A formula cannot tell you what the temperature in your city is going to be 3 days from now; you can get a range of possible values and a likelyhood; but no 100% surety.

Turn the temperature up on the tank.

Bottom line, who would be prepared for prolonged sub-freezing temperatures in Texas, California, Florida or any other place that typically doesn't see those temperatures?

You may also want the PM to validate the temperature in the units. 

Three days without any water or electricity during freezing temperatures. 

When I asked her if she let the water drip she said she didn't know she had to let the water drip although on the news they constantly told people to let the water drip due to the temperature dropping to the 20s for 2 days and ice was on the ground.

I have a remote vacation rental so I put a smart thermostat in it so I could monitor/control temperature remotely.

Meanwhile, a mini split is more like 250% efficient, because it uses one unit of energy to move 2.5x as much heat.This is effected by temperature.

A landlord must repair a condition if a tenant gives notice of a condition, the tenant is not delinquent in rent payment, and the condition materially affects health or safety of the tenant or is due to the landlord’s failure to supply hot water at a minimum temperature of 120 degrees Fahrenheit.

@Justin BrickmanMy recommendation would be to get WiFi thermostats and control the temperature remotely.Depending on state solar does not provide a significant ROI