How Much Energy Can You Get from Solar Electricity
Here is a map that shows the average amount of kwh produced by a 1 kilowatt solar power system each day in each state of the USA. The amount of electricity that is produced by each kw of solar power that you install varies greatly depending on your location. It also varies by the season and so the estimates given in the map above are annual averages. Please also note that in some states different parts of the state have very different climatic conditions and so whilst the amount quoted above may be true for some parts of the state it may not necessarily be the case where you live. You should speak to your local installers about exact production at your specific location. Across the USA daily production per kilowatt installed varies from as little as 2.9 kwh per kw per day to close to 4.7 kwh in very sunny locations.
The amount of electricity produced by each kw of solar you install is primarily a function of the solar irradiation that falls on your home or business. Solar irradiation is often measured in Sun Hours. To work out how many Sun Hours falls on each area meteorologists measure the total amount of irradiation that falls on a place in a day (in mega joules). They then work out what this is equivalent to if it were converted to complete hours when there was 1000 mega joules per square metre of solar irradiation falling on an area. So if there was 500 mega joules falling on average over a 12 hour day then the Sun Hours would be 6. The reason this ties back into the output of solar panels is that solar panels are rated based on the power they produce with 1000 mega joules per square meter of irradiation falling on them. Once we have the Sun Hours as a way of measuring the irradiation we can then work out the actual amount of power we will produce by allowing for the many factors that cause real world solar power systems to produce less than their maximum rated output. These derating factors include:
- Inverter inefficiency- most inverters will lose 3-5% of electricity in converting it from DC to AC
- Cable Losses- small amounts of power are lost through resistance in the cables;
- Dirt- dirt and grime on solar panels will reduce their real world performance;
- Temperature losses- solar panels are rated based on what they produce at 25 degrees celsius. As the cells in solar panels get hotter there is more resistance to the flow of electrons across the cells and so their power output reduces compared to when they are getting the same amount of irradiation at a lower temperature.
All up total losses due to these derating factors will generally be between 20-30% but when we are working out the real world power of a system we usually use a derating factor of around 25%. Please also note when using the map above that the estimates assume a perfect installation being due south, at an optimal tilt angle, and unshaded.