BTU (British thermal unit)
A BTU is the energy needed to raise one pound of water 1 °F; stove heat output is rated in BTU per hour. Certified stoves publish an output range — roughly 10,000–20,000 BTU/h at low burn up to 30,000–80,000 BTU/h at peak, with about 25–35 BTU/h per square foot as a sizing rule of thumb.
The range matters more than the peak: a stove spends most of its life at low-to-medium output, and the low end determines whether it can idle in mild weather without cooking the room. The EPA database records the output range measured during certification testing.
For fuel comparisons, a cord of seasoned hardwood holds about 22 million BTU (MMBtu) and a ton of wood pellets about 16.4 MMBtu — before multiplying by the appliance's efficiency.