Well the siemens Logo PLC doesn't use much power. The tanks are on regardless. However we have a free power/gas day sunday 9am till 5pm. you wont be surprised to hear thats also WC day. Also on main tank water temp is reduced at night by a degree or so by the software, so a little saving there as when it returns to normal temp the heating is on. Minor in real terms. As to the cost.... dont look. Do I dream the DIY nope. see an issue and put it to back of mind, answer just pops like a flashing Icon. sleepless nights at times yes but that is rarely productive.