You’ll have no hot water in reserve when the power goes out.
You’ll have to put in new circuits and run a dedicated line to each unit you install if you do multiple electric units.
These work better if located very near the point of use.
You may need a permit/inspection in your location or with your insurance company before you can install.
Any savings depends on how much water you use.
Have you been using a timer on your tank heater? You can cut your heating timme from 24 to 2 hours a day and save a lot of money.
This is a big deal. During Hurricane Andrew, I was without power for a week, yet I was able to take a warm shower every day.
I have a 3500W generator hot wired into my panel box (mobile home) and I have practiced with what I can run and what I can’t during a SHTF episode ... 30 gallons of conditioned gasoline in storage, rotated every 4-6 months or so (like, when I feel like it)