The power source will heat water that's pumped through a heat exchanger (as in your domestic immersion heater). Here, hot water flows past the piped pond water so the heat can be transferred to the cooler pond water.

There are various stainless steel heat exchangers with different heating capacities (quoted in kilowatts (kW) or British Thermal Units (BTU)) suitable for raising the temperature of different pond volumes above the ambient temperature. These are available to Koi keepers through specialist suppliers.

Power sources include solar panels (but these are still quite obtrusive and expensive, although they are improving), gas and electricity. The risk of power cuts is a major downside of electricity, so you may need a back-up generator.