Would it be better than tankless?
First of all, there’s no “guidelines” for horsepower-Watts conversion. One horsepower is always exactly 745.7 Watts. They’re just two different units for measuring the same thing.
Second of all, “a couple hundred watts a month” is a meaningless unit for almost any purpose. The unit you’re looking for is just Watts: They already have the “per-time” part included in them. If you get a generator that provides you with, say, 200 Watts, then over the span of a month, it’ll give you 527 megaJoules of energy. If you run it for two months, you’ll get 1054 megaJoules of energy, and so on. Power (which is measured in Watts or horsepower) is energy (measured in Joules, BTU, calories, or a bunch of other units) per time, and something like a generator will generally have a fixed power value, not a fixed energy value.
I second this, and what it adds will pay you back when you sell. There maybe tax value for doing it, too.
A solar powered “attic” fan/vent is a good purchase, and will keep the home cooler too. Saves on AC costs.
Well tankless is very efficient if you are going to continue heating your water with gas or electricity. But, it’s expensive and may be difficult to install. Solar can give you essentially free hot water (at least in the summer). The best situation is solar boost with a tankless backup, but then you are talking real money.