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PV Costs and Output Calculations
by Steve Spence
sspence@green-trust.org
I have put together a spreadsheet that helps you calculate the costs and output of a Photovoltaic Power System. Follow these four steps to determine your loan payments, output, and costs.
1. Determine the size of your array
On our PV Sizing page, you should have determined the number of panels your system will need. Lets assume you needed a 5kW array. This is sometimes configured as fifty 100 watt panels. In cell B3 enter the size of your array in kW. We entered a 5 for our example.
2. Determine the amount of Sun Hours you receive in a day
On our PV Sizing page, we have a chart called "Sun Hours / Day - Chart" where you can find the number of hours of full sun for your area. We will enter 2.53 for our example (Schenectady, NY - Winter).
3. Enter annual interest rate for your loan
Now enter your interest rate in cell B11, and the number of years the loan runs in B12.
4. Compare the cost per kWh from your system vs. the utility grid
We have calculated the cost per kWh from the pv system in cell B29, and have entered our utility price in B30 for comparison. To compare your utility price look at your electric bill. You want the toatl electrical usage for the month in kWh's, and the total amount to be submitted in $. Divide the kWh's by the dollars like this:
Bill total = $36
kWh total = 90036 / 900 = .04 or 4 cents per kilowatt hour.
5. Assumptions and formulas
We have made some assumptions in the calculations that need to be explained.
In B21, we calculated the total system cost by multiplying the system size in watts by $8
In B22 we assumed the tax credit is 50% of the system cost. In some states, you get reimbursed for $6 / watt, up to 50% of the system cost. Visit the incentives page to determine your rebates, and substitute this value.
In B24 we made the assumption that system maintenance (batteries, inverters, etc.) would be 30% of the system cost over the length of the design life. Feel free to adjust this factor.