Friday, July 12, 2013

Solar Power Calculator

A reader pointed me to the National Renewable Energy Laboratory PVWATTS 2 calculator, which uses your local weather information to calculate likely power output, including adjusting from nominal DC output to AC (because of inverter losses).  I ran the data for a nominal 1 KW system (four of those Westinghouse panels), using the default inverter efficiency, and our current 7.8 cents/KW-h.  I also assumed a fixed tilt array.  (Changing the angle four times a year helps, but not dramatically.)  I am not assuming that I will get my proprietary improvements working.

"Station Identification"
"Lat (deg N):", 43.57
"Long (deg W):", 116.22
"Elev (m): ", 874
"PV System Specifications"
"DC Rating:"," 4.0 kW"
"DC to AC Derate Factor:"," 0.770"
"AC Rating:"," 3.1 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 43.6"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 7.8 cents/kWh"

"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 2.88, 276, 21.53
2, 4.16, 359, 28.00
3, 4.93, 463, 36.11
4, 5.77, 507, 39.55
5, 6.12, 542, 42.28
6, 6.47, 541, 42.20
7, 7.05, 582, 45.40
8, 6.93, 587, 45.79
9, 6.40, 536, 41.81
10, 5.30, 481, 37.52
11, 3.48, 313, 24.41
12, 2.80, 272, 21.22
"Year", 5.20, 5459, 425.80

I should point out that the 7.8 cents is for the 0-800 KW-h consumption rate -- Idaho Power actually charges us more than that above 800 KW-h per month.  Sixteen of those panels would cost $9600 plus shipping; I am going to have to pay something to my electrician to put in a 20 amp breaker to connect this to the main bus.  I will also have to spend some money and time build a support system to position these on the hillside that allows me to change angle four times a year.  Still, it is hard to imagine that this plus shipping will exceed $600.  A $10,000 investment thus takes 23 years to pay for itself, ignoring rising electricity prices and leaving out the 30% tax credit from the federal income tax return and the relatively trivial tax deductions on the state income tax return.  It also does not include the inevitable drop in output over the life of the panels.

This is beginning to make less and less sense.

UPDATE: Of course, if the cardiologist calls me back next week and says that I need an angioplasty, all of this will need to be delayed.

UPDATE 2: This isn't California.  Idaho Power's net metering program is somewhat limited:

Is there a limit to the size of generation from the customer? 
Yes. For residential and small commercial customers, generation is limited to 25 kilowatts of nameplate generation or less.
I am guessing that they mean 25 KW-h per month.  Not a problem; I can't imagine investing that much capital in solar panels.
Is there a limit to how many Idaho Power customers can participate in Net Metering? 
Yes. At this time Idaho Power limits the total amount of customer-owned generation to 2.9 MW. The actual number of systems will vary depending on the size of the individual installations. 
I have no idea if they are at limit yet; I've put in a call to their office to find out.  Worst comes to worst, we skip net metering and just settle for wasting the extra energy. Perhaps this would encourage not putting in more panels than I would likely use at any given moment.  Or it might make sense to look at using excess power to charge batteries for use after dark.

UPDATE 3: Details on Idaho's alternative energy tax incentives are here.  You are allowed to deduct 40% of the purchase price of the system in the first year from your income taxes, and 20% in the following three years.  Idahoans who can afford to consider alternative energy are going to be in the 8% marginal state income tax bracket, so a $3000 investment means a $1200 reduction in your taxable income, or about a $96 tax reduction the first year, and $48 tax reduction for the next three years.  The website previously linked isn't very clear on how the $5000 maximum incentive per year, $20,000 maximum incentive total, works; Idaho Code 63-3022C gives the details.  It appears that you are not allowed to take more than $5000 of deduction in any year.  The Idaho Code says nothing about a $20,000 maximum incentive.

If you are a greenie, Idaho law is a sign of what a backward, primitive, stingy state we are.  If you are anyone else, they are a sign that Idaho legislators like the idea of alternative energy, but didn't drink the green Kool-Aid.  The incentives are so tiny that they are essentially a sop thrown to greenies to make them think that Idaho cares about Mother Earth.


  1. When the president visited Denver he was shown solar panels on top of the Natural History Museum. These were expected to pay for themselves in 15-18 years. Problem was that the life expectancy of the panels was only 8-10 years.

    Presumably the rest of the system would last a good long time, but check the life expectancy and output/time curves for those panels.

  2. That's a bit hard to believe. Nearly all solar panels come with 25 year warranties to hold 80% of their original output. Even if these are optimistic, I can't imagine anyone actually promising 8-10 years on panels.

    This is one of the reasons to buy from Westinghouse -- I can be pretty sure that they will be in business in 20 years. These other companies? Maybe not.

  3. 1. Unlike your average journalist, it is likely your power company knows the difference between kWh and kW.

    2. Your calculation is off by more than a factor of 4. You used the default DC rating of 4 kW.

    You should use 235W per panel *4 panels = 0.94 kW. Keeping all else the same, I got an annual energy value of $100.18

  4. Westinghouse is mostly a licensed brand name nowadays, Westinghouse Solar is one of those active licenses and has only been in business for 10 years (and they use PHP :-).