I'm upgrading to a tankless water heater in my home and the one I got requires 2 d batteries to function. I have 120v and 12v available at the location so I was thinking that it would make sense to put in a converter or transformer to step down the 12v to 3v and hardwire that in so I'm not replacing batteries all the time. I estimated that about 3 amps of 3v DC would be similar to 2 d batteries. Are there any unforeseen consequences of doing this and would 3a be reasonable or would dropping it lower make sense.