The US Energy Information Agency has estimated that the average U.S. household uses 11,000 Kwh per year or about 30 Kwh per day.
Solar Panel Output
How much power can you get from solar roofing panels? This will depend on the efficiency of the panel’s solar photovoltaic (PV) cells in converting solar light energy into electrical energy.
The efficiency of a panel is defined by a complicated formula that assumes optimal sun exposure. An 8% efficient panel will require twice the area of a 16% efficient panel to produce the same output.
Although, we often hear of marvelous advances in solar cell technology in the laboratory, the commercial solar cells available today represent the current state of the art.
Commercial solar roof panels average 14% efficiency and produce about 14.4-w/sq ft (155 w/sq m). Standard solar roof panel sizes are about 13.5 sq ft., and each panel produces about 200 watts. Wholesale prices for these panels average about $1 per watt.
Is That What You Get?
That is a lot of power if you can utilize it cost effectively. Actual panel power output will depend on your homes geographical location, roof orientation and the chosen solar power system. Is your goal a completely off-grid system that fulfills all your electrical energy needs, or is your goal a grid-tie system to decrease energy bills?
The amount of yearly sunlight available across the continental U.S. averages about 5 hours. The northeast sees only 4.2 hours of full sun, but the southwest enjoys 5.5 hours. Assuming an average of 5 hours of sunlight, a 200-watt panel would produce 1 Kwh/day or 365 Kwh/year. Panels sell for about sunlight.
Solar System Type
For an off-grid system, a Derate Factor must be included that adjusts for the efficiency of an off-grid solar power system in transferring panel output to your home outlet. A common value used is 0.77. Therefore, a 200-watt panel will ultimately produce only 154 watts – 281 Kwh/year.
In a grid-tied solar power system simpler set-up, power goes straight to your utility meter. The power generated from your solar panels goes into the energy grid and offsets the cost of power you use during sunless hours. With this direct connection, a grid-tied system does not suffer from this loss.
Assuming your home uses the U.S. average of 11000 Kwh/year, an off-grid system would require 40 panels spread over 540 sq ft of optimally oriented roof space. Cost for panels would be about $7800. With an off-grid system, you would need inverters to convert the power to AC and batteries for storage. A complete off-grid system could easily cost $20,000.
A grid-tie system would only require 30 panels. With an inverter to connect to the grid, a grid-tie system would cost about $12,000 and – in theory – totally offset your energy bill. However, a grid-tie system can be scaled down to only offset a portion of utility costs.
According to the US Energy Information Agency, the current average price of electricity is about 11.3 cents per Kwh resulting in a monthly bill of $103.67 or $1,244.04/yr. However, the variation in cost of electricity by location is greater than for the hours of sunlight and may be a more important variable.
The necessity for government incentives probably shows that solar energy simply has not reached the ROIs making them practical for the majority of homeowners.