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Grid Tie and Net Metering A bit of Renewable Energy History
15 years ago before utility grid tie technology became advanced, most solar and wind systems had batteries. Batteries were the weak link in the system. They required maintenance and were as much as 1/3 the cost of the system. These were stand alone systems. With stand alone systems you had to use the energy at or near the time of production or the excess energy would be wasted. At times of high demand and low production backup energy from a generator was often needed. Therefore, the system had to be designed to maximize production at the time of greatest demand. Homes generally have the greatest demand for energy in the winter. Therefore, solar panels were often tilted up to steep angles to be perpendicular to the low winter sun angle. The peak production of solar energy, however, happens in the summer. Batteries can not store energy from one season to the next. They can only store for a few days.
Then came highly efficient grid interactive battery-less inverters and net metering. Net metering is an arrangement with your electric utility where you can send power to the utility for which the utility credits you at the same retail rate that you pay them. During times of over production during the day if your loads shut down the meter spins or records backwards and automatically credits you directly reducing your bill. At other times when you need more than you are producing you buy energy from the grid as usual. During the summer over production happens more than in winter. Thus you can build credits during the summer towards your winter bills. This is like having a giant annual storage battery without the cost and reduced efficiency of batteries. The down side is that during a utility power outage the system can not back feed a dead grid for safety reasons and must automatically shut down even if it is sunny. Therefore during outages you do not have power from the system unless you install optional batteries. If you choose to have batteries for backup power from the system you have to realize that you loose efficiency and the system will not save as much on your electric bills as without them. Batteries will need to be replaced at least twice during the life of the system. So it is best to get your backup power from a generator and delete batteries unless you believe the utility company is soon going the way of Iraq. Batteries can also be helpful if you have critical loads that must be powered without even momentary interruption.
The design philosophy of grid tied solar PV systems becomes completely different from that of stand alone systems. Unrestrained by the limitation of batteries one can design to maximize annual production. It turns out that the ideal tilt angle for a grid tied solar array is a low tilt that favors summer sun. One great advantage is that this tilt is similar to the standard roof tilt. This favors summer sun during the peak production time of year building more credits toward winter bills and actually increases the total annual production of energy. Also it is more aesthetic to have panels parallel to the existing roof plane than trying to raise them up.