Question about restarting water heaters

[Warpy]

1. it's going to have to heat that 40 gallons no matter what

and the only difference I can see in starting with an empty tank is speed on those first few gallons. If you are absolutely desperate for a hot shower and don't mind running out in the middle of it, finishing in cold water, it might be a plan.

However, you are going to use the same amount of energy to heat 40 gallons of water, whether you do it all at once or start with a dry tank.

The best way to save money is to turn down the temperature in the summer, when you're not using the hot water for heating.