With MECO's "Customer Grid Supply" (CGS) program, the solar pv system sends all excess energy back to the grid, after powering the loads in your house and charging your battery, if you have one. You receive credits for the energy you send back to the grid at a rate of .17 cents per kwh. This credit rate is subject to change, which is why a battery maximize your savings. Credits are banked on a monthly basis. Excess credits are not rolled over to the next month, they are lost. During the day, much of the Sun's energy goes to power the loads in your house, bypassing the meter. Only the excess is what you receive credits for. This program is back open temporarily, until October 21st, 2017. After that date MECO will no longer be accepting Grid Supply applications. Getting your application in now, will keep your options open. If you missed the opportunity, not to worry, MECO's Customer Self Supply is still open, but will require a battery to maximize savings.
The downsides of the CGS program, is that the rates are not a full credit and the current rate can change at any time. If you installed a Solar PV system, in order to cover 95% your MECO electric bill and the credit rate is lowered from .17 cents to .10 cents per kwh, your bill will go up and now your Solar PV system may only cover 85-90% of your electric bill. The Tesla Powerwall battery will insulate you against those future rate changes.
This issue can be mitigated by installing a battery to store some or most of the energy that was produced during the day, for use at night. This way you are getting almost 100% of the kwh credits, because you are sending much less energy through the meter, to MECO. Due the the "Round Trip" efficency of charging and discharging your battery, those kwh credits will be closer to 95%. This is very similar to MECO's Customer Self Supply program (currently open). Only, once your batteries are fully charged, you send any excess energy back to the grid and receive the .17 cents per kwh rating. While, the batteries will greatly reduce or eliminate this situation, it is better to get a partial credit for any excess power, than no credit at all.