DAILY VARIATION OF SOLAR ELECTRICITY PRODUCTION AND USAGE

One logic suggests that since solar operates during the day, when the peak power requirements exist. that solar power should get extra credit for meeting the peaking / backup requirements. While this might be partially true, this effect is diminshed by the fact that solar power is not constant throughout either the year - or even during any single day, requiring back up peak power.

The following logic and calculations quantify this fact.

The amount of electricity produced by any solar panel is related to the amount of solar energy that it sees.

If the solar panel is rotated so that its face is always perpindicular to the sun then it will produce a constant amount of energy as long as the sun is shining. (This ignores the relatively small reduction that occurs for the hour or so just after sunrise, and the hour or so just before sunset, during which time the sun light must travel through much more of the atmosphere.)

However, if the panel is fixed rather than rotating, as most panels are, then the amount of energy produced will be related to the degree to which the panel is not perpindicular to the sun. The reason is simply that the panel presents less of a surface area to the sun's rays.

Of course, solar panels do not produce any electrical power after the sun completely sets.

Unfortunately, the peak rate of electrical energy production from a solar panel does not match the peak rate at which electrical power is needed.

The following CAISO chart for September 29, 2010 shows how power usage varies throughout a typical hot summer day. Note that the peak usage (the red line) lasted from 4:00 to 5:00 pm, well after the sun was well down in the sky, and is still over 90% of the peak as late as 8:00 pm.

For comparison of summer use to todays use click here to view the CAISO's home page containing a graph showing the actual and predicted system power needs for today. Note that this usage is for the whole State, not just Marin County.

This shift is largely caused by the increased air conditioning load, which in turn is largely dominated by air conditioning in the hotter portions of the State. There is no readily available data on the timing of the mid-summer peak for cooler Marin County although it undoubtedly is not shifted away from the normal 8:00 pm peak as it is for the hotter regions.

To be valuable as peaking power, the solar panels must provide their calculated amount of power at all times during that specific peak demand time period.

The elevation of the sun, the angle that it is above the horizon, varies throughout the year, and of course during the day.

Click here to see a table of sun elevation data provided by the National Oceanic and Atmospheric Administration (NOAA) Solar Position Calculator.

This calculator shows that the altitude of the sun in late August at 5:00 pm Daylight Savings Time in Marin County is only about 10 degrees above the horizon.

Thus, at the end of the peak daily period a panel located on a flat surface the solar intensity would be only 17 percent of the maximum and the solar panel would be generating less than 20 percent of its maximum noonday power, severely limiting its use for producing peak power at the 5:00 pm peak.

However, if the panel is mounted on a South Westerly oriented roof with a roof slope of 20 degrees, the angle of incidence would be 40 degrees and the solar power production would be about 50 percent of the maximum noonday power.

Also, if the solar power maximum rating were already being counted on as part of the base supply, there would be no peaking power advantage and would be a significant requirement for other power when it was at less than its maximum capacity.

Note that the above logic would not necessarily apply if the solar system included a very expensive battery power storage system capable of providing all of the power that was needed when the solar panels were not working. This expensive system is not what is being installed in most cases, where the user continues to be connected to the power grid for power when the panels are not providing the full need. Alternatively, there are under development methods to store thermal energy from large arrays of solar reflectors so that the stored thermal energy can be used to provide power after the solar source has gone down.