Going by this figure of 65W per square metre - I would be looking at 13kW per hour though the night on fine days, but if we expect bad weather, we can light the fire in the dinning/lounge kitchen area and drop this to 3-5kW per hour in the bedrooms.
So planning on 16 hours x 13Kw that is 208kw for overnight on fine days, or two winter days with the fire going. We are setting up in-home childcare, so we need to heat most rooms for most of the day. Where do I go from there in terms of Wattage of risers and size of tanks, as there are other factors such as
* how effective the radiators will be as the water cools (what is the minimum temperature you would want your water to drop to?)
* How many days of cold water weather is it worth storing up (that is, how quickly will the tanks lose their heat naturally?)
Don't count your weasels before they've popped. And now for a mulberry bush related tiny ad: