Garage Mini Split vs Gas Unit Heater Lifetime Cost Calculator

Related Calculators

Table of contents

• How to use
• Common mistakes
• Definitions
• Methodology
• Limitations & assumptions
• Sources

HSPF2: A seasonal mini-split heating efficiency rating. Higher HSPF2 means more heat delivered for each watt-hour of electricity used.

Gas heater efficiency: The percentage of natural-gas heat that reaches the garage as useful heat. An 80% value means about 80 of every 100 fuel BTUs become delivered heat.

Equivalent full-output hours: A way to combine partial-load heating time into one annual number. Two hours at half output equal one hour at full output.

Therm: A natural-gas energy unit equal to 100,000 BTU.

Net project cost: Base project quote plus extra site work minus rebates and tax credits entered for that system.

Discount rate: A yearly rate used to convert future costs into their value in today's dollars.

Mistake: Entering a gas heater's fuel-input BTU rating as Garage heating output needed .
Fix: Enter the useful heat required by the garage from a load estimate or quote, in kBTU/h.

Mistake: Treating every hour the heater runs as full output in Hours per year at full heating output .
Fix: Use equivalent full-output hours. Two hours at half output count as one full-output hour.

Mistake: Putting a fixed monthly customer charge into Natural gas rate .
Fix: Put only usage-based per-therm charges in Natural gas rate and use Added gas service charge only when this choice creates or preserves that bill.

Mistake: Comparing base quotes while leaving out wiring, gas piping, venting, permits, or openings.
Fix: Add omitted work under Extra mini-split site work or Extra gas heater site work .

Mistake: Assuming a high Mini-split HSPF2 proves the unit can heat the garage during the coldest weather.
Fix: Check the proposed unit's delivered heating capacity at your local cold-weather design temperature.

Mistake: Reading a negative Gas cost minus mini-split cost over the selected years as an error.
Fix: A negative value means the gas heater has the lower estimated cost under your entries.

• This is a heating-only planning comparison. It does not assign a dollar value to mini-split cooling, dehumidification, or air circulation.
• The calculation uses one annual useful-heat estimate based on Garage heating output needed and Hours per year at full heating output. It does not model hourly weather, thermostat setbacks, garage air leakage, or door openings.
• HSPF2 is a seasonal rating and does not confirm that a mini-split has enough delivered capacity at the coldest local outdoor temperature. Verify cold-weather capacity before choosing equipment.
• Energy price increase per year applies only to usage-based electricity and gas costs. Maintenance and the added gas service charge stay level except for discounting.
• Project quotes, incentives, repair costs, equipment replacement, financing, taxes, and resale value are excluded unless you include applicable items in the available project-cost or maintenance fields.
• Enter Added gas service charge only if the gas heater decision changes whether the property keeps gas service. Do not count a charge that will be paid either way.

Calculation method

The calculator first estimates the same annual useful heat for both systems. It then converts that heat into mini-split electricity use and gas-heater fuel use using the entered efficiency ratings.

annual_heat_BTU = heating_output_kBTU_per_h * 1000 * full_load_hours

mini_kWh = annual_heat_BTU / (mini_HSPF2 * 1000)

gas_therms = annual_heat_BTU / ((gas_efficiency_percent / 100) * 100000)

First-year energy costs use the entered usage-based utility rates. The gas calculation keeps any fixed monthly service charge separate from the per-therm fuel rate.

mini_first_year_energy_cost = mini_kWh * electricity_rate_cents_per_kWh / 100

gas_first_year_energy_cost = gas_therms * gas_rate_dollars_per_therm

Net project cost equals the base quote plus extra site work minus entered incentives. Recurring energy costs can grow each year, while future costs are discounted back to today when a discount rate is entered.

energy_factor = sum from year 1 to years of ((1 + growth)^(year - 1) / (1 + discount)^year)

level_factor = sum from year 1 to years of (1 / (1 + discount)^year)

mini_lifetime_cost = mini_net_project_cost + mini_first_year_energy_cost * energy_factor + mini_annual_maintenance * level_factor

gas_lifetime_cost = gas_net_project_cost + gas_first_year_energy_cost * energy_factor + (gas_annual_maintenance + added_gas_service_charge * 12) * level_factor

lifetime_cost_gap = gas_lifetime_cost - mini_lifetime_cost

Worked example

With 24 kBTU/h of needed heat for 1,000 equivalent full-output hours, the annual useful heat is 24 million BTU. At HSPF2 9.0 and 17.3 cents per kWh, the mini-split first-year electricity cost is about $461. At 80% gas efficiency and $1.53 per therm, the gas heater first-year fuel cost is about $459. With $5,000 and $4,500 project quotes, no added costs, no maintenance, and a 10-year period, the gas cost minus mini-split cost is about -$523, meaning the gas heater costs less under those entries.