Convert natural gas therms to kWh or kWh to therms, with optional equipment-efficiency and price checks.
Advanced options
Table of contents
How to use our Therms to kWh Converter
- Pick the Convert direction: Therms to kWh if you have a gas-bill amount, or kWh to therms if you have an electric amount.
- Enter the Amount in the unit named by your Convert choice; commas and spaces are okay, but negative energy amounts are not.
- Open Advanced options only if you want a heating comparison, then set Gas equipment efficiency and Electric heat COP.
- Enter Gas price and Electric price only if you want cost results; leave either price blank to skip that cost card.
- Sanity-check the result by confirming that 1 therm is about 29.307 kWh, then remember that efficiency and COP change useful heat, not the raw unit conversion.
Definitions
Therm: A gas energy unit equal to 100,000 Btu, commonly seen on US natural gas bills.
kWh: A kilowatt-hour, the energy used by 1,000 watts running for 1 hour.
Conversion rate used: The factor 29.307107017 kWh per therm, calculated from Btu and joule conversions.
Gas equipment efficiency: The percent of gas energy estimated to become useful heat after losses in the furnace or boiler.
Electric heat COP: Coefficient of performance, or useful heat out divided by electricity in; electric resistance heat is 1, and heat pumps can be higher.
Useful heat: The estimated heat delivered after applying Gas equipment efficiency or Electric heat COP.
Cost gap: Electric cost minus gas cost for the same useful heat; positive means electric costs more, and negative means electric costs less.
Common mistakes and quick fixes
Mistake: Entering a gas-bill number in Amount while Convert is set to kWh to therms.
Fix: Set Convert to Therms to kWh when Amount is in therms.
Mistake: Using Gas equipment efficiency as 80 instead of 0.80, or 0.80 instead of 80.
Fix: Enter Gas equipment efficiency as a percent, such as 80 for 80 percent.
Mistake: Leaving Electric heat COP at 1 when comparing a heat pump.
Fix: Change Electric heat COP to the COP you want to test, such as 3, if you are estimating heat-pump electricity use.
Mistake: Typing a monthly bill total into Gas price or Electric price.
Fix: Use the per-unit price: Gas price is $ per therm and Electric price is $ per kWh.
Mistake: Reading Cost gap as a raw energy result.
Fix: Use Converted amount for the unit conversion; Cost gap is only a price comparison for the same useful heat.
Limitations & Key Assumptions / Boundary Conditions
- This is an energy conversion first. It does not prove that two real appliances will cost the same to run.
- Gas equipment efficiency is limited to more than 0 percent and up to 100 percent for this simple delivered-heat estimate.
- Electric heat COP must be more than 0. Real heat-pump COP changes with outdoor temperature, equipment, and operating conditions.
- Gas price and Electric price should include the full per-unit charge you want to compare. Fixed monthly fees, taxes, and tiered rates may change real bills.
- Useful heat assumes all compared heat is needed and usable in the same way. Duct losses, standby losses, defrost cycles, and room-by-room comfort are not modeled.
- Very large inputs are formatted for readability, but results are still estimates based on the entered numbers and the fixed therm-to-kWh factor.
Methodology
Raw energy conversion
The converter uses 1 therm = 100,000 Btu and the International Table Btu in joules to compute the therm-to-kWh factor. The kWh is based on energy in watt-hours, where 1 kWh equals 3,600,000 joules [1][2].
K = 100000 * 1055.05585262 / 3600000 = 29.307107017 kWh per therm
kWh = therms * K
therms = kWh / K
Useful heat comparison
For Therms to kWh, the raw conversion is first changed into useful heat by multiplying by Gas equipment efficiency. Then the calculator finds the electric kWh needed to deliver that same useful heat at the selected Electric heat COP.
useful_heat_kwh = therms * K * (gas_efficiency_percent / 100)
electric_kwh_needed = useful_heat_kwh / electric_cop
For kWh to therms, the electric input is first changed into useful heat. Then the calculator finds the gas therms needed to deliver that same useful heat at the selected Gas equipment efficiency.
useful_heat_kwh = input_kwh * electric_cop
gas_therms_needed = useful_heat_kwh / (K * (gas_efficiency_percent / 100))
Cost comparison
Cost cards appear only when the related price field is filled in. Cost gap is electric cost minus gas cost, so a positive value means the electric option costs more for the same useful heat.
gas_cost_dollars = gas_therms_needed * gas_price_per_therm
electric_cost_dollars = electric_kwh_needed * electric_price_per_kwh
cost_gap_dollars = electric_cost_dollars - gas_cost_dollars
Mini-example
If Amount is 5 therms, Gas equipment efficiency is 80 percent, and Electric heat COP is 3, the raw conversion is 5 * 29.307107017 = 146.535535085 kWh. Useful heat is 146.535535085 * 0.80 = 117.228428068 kWh of heat. The electric energy needed for the same useful heat is 117.228428068 / 3 = 39.076142689 kWh. If Gas price is $1.50 per therm and Electric price is $0.20 per kWh, gas cost is $7.50, electric cost is about $7.82, and Cost gap is about $0.32.