Domestic & Shire Swimming Pool Heating
Solar is great for small domestic (potable) hot water, however, if you'd like to reliably and quickly heat a domestic swimming pool of eg: 60,000 litres from 15 C to 25 C within a day or two (and keep it at 25 C) you really need some serious power to make it practical. This tends to leave the energy choices to either diesel, LPG or waste oil, especially in rural areas where natural gas is unavailable out of town or on the farm.
If you do an approximate cost analysis, diesel is around $1.30 a litre, LPG is around $0.80 a litre with waste oil typically $0.20 a litre. Waste oil prices tend to remain the same, whereas diesel and LPG fluctuate depending on international trading circumstances.
LPG looks good so far, however, it's heating value (Mj rating) is 22 Mj per litre, whereas waste oil has around 38 Mj per litre. So, practically then, you need nearly twice as much LPG to generate the same amount of heat. Diesel and waste oil are normally around the same heating value. Go figure it out yourself, as prices constantly vary depending on supply & demand and your own location.
A 50 kW waste oil compatible water heater will typically heat the home, swimming pool, spa and domestic hot water with fuel cost at $0.20 per litre.