There are lots of systems that you can remotely turn on when desired; i.e., a demand, if you wish, you can do it that way with readily available bits.

Assuming you are on pubilic water and sewer, part of your bill includes pumping the fresh water to you, and then back to the treatment plant. If you have your own well, you pay to pump it out of the ground. Essentially, the water is free, but you are paying for the infrastructure to get it to you then once soiled, away. Pumping the hot water around in your house if your pipes are insulated is a very minor cost. Mine is on a thermostat and a timer...if it runs 1/2-hour a day, I'd be surprised - that's probably in the order of 25W a day for the pump and maybe 200-300BTU in lost heat (good in the winter). Well worth the convenience. Many locales are now requiring them, since they feel they do save resources. It depends a lot on how your pipes are layed out and if they are insulated. From my WH to the furthest fixture is about 40' of pipe, some of it 3/4", and to get warm, not hot water takes close to a minute or more.