Heat loss & insulation calculator

Before you argue about stoves, heaters, or fuels, there is one number underneath the whole conversation: how much heat your building loses. This calculator estimates that loss two ways, the peak load on the coldest design night, and the total energy a whole season takes, then translates the season into the three currencies off-grid people actually spend: cords of wood, gallons of propane, and kilowatt-hours.

Design load versus seasonal load

The design load, in BTU per hour, is what your heater must be able to produce when it is as cold outside as your climate realistically gets. It sizes the equipment. The seasonal load, in millions of BTU, is the sum of every hour of the heating season weighted by how cold each hour was, which is what heating degree days measure. It sizes the woodpile and the budget. Two buildings can share a design load and have very different seasonal loads if one sits in a longer winter.

The calculator scales a baseline loss rate by your climate's design temperature difference, your ceiling height (more volume, more air to keep warm), and the build-quality multiplier, which is doing the heavy lifting. A drafty camp can lose three times the heat of a super-insulated build of identical size. If you want the fuller version of this math applied specifically to wood heat, the firewood calculator runs the same logic straight to a stove size and cords.

The fuel row is a price list

A million BTU delivered into the room costs roughly: one fourteenth of a cord of hardwood through a decent stove, about 13 gallons of propane through an efficient heater, or about 293 kWh of electric resistance heat. Off-grid, that last one is the killer, resistance heat from a battery bank is the most expensive warmth money can buy, which is why serious off-grid builds heat with wood or propane and save electricity for everything else. A cold-climate heat pump changes the electric math by roughly a factor of 2.5, but only if you have the array and bank to feed it, which the solar calculator can size honestly.

Why the last row is the most important number on this page

The "one insulation level tighter" row prices the upgrade nobody gets excited about. Air sealing and insulation are permanent: they cut every future winter's fuel, shrink the stove you need, quiet the building, and never break or need refilling. A step from average to tight construction typically cuts heating energy about 30 percent, every year, forever. Compare the one-time cost of cellulose, foam board, and a weekend of caulk against buying that percentage of your woodpile or propane annually, and insulation is usually the best-paying investment on the whole homestead.

Chase the leaks in order: air sealing first (rim joists, penetrations, attic hatches, door sweeps), then attic depth, then walls and windows. The Shelter & Structures archive and Heating & Cooking archive walk through the projects themselves, in the order that pays.