The perpetual issue in battery-based system design is how to charge the battery bank to 100% full, preferably daily, but at least once a week. Problems arise when there isn’t enough sun to meet the daily load, and after a few days, the battery has less opportunity to get completely full.
Whenever budget allows, it can be beneficial to oversize the PV system. During sunny weather, the modules will stop charging the battery earlier in the day—but during periods of cloudy weather, there will be more PV production to boost the batteries and less reason to turn the generator on.
With a larger array, specifying a smaller battery bank becomes possible, since it is also more likely to get fully charged on a regular basis. If the loads can be coordinated to operate during the daytime while the PV array is producing, then a smaller battery capacity can be used successfully.
With module prices significantly declining over the last few years, and battery prices climbing, increasing PV array size is viable for more system owners. Additionally, diversion loads (such as hot water elements) can be utilized to take advantage of excess PV system output on sunny days.