/diy/ - Do It Yourself

How difficult would it be to design a system like this for my pumps?
>A ~100w pump that needs to run 24hrs
>It's connected to a solar panel and battery
>during the day it runs off of solar power and the battery charges
>at night or when there isn't enough power from the panel it receives power from the battery
>when the battery is nearly depleted and there isn't enough power from the solar panel it begins using mains electricity to run and charge the battery at the same time.

The last one is the one that scares me, I've seen people run small aerators with batteries and solar but messing with mains electricity scares me