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Sizing and pricing of a solar panel
ENV 170, Due: Thursday, June 9th
Read through the following text and complete the bold calculations in order to figure out how much
sun we get in Seattle, how big a solar panel you would need to offset some of your power needs and
how much that panel would cost.
SUN!
The amount of electricity you can generate from solar radiation depends on how much sun you get,
which is measured in peak-sun hours (technically flat panel surface area facing the equator and
tilted at an angle equal to your latitude, or 48°N). In Seattle we get on average 4 peak-sun hours
per day. (If you are thinking of installing solar panels on your future vacation home in Colorado, you
can see a cool map of peak sun hours at: http://www.wholesalesolar.com/InformationSolarFolder/SunHoursUSMap.html).
PANEL SIZE
How much of your roof do you need to cover in solar panels? First of all, the sun generates 1000
watts of energy per square meter but our technology can only capture around 30% of that energy.
Therefore,
1. Solar panel output (W/m2) = __________
For every square meter of solar panel, your annual estimated power output should be,
AEO (watt-hours/yr/m2) = solar panel output (W/m2) * Seattle’s peak sun (hours/day) * 365 (days/year)
2. AEO (watt-hours/yr/m2) = __________
How many square meters of solar panel do you need to meet half of your annual power needs? The
average US house consumes around 11,000 kWh/yr. Let say you are energy conscious and half of
your needs would amount to 4,000 kWh/yr. Therefore,
Solar panel area (m2) = 4,000,000 watt-hours/yr
AEO (watt-hours/yr/m2)
Or, for square feet,
Solar panel area (ft2) = solar panel area (m2) * ___1 ft2__
0.0929 m2
3. Solar panel area (m2) = __________, or solar panel area (ft2) = __________
(Just for comparison, a typical roof area is 1000 ft2 or 92.9 m2.)
COST
What power capacity did you end up installing?
Watts installed (watts) = solar panel output (watts/m2) * solar panel area (m2)
4. Watts installed (watts) = ___________
It costs between $4-5/watt of installed solar panels. At $4/watt, what is the initial cost?
5. Cost ($) = __________
What is the cost minus the 30% federal tax credit (with no cap! Thanks, Obama!)?
6. 30% tax credit ($) = ___________
7. Cost minus 30% tax credit ($) = __________
WHAT YOU PAY NOW
Your power is cheap! You pay 6¢/kWh in Washington State. What would you pay if you didn’t
install solar panels?
Cost = 8,000 kilowatt-hours/yr * 0.06 $/kilowatt-hour
8. Cost of power without solar panels ($/yr) = __________
What will you need to pay for power you’ll still need if you DID install solar panels?
Cost = 4,000 kilowatt-hours/yr * 0.06 $/kilowatt-hour
9. Cost of the remaining power you’ll need to buy ($/yr) = __________
PAYBACK PERIOD
How long would it take for your solar panels to be “worth the cost”? This concept is called the
payback period.
Payback period (yrs)
=
Cost minus 30% tax credit ($)
Cost of power without solar panels ($/yr)
10. Payback period (yrs) = __________
FYI: You can also earn money from selling power back to the grid, called net metering, IF you
installed more power than you need (which you didn’t in this example) or you generate power at a
time of day when you don’t need it. If you buy solar panels manufactured in WA, you will earn
36¢/kWh in net metering (6 times the amount you pay for power!), if you also purchased an inverter
manufactured in WA the whole package will pay you back at 54¢/kWh (9 times what you pay for
power!) Also, the whole system is exempt from state sales tax.
Remember, you can find all the state and federal decentralized power production incentives at the
Database of State Incentives for Renewables and Efficiency, http://www.dsireusa.org/.