Do not power an AC hot water heater with DC, as it will fuse the thermostat contacts. It is a fire hazard.
The contacts in an AC thermostat don't part wide enough to break DC, as it will arc, create heat and fuse the contacts shut.
Solutions:
1. Replace AC thermostat with DC thermostat (Limit: 50VDC): $12.83 @
https://windandsolar.com/dc-water-heating-elements/
2. MPPT controller: $299.95 + $18 S/H @
http://techluck.com/
3. DC -> AC inverter: $1697.00 @
https://greenetechenergy.com/product/sun-bandit-sb-1200-b-ppk-off-grid-solar-water-heater-inverter-system-1-4-pv-panels/
