Coincidentally, last week I ordered
PWM regulator for heaters with an algorithm for finding the maximum MPPT power point from photovoltaic panels. The regulator enables powering standard electric DHW boilers from solar panels. It is fully automatic and starts working as soon as you plug it in.
www.ebay.com
I got lucky and found a new 50-gal water heater on Craiglist for $200. This will be a preheat tank feeding my existing water heater, which I will leave intact. I've been doing research and thinking on the pros and cons of different methods for a couple of months, and settled on this approach.
Selling points of this controller are the mppt, and that it rapidly switches the dc to simulate ac, to prevent arcing on the heater's thermostat contacts. It also has a settable temperature sensor which I don't think I'll need. I don't know when I'll start on the installation, but I will post the progress.