ESTIMATED SYSTEM RUN TIME
BATTERY CAPACITY:
660 Ah @ 8 hour discharge rate. 48 volts x 660 Ah = 31.7 kWh.
31.7 kWh X .93 (inverter conversion efficiency) = 29.5 kWh
29.5 kWh X .25 (maximum 25% depth of discharge) = 7.4 kWh available capacity
ELECTRICAL USAGE:
420 kWh per month / 30 days = daily average usage of 14 kWh
14 kWh per day X .75 (75% emergency load) = 10.5 kWh/day emergency usage
7.4 kWh / 5.5 kW (100% inverter capacity) = 1.3 hours full-load run time
7.4 kWh / 14 kWh (daily average usage) = 12.7 hours normal load run time
7.4 kWh / 10.5 kWh (est. daily emergency usage) = 16.9 hours emergency run time
ASSUMPTIONS & VARIABLES:
Since the cells are used surplus, actual capacity will probably be less than indicated by the nameplate rating. However, since the rate of discharge in actual service is spread over 12-17 hours instead of the 8 hour period used for the nameplate rating, the actual amp-hour capacity of a new cell at the C/20 discharge rate is probably closer to 725-750 Ah. Finally, the emergency load estimate (75% of normal load) is probably on the high side. Actual emergency loads may be as low as 50% of normal loads. My guess is that most of these factors may cancel each other out.