Solar Energy Calculator for the Austin Energy Solar Rebate Program
Sol, our sun, shines about 1000 watts of power onto each square meter of the Earth. This is used by plant life to sustain the ecosystem of life. The remaining power is reflected into space, drives the weather, and eventually turns into heat. If we take a portion of this energy from the sun, it can eventually replace our energy needs currently met by fossil fuels, which life took millions of years to make. The use of solar energy does not add more heat to the earth, does not create green house gases which can affect the climate, and it leaves fossil fuels for their more valuable uses as raw materials.

Photovoltaic solar cells are reaching the cost levels that will make them the most economical and environment friendly source of energy. As demand for them grows, the efficiencies of scale will continue to lower their costs. If every rooftop were covered with solar cells optimally pointed toward the sun, and if this energy were fed into the grid and into storage, this could satisfy a major portion of our energy requirements.

"A 100 mile by 100 mile square of Texas covered with solar cells would on average produce enough electricity to run the country."

The city of Austin Texas has taken a wonderful, forward looking step toward using renewable energy and recently passed one of the best incentive programs for encouraging the use of roof-top Photovoltaic solar cells. Some raw solar cells are down to $3 per watt while the subsidy pays $5 per watt. Mounting, installation, and the inverters which connect them to the electric grid cost more. However, with a 25 year warranty, these cells are an attractive investment in our future. Try out the unofficial Solar Energy Calculator.

This unofficial calculator can be used to figure out your costs and benefits for a residential grid-tied photovoltaic solar panel electricity generating system, specifically considering the Austin Energy Solar Rebate program (although you can ignore this part if you are not in Austin).

Photovoltaic solar panels generate electricity when the sun shines on them, typically generating somewhat more than 100 watts per square yard or square meter in full, direct sunlight. They have no moving parts and thus are relatively maintenance free. They are flat panels mounted a few inches off the roof. This lets air pass between them and the roof, cooling both the panels and your roof, since the panels shade your roof. The panels generate slightly more electricity when they are cooler so the extra air passing behind them helps. The fronts of the panels are typically tempered glass able to withstand strong winds, weather, and hail about an inch in diameter. Presumably your home owner's insurance would cover any damage. The panels typically have a 25 year warranty. Their efficiency may drop to about 80% in that time.

A grid-tied system uses a device called an inverter to convert the direct current (DC) coming from the solar panels into alternating current (AC) matching your normal power line electricity. Any energy generated by the panels that you do not use goes into the electric grid, running your electric meter backwards. Thus, you get credit for the kilo-watt hours of energy you contribute. If you are very frugal and don't use more energy than your solar panels generate, Austin Energy will end up paying you at the end of the month. The inverter would normally cost just a few hundred dollars, but Austin Energy requires you to use a more expensive model which reports back to them the amount of electricity you generate. This inverter also has safety provisions to disconnect it from the grid when there are power line failures, for example. The alternative to a grid-tied system is a battery system which stores your generated electricity in batteries for use when the energy is needed. However, batteries are quite expensive, not as efficient, and more suitable to remote locations where electric grid energy may not be available. For these reasons, this calculator will not consider battery storage system calculations.

Fill in the values specific to your situation and then click on the compute button at the bottm. Further information can be seen by clicking on the "info" link next to any item.

Solar panel width = Centimeters Info
Solar panel length = Centimeters
Peak power per solar panel = Watts Info

Inverter efficiency = Percent Info
Average yearly solar angle, daytime, temperature and weather derate = Percent Info

Austin Energy rebate rate per solar generated watt = Dollars Info
Austin Energy cost per kilowatt-hour (kwh) = Cents Info

Each installed solar panel costs = Dollars times Panels = ? Dollars Info
Inverter and fixed installation costs = Dollars Info
Total cost of system = ? Dollars
Austin solar rebate program subsidy = ? Dollars
Cost to owner = ? Dollars
Break even time = ? Years Info
Earned after break even = ? Dollars per year Info

Solar panel efficiency = ? Percent Info
Area of all panels = ? Square meters or about a ? foot by ? foot area Info
Net power at peak sunlight considering inverter efficiency = ? Watts
Average generated energy = ? kilowatt-hours (kwh) per month

If you see "NaN" somewhere, this means "Not-a-Number" and is caused by an invalid input value in some entry field above. Check what you entered or click on the "Reset" button to go back to the default values.

This photovoltaic solar panel installation by Texas Solar Power Company of Austin, Texas.