I use an Ariston 4 gal. Got it at Lowe's for about the same price mentioned in the previous message. Takes about 20 minutes to heat the water from dead cold to when the thermostat shuts it off. Has an on-off switch and I just flip it on in the morning when I run the generator to charge batteries and make coffee. Water stays useably hot all day, but I don't use much. Easy to install, just a hot and cold connection and comes with brackets so you can mount it on the wall, mine's in the cabinet under the kitchen sink. If you just want hot water at the sink, just tee into the cold water line under the sink, add a male adapter to the tee, connect two supply tubes, plug it in and you're done. Takes up about as much room as a dishpan stood on end.
Long as you don't have a particular affection for long, luxurious showers it seems to be adequate.