Heat Pump Water Heater Savings Calculator

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Table of contents

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

Heat pump water heater (HPWH): A water heater that moves heat from nearby air into the water, so it can use less electricity than standard electric resistance heating.

Current water heater type: The fuel or technology you have now. It sets whether Current heater energy use is shown in kWh, therms, propane gallons, or heating oil gallons.

Water temperature rise: The hot water temperature minus the incoming cold water temperature, measured in degrees F.

Efficiency factor: Delivered hot water heat divided by energy input. A higher factor means less purchased energy is needed for the same hot water.

Current heater energy price: Your price for the fuel used by the current heater, entered in the unit that matches Current water heater type.

Simple payback time: Net project cost after rebates divided by positive annual energy cost savings.

Mistake: Using the wrong unit for Current heater energy price.
Fix: Match Current heater energy price to Current water heater type: $ per kWh for electric, $ per therm for natural gas, or $ per gallon for propane or heating oil.

Mistake: Leaving Current heater efficiency factor at an electric-tank value when Current water heater type is Natural gas, Propane, or Heating oil.
Fix: Replace Current heater efficiency factor with the rating from the EnergyGuide label, product data, or your installer estimate.

Mistake: Entering total home water use for Hot water use per person.
Fix: Use hot water only for Hot water use per person, not outdoor water, toilet flushing, or cold-water laundry.

Mistake: Guessing Water temperature rise from the water heater setting only.
Fix: Set Water temperature rise to hot water temperature minus incoming cold water temperature, such as 120 F minus 50 F equals 70 F.

Mistake: Entering the full installed price in Net project cost after rebates when rebates or tax credits are expected.
Fix: Use the amount you expect to pay after rebates, tax credits, and discounts in Net project cost after rebates.

Mistake: Reading a negative Net savings over chosen years as a broken result.
Fix: Negative Net savings over chosen years means the entered energy savings do not recover the project cost within Years to compare.

• The calculator estimates energy cost only. It does not include maintenance, repairs, financing, water-heater lifespan, demand charges, or future energy price changes.
• It uses one blended Current heater energy price and one blended HPWH electricity price. Time-of-use rates and tiered rates can make actual costs different.
• Hot water demand is estimated from People in the home, Hot water use per person, and Water temperature rise. Real use can change with shower length, laundry habits, dishwasher use, and fixture flow rates.
• Efficiency factors are treated as steady yearly averages. Real HPWH performance can change with room temperature, installation location, ducting, settings, and backup resistance heat use.
• Net project cost after rebates should include only rebates, tax credits, and discounts you expect to actually receive.
• Simple payback time is shown as N/A when annual energy savings are zero or negative because energy savings alone do not recover the project cost.

Demand and heat load

The calculator first estimates daily hot water use from the number of people in the home and the gallons of hot water used per person per day.

daily_gallons = household_members * gallons_per_person_day

It then estimates the yearly heat needed to warm that water. The heat formula uses 8.34 lb per gallon for water and the rule that 1 Btu raises 1 pound of water by 1 degree F.

annual_heat_btu = daily_gallons * 8.34 * temperature_rise_f * 365

Energy use by heater type

The current heater uses the selected fuel unit. The calculator divides the hot water heat load by the current efficiency factor and by the Btu content of the selected fuel unit.

current_energy_use = annual_heat_btu / (current_efficiency_factor * btu_per_selected_fuel_unit)

The fuel constants are 3412.142 Btu per kWh, 100000 Btu per therm, 91500 Btu per propane gallon, and 138500 Btu per heating oil gallon.

The heat pump water heater is always calculated as electricity use.

hpwh_annual_kwh = annual_heat_btu / (hpwh_efficiency_factor * 3412.142)

Costs, savings, and payback

The calculator multiplies each yearly energy use by the matching user-entered energy price.

current_annual_operating_cost = current_energy_use * current_energy_price

hpwh_annual_operating_cost = hpwh_annual_kwh * hpwh_electricity_price

Annual energy cost savings are the current heater yearly cost minus the HPWH yearly electricity cost. Negative savings are kept because they mean the HPWH costs more to run with the entered prices and efficiencies.

annual_operating_cost_savings = current_annual_operating_cost - hpwh_annual_operating_cost

simple_payback_years = net_project_cost_after_rebates / annual_operating_cost_savings, when annual savings are positive

net_savings_over_chosen_years = annual_operating_cost_savings * analysis_years - net_project_cost_after_rebates

Worked example

For 4 people, 12 gallons per person per day, and a 70 F water temperature rise, daily hot water use is 48 gallons and yearly heat needed is 10,228,176 Btu. With an electric tank efficiency factor of 0.93 and electricity at $0.16 per kWh, the current heater uses about 3,223 kWh per year and costs about $515.71 per year. With an HPWH efficiency factor of 3.5 and the same electricity price, the HPWH uses about 856 kWh per year and costs about $137.03 per year. Annual energy cost savings are about $378.68. If Net project cost after rebates is $2,500, simple payback time is about 6.6 years and 10-year net savings are about $1,286.81.