Currently: temporary 500gal intex pool. 50w pond pump, can't remember what brand but it is lower wattage for the 8,000ltr per hour turnover than others i could find. The pool has a layer of bubble wrap around the outside and a plastic sheet over the top to help keep the heat in and 3 300w tank heaters in the filter so all the water runs over them. Since putting the bubble wrap around and the plastic sheet over the top the heaters are on far less than they were. Water change is about 20% per week (100gal ish) syphoned out then slowly topped up with a hose. I don't use dechlor and have never had any issues with not using it even when doing 75% water changes on tanks in the past in this area.
The permanent 1000gal wood framed tank/pond i'm building has much more insulation in the walling to retain more heat. Insulation seems to be a much more important factor with these large builds than small tanks to help reduce heating costs. The same filtration and pump will be moved over from the 500 to the 1000 and still use 3 heaters. I have a couple of 55gal plastic drums ready to turn into water butts for rain water to help with the water changes but will probably need more than 2! When the indoor pond is complete i'll probably look into digging a heavily planted pond in the garden to drain the water change water into to help recycle it and if possible use it for future water changes if it all works right. Any ways of reducing bills is a good thing!