Huh. Never seen that AirPump before. Sez it draws 600+ watts, tho -- depending on its cycle time, that's a whole bunch. More efficient than electric resistance (which is awful), but that's a load of juice for RE to supply.
Even a small solar hot water panel can do wonders for heating water, and if you use a counterflow (external) heat exchanger you can add it to any tank water heater. A little Taco circulating pump draws less than one Amp. We only have a single 4x8 low tech panel, but over the course of the day it heats our well water from 45F to 105F. At that point, a second pump/heat exchanger turns on and moves some of that heat into the floor slab. It is really nice to have your domestic hot water most of the way to shower temperature; our Bosch/Aquastar demand heater barely needs to fire up.
The hot water heater element was an interesting quest. I had the storage tank custom made by Advance Metalpres in Canada; it's all stainless, because our water is so aggressive. They can add as many bulkheads as you like. But low-voltage DC elements are brutal, around $90 each. And you need huge wire to handle the amps. Thought about it awhile, then stuffed a $10 hardware store AC element into the tank. Wired it for 120VAC run thru a solid-state relay (silent, no moving parts). Even tho this dump load is technically for the sake of the wind turbine -- you can't let them run unloaded -- the relay is triggered by the v.smart solar controller. The turbine catches fire much less often, now that it has good dump load on it.
