Here is some additional information about the battery technology directly from the manufacturer
Eliiy Power wrote:Battery Cell Specifications
Nominal capacity: 50 Ah
Nominal voltage: 3.2 V
Weight: 1.5 kg
Size: W 170.5 x D 43.5 x H 111.9 mm
Energy density: 106 Wh/kg
Operating temperature: -20 to 60C (guaranteed operating temperature: 0 to 45C)
Cathode material: Lithium iron phosphate
Anode material: Carbon
This data makes me wonder whether Enphase' AC Battery will store 1.6k Wh of energy rather than the 1.2 kWh originally indicated. The reason is that each of these cells stores 160 Wh of energy and ten cells in series gives you an overall battery output voltage ranging from 28 V to 36 V, which fits perfectly with the S280's "Peak power tracking voltage" range of 27 V to 37 V. Ten cells would weigh about 33 lbs. Add in 15 lbs. for two S280s, a BMS, wiring and connectors and another 10 lbs. for packaging and you are right around 60 lbs for the unit, which should be manageable.
So, the question becomes: Is a ratio of 1.6 kWh/560 W proper for the intended purpose of time-shifting, or is that too much energy storage? For my system, I would need about 18 of these (~10 kW) to keep nearly all of my extra production off the grid, which would equate to 28.8 kWh of batteries. That seems to be a bit much storage without the ability to provide backup (though about right for backup), so perhaps they really are going with the smaller cells. Perhaps sizing the AC battery for time-shifting is similar to sizing microinverters to modules: handling the 100% case is overkill and you make out fine handling the 70% case. I suppose with the battery it depends on the regulations where you live.
The cycle life claimed for this battery is very impressive:
Eliiy Power wrote:What’s more, they offer an extended lifespan: even if charged and recharged repeatedly for 10 years (approx. 12,000 times), they will retain 80.1%* of their electricity storage capacity.
*Estimated value assuming 23ºC room temperature and three full charge and recharge cycles per day (depth of discharge (DOD) = 100%).
Hopefully those numbers are real and include the effects of calendar life as well and not just the cycling.