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Heat Load Calculator from Utility Bills (BTU/hr Without a Manual J)

Estimate the design-day heating load of your home in BTU/hr from a real heating bill: enter the fuel used in a billing period, the average outdoor temperature, and your indoor setpoint, and the tool scales that usage to your design temperature. Includes BTU-per-square-foot benchmarks by house age and a quick look at envelope upgrade savings.

A heating-load estimator that works backward from fuel you actually burned instead of forward from construction assumptions. Enter one or more heat sources for a billing period (natural gas in therms or CCF, propane, heating oil, electric resistance, or electric heat pump), each with its fuel quantity and appliance efficiency, plus the billing period length, your indoor setpoint, and the average outdoor temperature for the period. The tool converts fuel to delivered BTU, computes the average heating load, and scales it linearly by temperature difference to your design temperature, which defaults from your ASHRAE climate zone and can be overridden with your local design value. The result is an estimated design-day BTU/hr figure for furnace or heat pump sizing conversations, with a BTU-per-square-foot comparison against typical benchmarks by house age, diagnostics that flag inputs likely to skew the estimate, and a quick estimate of common envelope upgrade measures. It is an estimate from your bills and your entered temperatures, not a room-by-room Manual J load calculation, and it does not automatically remove non-heating fuel use such as water heating or cooking from the bill. PDF export, share links, and light/dark themes are included.

Pro Tip: Use a cold mid-winter billing period, not a shoulder month, and subtract non-heating use before you enter the fuel quantity: compare the winter bill against a summer bill on the same fuel and the summer usage is roughly your water-heating and cooking baseload. Entering raw bill totals with a gas water heater on the same meter is the most common way to oversize from this method.

Compare heat pump operating cost against your current fuel using the load you just estimated

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Size a shop or garage heater where bills are not available

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Heat Load from Bills Calculator
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How It Works

  1. Pick a Billing Period

    Choose a cold billing period, ideally the coldest full month you have a bill for. Enter the number of days in the period. The colder and longer the period, the less the estimate is distorted by baseload and solar gain.

  2. Enter Your Heat Sources

    Add each fuel that heated the home during the period: natural gas (therms or CCF), propane, heating oil, electric resistance, or an electric heat pump. Enter the quantity used and the appliance efficiency (AFUE for combustion equipment; the defaults are editable). Multiple simultaneous sources are supported for dual-fuel homes.

  3. Enter the Temperatures

    Enter your indoor setpoint and the average outdoor temperature over the billing period (your utility bill or a weather-history site usually has the monthly average). Then pick your ASHRAE climate zone for a default design temperature, or override it with your local 99 percent design temperature if you know it.

  4. Read the Design Load

    The results show delivered BTU for the period, average load in BTU/hr, and the headline number: estimated design-day heat load in BTU/hr, scaled from the average load by the ratio of design temperature difference to billing-period temperature difference.

  5. Check the Benchmarks and Diagnostics

    Enter your floor area to see BTU per square foot compared against typical ranges by house age. The diagnostics panel flags conditions that weaken the estimate, such as a mild billing period or an unusually high or low result.

  6. Check Upgrade Savings and Export

    The upgrade section gives rough percentage-savings estimates for common envelope measures (air sealing, attic insulation, and similar) applied to your estimated load. Export a PDF report or share the inputs by URL.

Built For

  • A homeowner getting furnace replacement quotes who wants an independent BTU/hr estimate to compare against the contractor proposal
  • Checking whether the 100,000 BTU furnace in a 1,400 sq ft house is oversized before a like-for-like replacement repeats the mistake
  • Estimating the design load for heat pump sizing conversations in a house where no Manual J has ever been run
  • A landlord estimating loads across several older units from gas bills without paying for an engineering visit on each
  • Comparing your BTU per square foot against house-age benchmarks to gauge whether envelope upgrades or equipment should come first

Features & Capabilities

Bills In, BTU/hr Out

Converts gas, propane, oil, and electric usage to delivered BTU using standard fuel heat contents and your entered appliance efficiency, then derives average and estimated design-day load from the billing-period temperatures.

Multi-Source Billing Periods

Dual-fuel homes can enter several heat sources for the same period (for example a heat pump plus propane backup) and the delivered BTU is summed across sources.

Climate-Zone Design Temperature with Override

Design temperature defaults from your ASHRAE climate zone selection and accepts a manual override, since climate zone alone does not pin down a site design temperature.

House-Age BTU/sqft Benchmarks

Compares your estimated load per square foot against typical ranges by construction era, a fast sanity check on both the estimate and the house.

Diagnostics That Flag Weak Inputs

The tool warns when the billing period is mild, the temperature difference is small, or the result lands outside plausible ranges, instead of presenting every output with equal confidence.

Upgrade-Savings Estimate

Rough percentage estimates for common envelope measures show how the estimated load and fuel cost would shift, as a conversation starter for an energy audit rather than a substitute for one.

PDF Report and Share Links

Exports a report with inputs, results, assumptions, and source notes, and encodes the full state into a share URL with no account required.

References

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Frequently Asked Questions

The honest answer is whatever your house actually loses on a design-cold day, and your heating bills already contain that information. Enter a cold-month bill, the average outdoor temperature for that month, and your setpoint, and the tool scales the measured usage to your design temperature. Rules of thumb like 30 to 60 BTU per square foot vary by a factor of two or more with age and climate; a bill-based estimate is anchored to your actual building.
No, and it is not meant to be. Manual J models the building room by room and is required for ductwork design and many permits. This method estimates the whole-house load from measured fuel use, which makes it a strong reality check (it reflects the building as it actually leaks, not as drawn) but it gives no room-by-room breakdown and depends on the quality of your billing inputs. Use it to sanity-check quotes and size conversations, then verify with a contractor before purchase.
Enter the therms or CCF from your coldest bill, the billing days, the average outdoor temperature, your setpoint, and your furnace AFUE. The design-day BTU/hr output is the load estimate; furnaces are then typically selected with a modest margin above it. If the result says 45,000 BTU/hr and the quote is for 120,000, ask the contractor to justify the difference.
No. Non-heating baseload is not automatically removed, and on a shared gas meter that can inflate the estimate meaningfully. Compare against a summer bill on the same fuel: the summer usage approximates your non-heating baseload, and you should subtract it from the winter quantity before entering it.
The coldest full period you have, typically January. A large indoor-outdoor temperature difference makes the heating signal dominate the bill and shrinks the influence of baseload, solar gain, and internal gains. The diagnostics panel flags mild periods where the estimate is weak.
The 99 percent heating design temperature for your location: the outdoor temperature your site stays above 99 percent of the hours in a year. The climate-zone default is a reasonable starting point, but local design temperatures vary within a zone, so override it with the value for your town (ACCA and ASHRAE publish tables) when you have it.
Yes, as the load side of the conversation. The design-day BTU/hr estimate tells you what the house needs; a heat pump must then be checked against its published capacity at your design temperature, since heat pump output falls as it gets colder. The Mini-Split Efficiency Calculator and your contractor handle the equipment side.
Disclaimer: This is a sizing estimate computed from the bills, temperatures, and efficiencies you enter, not a Manual J load calculation, Manual S equipment selection, or permit document. It does not automatically separate water heating, cooking, or other baseload from your bill, and it scales one billing period linearly to design conditions. Verify any equipment purchase with a qualified HVAC contractor and a proper load calculation where required by code.

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