Pick the one closest to yours — it changes which questions we ask next.

Measure the inside of the area you want warm — the living space, not the whole vehicle. Or pick a common model and we will use typical figures.

Pick a model above, or fill in a length, width and height.

Be honest here — it moves the answer more than anything else we ask.

Pick the coldest night you actually want to be comfortable in, not the average.

Windows, portlights and rooflights lose heat several times faster than an insulated panel.

Water is usually a few degrees above freezing even in winter, so a submerged hull loses far less heat than a panel facing the night air.

A single-glazed windscreen leaks heat badly, so it is worth counting separately. If you say yes, make sure the length you gave includes the cab.