maroda i eventually invested in an instant hot water machine. My dh is a tea monster...
So in the old days, I'd make up a load of bottles with powder, and previously boiled, cooled water. But i came to figure that wasnt the point. The point is not the purity of the water, but the powder.
So we did this.
Boiled a load of water, and kept it in the fridge in a previously sterilised container. Anything will do as long as it's clean/ miltoned. You can use the cold water for a day. We did this every day to have enough cold water available.
When ds cried for a bottle, 3am or whatever, we boiled the kettle. (after a while , we bought an instant hot water machine. Still use it. Bosch- has variable temperatures, 70C is plenty to sterilise the powder)
Chucked x scoops of powder in the bottle, fired enough hot water in to mix from kettle or machine (lets say 60ml) and topped up to correct powder-to-water measure from the cold water in the fridge.
If it was too hot we employed the running water method. If too cold, a quick blast in the microwave. But we got really good at the ratios and made a perfect temp bottle in less than a minute after some trial and error.
After 4-6 months, make a load of bottles from boiling, chuck them in the fridge, use within 24 hours. Clearly that last bit is not typical advice but its what i did 