DIY Heat Load Calculator from Utility Bills (BTU/hr)
Size Your Furnace or Heat Pump
Use your real-world energy bills to estimate your home’s heating load in BTU/hr.
This free calculator works with natural gas, propane, or electric bills, and factors in your ASHRAE climate zone so you can size a furnace, boiler, or heat pump without guesswork. Perfect for DIY homeowners comparing systems or checking contractor quotes.
Enter your billing period, average outdoor temperature, and fuel usage to see your design heat load, plus your home’s BTU/hr·ft² and benchmarks by house age and efficiency. You can also adjust the outdoor temperature slider to see how your burn rate per hour changes at different conditions—not just the design point. Pair it with the energy efficiency calculator to compare your actual run rate against fuel costs and system performance.
Works for older, draftier homes as well as recently insulated houses with high attic R-values and good air sealing.
Common searches this tool helps with:
“heat load calculator from gas bill,” “propane usage to BTU/hr,” “how many BTU to heat my house,” “BTU per square foot by climate zone,” “size furnace from utility bills,” “DIY home heat loss calculator,” and “design temperature ASHRAE heat load.”
BTU Load from Utility Bills
Inputs
Results
Benchmarks & Diagnostics
Quick checks: look for duct leakage (attic/basement), attic bypasses, door seals, and furnace efficiency. Consider a basic thermal camera and a simple airflow/RTD probe to spot cold air infiltration and low-temp supply issues. Thermal camera guide · Airflow + RTD basics
Best Bang for Your Buck Changes to Make
Enter prices to see rough payback. Savings depend on home specifics; this is a directional guide.
Heat Load from Bills — Frequently Asked Questions
What does this calculator do, in plain English?
It estimates your home’s heating design load (BTU/hr) from real utility bills (natural gas, propane, or electric). You enter a billing period, average outdoor temperature for that period, and basic system info. We scale your real heat use to your ASHRAE 99% winter design temperature to approximate the peak load a furnace, boiler, or heat pump must cover.
How accurate is this vs. a Manual J?
A Manual J is a detailed room-by-room model. This calculator is a data-driven estimate from your bills. It typically lands in the right ballpark for existing homes and is great for sanity-checking contractor quotes. Expect some variance if your bill includes non-heating loads (water heater, cooking), if your thermostat has big setbacks, or if weather during the billing period was unusual.
How do I find the average outdoor temperature for my billing dates?
- Your utility bill: many bills include “Average temperature.”
- NOAA NCEI: use daily Mean Temp (TAVG) or (TMIN+TMAX)/2.
- Meteostat or Weather Underground – History: average your billing dates.
- Degree-days shortcut: if you have HDD (base 65°F), average temp ≈
65 − HDD ÷ days.
Natural gas vs. propane vs. electric — how do you convert usage to BTUs?
- Natural gas: 1 therm = 100,000 BTU (or convert CCF/MCF via utility therm factor).
- Propane: ~91,500 BTU per gallon (varies slightly).
- Electric: 1 kWh = 3,412 BTU; for heat pumps we use COP to infer delivered heat.
What is a “good” BTU/hr·ft² number?
- Post-2010 / very well sealed: ~10–20
- 2000s–2010 upgrades: ~15–25
- 1970s–1990s typical: ~20–35
- Pre-1960 or leaky: ~30–45+
Your upgrades can beat these ranges.
How do I use this to pick equipment size?
Choose output close to your design load. Fixed-stage gear often uses a small buffer (next standard size, ~10–20%). Modulating/variable-speed aims closer for comfort and efficiency.