Is there a commercial AC inverter that will use grid power if the DC input is too low (not grid-tie)?
For example: If you had a refrigerator that averages 500 watts, and a 700 solar watt array, the device would power the refrigerator for 4 or 5 hours during the day and switch back over to grid power at night. It would not have to be any more electrically complicated than a 1000 watt UPS. The inverter would power the fridge when it is on and store extra power in the battery when the fridge is off. Once the output from the solar panels drops and the battery voltage drops, the device would switch to grid power. I believe this device would be cheaper because a 1000 watt UPS (with a bigger battery than would be required for this application) costs about $500, while a grid-tie inverter from a reputable company costs well over $1,000 + installation by an electrician.