Auber provides many options.
There is a pure power controller that can control up to SIX SSR's by percentage of power.
Digital SSR Power Regulator for Wort Boiling Control $33.95
http://www.auberins.com/index.php?main_ ... cts_id=444" onclick="window.open(this.href);return false;" rel="nofollow
Then there is the fancy stuff...can be set to reach a temp and alarm, stabilize at that temp or hold a temp for a given time, run element at a percentage of power,...PID thermo controllers, but even they have a manual mode for percentage of power.
EZboil, Power Regulator for Boiling Process Automation Starting at: $46.95
http://www.auberins.com/index.php?main_ ... cts_id=560" onclick="window.open(this.href);return false;" rel="nofollow
These seem expensive, because they are ten times the price of a rotary Potentiometer, but if you added the price of a volt/amp meter and they get more competitive.