Billing is done on the basis of net electricity used (or generated) during the month. This net amount is a function of the size of the solar collectors, the insolation in the area, the weather conditions and other factors (dirt build-up or collector maintenance, etc.)
The economics of the operation is a function of the contract with the electric utility and is also affected by government support. The utility either charges (or pays) a flat rate for the electricity or a rate that changes with the time of the day. If the second is the case, it's likely that the night rate is the lowest, and the "peak rate" is the highest. Peak rate is usually applied when the power plant is operating at nearly full capacity (usually because of high air conditioning loads on hot summer days). Under these conditions, receiving the excess solar energy is advantageous to the utility because it does not need to start up its emergency generators. Sending "peak electricity" to the utility is also advantageous to the homeowners if the rate paid is higher.
It is the utility company that takes government subsidies and regulations into consideration, so that the homeowner just receives a monthly statement without requiring any additional paperwork. Besides direct subsidies, the government in some countries guarantees a fixed rate for the solar energy sent to the grid. This rate is often higher than its "market value." In such cases the utility is required to distribute that extra cost among the "non-solar" households.
As shown in Figure 2, the electric meter is an intelligent one that automatically considers the electricity flows in both directions and displays the net total (cost or payment) at any time during the month. The software provided by the utility is automatically adjusted for times of day variations in cost rate or times of "peak periods," so the homeowner is not burdened with adjusting them.
The software in the intelligent electric meter should also be able to perform other control tasks, such as to automatically charge the batteries of the electric car(s) when the electricity is inexpensive ("night time"), and to maximize the amount of electricity sent to the grid (by temporarily turning off optional users) during periods when peak electricity rates apply on hot summer days.
None of this software, nor the intelligent net-billing utility meters themselves exist today, but as energy costs increase, they will. The purpose of this series of articles is not only to describe software that is available, but also to describe the control software needs of the future as renewable energy systems become more complex and sophisticated.
Our tax dollars should be invested in new technologies that guarantee an energy future that is inexhaustible, clean and free. The use of solar shingles is already cost-effective in the southern half of the United States and installing them would not only eliminate unemployment, oil imports, energy wars and the destruction of nature, but would also lower unemployment, because of the millions of carpenters, electricians and laborers that their installation would require.