellises around the house
  6. Consider positioning the PV on parts of the house that need more cooling, because the PVs will keep the roof underneath shaded and cool.
  7. Consider the merits of adding a bit more PV for hot water, rather that messing up you plumbing system with solar water heaters – if your hot water needs are small
  8. Ask if contractor will provide State and Fed Tax Credit forms
  9. Ask that the PV disconnect switch is of the easier-to-operate “Lever” type rather than the “Plug-in” type
  10. Check that the contract includes the ~$200 monitor and display (Enphase or equivalent on-line data acquisition and monitoring; after the first 6 months of free monitoring service, Enphase charges $2/year per PV/inverter module)
  11. Consider the desirability of (and means to) being able to install battery back-up in the future, to charge your PHEV
  12. Insure you have papers ready to be submitted by contractor to your electric company ASAP after completion of installation: 1) County Inspector ( called by contractor) approval, 2) Signed engineering drawing by PE and 3) NEM agreement. (Hawaii's HELCO does not recognize your generated kWh until they change their meter, supposedly one that is more accurate, but is digital, w/o the rotating disk, which was nice because you could tell at one glance whether the PV system was making the meter go backwards)


[1]  I have recently contracted to install a 2-kW PV system on my house in Kona. The quotes I received during August 2009 are in the $/kW range shown, after including all fees and taxes. Beware of enthusiastic PV marketing folks, who base your payback time on more than the equivalent of 4 full sun hours per day in Kona! While wind capacity factors (i.e. fraction of time the turbine operates at 100% capacity) rarely reach over 35%, solar (fixed) PV capacity factors can theoretically not exceed 25%, and in Kona seldom reach over 15%, or the equivalent of 3.6 h/day or 1314 hours/year. 

Therefore, I expect my 2-kW(peak) system to generate about 2600 kWh/year or 219 kWh/month. A nice on-line solar-PV calculator outputs the needed PV size and cost (7.96 $/W) for a given input of needed kWh/month (although I think it underestimates by 20% the needed PV size) -- see http://solar.coolerplanet.com/Articles/solar-calculator.aspx


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