Heat pump
Look up Coefficient of Performance (COP). I recently bought a 2kW system for a 60,000L tank which has 6.5+ COP.
So using 1000 Watt of electricity will generate 6500 watts of heat. Make sure the top of tank is covered. Even a clear plastic sheet (over a PVC frame) will make a difference.
You can run off solar panels during the day as well, depending on your location.