2.8kw electric heater by Bestway

Box says heats up to 4,500 gallons but in our experience, a 2.8kw heater will easily cope with up to 2000 gallons (a 12ft round pool) and just take the chill off anything larger
Digitally controlled with soft touch control panel
Water flow sensor for added safety, no water flow, no heat
1.25" hose connections, attaches to cartridge or sand filters with a maximum flow of 1000 gallons an hour