Tools you might need next
Calculate how many solar panels you need from monthly kWh usage, panel wattage, sun hours, roof orientation, and system efficiency losses.
Calculate solar battery bank capacity in kWh for off-grid or backup power from daily load, autonomy days, depth of discharge, and inverter efficiency.
Calculate wind turbine power output in watts from rotor diameter, wind speed, air density, and turbine efficiency using the Betz limit formula.
Calculate heat loss through each surface: walls, roof, floor, windows, and doors. Sum BTU/hr lost through each component based on area, R-value, and indoor-outdoor temperature difference.
Q_cond = A × ΔT / R (BTU/hr)Air leakage carries sensible heat out of the building. Estimate infiltration BTU from building volume, air changes per hour (ACH), and temperature difference.
Q_inf = 1.08 × CFM × ΔT; CFM = (Volume × ACH) / 60Sum conduction and infiltration losses at the 99% design outdoor temperature for your climate zone. Add 10–15% margin for extreme events on non-modulating equipment.
Total BTU/hr = Σ Q_cond + Q_inf + MarginUpdated: July 2026
A 1,800 sq ft single-story home with R-20 walls, R-38 attic, double-pane windows, and 0.5 ACH at −10°F design temp loses roughly 45,000–55,000 BTU/hr. A 60,000 BTU furnace is appropriate.
Original R-3 walls, single-pane windows (30% of wall area), and 1.0 ACH push heat loss to 80,000+ BTU/hr for 2,000 sq ft. Insulation upgrades cut load by 40% before equipment downsizing.
A 2,200 sq ft passive house with R-40 walls, triple-pane windows, and 0.1 ACH may need only 12,000–15,000 BTU/hr at design conditions — often served by a mini-split or small heat pump.
Sizing for −20°F when design is −5°F oversizes equipment by 30% or more. Use ASHRAE 99% design temperatures for your nearest weather station.
Roof, floor, windows, and doors each have different R-values and areas. Calculate each surface separately — a single average R-value misses window and infiltration losses.
Proper furnace and boiler sizing starts with an accurate heat loss calculation. This tool estimates total building heat loss in BTU/hr from construction details, insulation R-values, window and door areas, air infiltration, and local design temperatures.