I am doing basically the same thing but with a 36 volt/ 2400 watt element. I have tried different scenarios with panel sizes and number of panels. I have the same heating element but split the contacts to only use 1/2 of it per panel string. Best combination that I found was 3 - 305 watt panels wired in parallel. which gave me appx 36 volts and 23 amps. 3 panels on the south east of my house and 3 panels on the west side. Two strings wired to the same element but with the element not tied together on the posts. Pulls solar from about 9am to about 7 pm in the summer. Both sets of panels (connected to different sides of the element) produce 600 watts each + when the sun is at best angle. This DC element is in the bottom of our regular hot water heater tank. Still have the upper element wired with grid tie. Thermostat set at 120 but the upper element is on a timer to come on at 5 am till 7 for early showers. And then back on at 5 to 10 pm if needed on cloudy days. Currently no thermostat on the DC element just direct connect. On sunny days it has no problem at all getting to 150 degrees even with my wife washing clothes or cleaning during the day. I have a temp probe on the tank to set off an alarm if it hits 155-160 so we can disconnect the panels with a 20 amp blade style breaker. I tried a few different dc thermostat type devices but they all failed. In the last week I found a 30 amp DC thermostat switch that can handle the amps and voltage and it is on the way. It is from a heating system in a tractor trailer heating rig.