15kw on a 50 amp circuit is not gonna happen either, hate to say it. Do the math, 15kw at 240 volts is over 60 amps, and that is before you count the power factor stuff on those cheap kits. So you would need a 70 or 80 amp breaker and wire to match, which gets pricey fast. Plus dimming the lights in the whole house usually means you have a loose neutral or a bad panel connection, that is a separate problem that a new circuit might not even fix. My old man ran a 10kw unit on a 60 amp sub panel out in his shop and still had to baby it on long heats. You could get a sparky to look at the panel for a couple hundred, but throwing more money at a $400 kit to make it work might just be chasing good money after bad.