right been testing out a heater that you can mount in the van for the last week, in the house to start with. I have had it set up in the celler, pluged in and with a thermometer sitting about 4 inches away from it. the start temp was 19. turned it on and after the while the temp peaked at 26.4 it was pluged into a monator to mesure the power it uses. it was pluged in for 48hours with out being turned off and only used 4.4Kwh. The average cost for a standard domestic user in UK is about 12.5p per KWh. depending who you are with. so 48 hours constaint use costs £0.55p this can also run on an inverter if neede without draining the battery to much depending what leasure battery you have. but only as a one off as it will drain the battery. going back to the costs, some people use an Immersion Heater to heat the water. a standard immersion heater is 3kw, so it would take 2 hours to heat 100 litres of water. only if there is no heat loss, so 6kw per 100 litres. 500lt tank - 10 hours - 30kw used costing £3.75 pictures and more after tests in van next week!