Metering hot water usage
 

Does anyone know how much hot water is used in an average house, compared to total water usage?

Like in most houses, we have a water meter, so I know how much we use, but have no idea what percent of it is hot water. When we install a hot water tank next year I plan to put a meter on it, but maybe someone has already done this?

Thats a good question. I don't believe I've ever seen a statistic showing that.

Someone with an on demand water heater could tell you based off their gas use.
I personally think it's about half and half, mostly due to toilets.

And that reminds me of another thing I'd like to do when renovating our plumbing: have the toilets on a separate supply pipe. This would not only allow me to put a meter on just the toilets, but would also ease switching to grey or rain water for flushing.

I read somewhere that a 300 liter (79 US gallon) hot water tank is recommended for a 3-4 person family. I doubt that's how much water is actually used during the whole day (our family of 3 uses an average of 250-300 liters total per day, both hot and cold). One of the reasons I started this thread is to have an idea of how to size the hot water part of our plumbing when we start renovating.