Net Metering is the excess electricity your system may produce and is sent back on to the grid for your neighbors to use. Your utility will credit each Kilowatt Hour you send, and convert the value to cash which is applied to your monthly electric bill, or if you choose, may be applied to another account (within the same utility).
Some electric meters will run backwards, while newer meters will record “purchase” kilowatts (usually position 4) and “sales” kilowatts (usually position 10) separately. With the newer meters the sales and purchases are itemized on your bill, and in the event of a on going credit, carry that amount to your next bill … utilities will not send you a check.
To see Dept. of Energy Resources info on Net-Metering go to: http://www.mass.gov/eea/grants-and-tech-assistance/guidance-technical-assistance/agencies-and-divisions/dpu/dpu-divisions/legal-division/dpu-and-green-communities-act/net-metering/net-metering.html