klapauzius said:
Also note that a battery backed up system is slightly less efficient (~5%) due to losses in the battery charge controller.
Efficiency would be an issue. Charge controller efficiency is about 98%, which is not bad, but the efficiency of the inverter can make a huge difference. IIRC, the efficiency of the 6 kW XW inverter at full load is about 89% while the new Enphase M215 CEC weighted efficiency is 96%. At 10 kW, that's a 700 W difference to the grid, even ignoring charge controller, battery, copper and shading loss differences. That equates to about a 1-MWh/year difference around here.
The other big difference is in system life/redundancy. If I use the following MTBFs: XW - 10 years (generous?), M215 - 230 years, PV - 600 years, I can expect the following for a 10-kW system:
XW battery-backed system: After 5 years, on inverter will have failed, taking down half the system. After 10 years, the other one will have failed.
M215 system: After 25 years, the warranty for the inverters and the panels finally expire. By that time 2 of the inverters and one of the panels, out of 42 each, should have failed. Even if I did not make any repairs to the system during that time, 93% of my system is still available. Of course, the failed inverters would have been replaced under warranty and might have been easy to replace, depending on their location in the array.
During a 30-year period, the microinverter system should produce an additional $3,000-worth of electricity (at $0.10/kWh!) and should save over $24,000 in repairs. I have ignored failures in the batteries, charge controllers and other components, which also likely will fail.
The bottom line is that central inverters have not achieved a level of reliability anywhere close to what can be achieved in a microinverter system. This is primarily due to the inability to derate the capacitors to a level which would give them a long life.