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Estimated heat pump running cost = current annual cost / 0.92 / COP x electricity rate. Annual saving = current annual cost - heat pump running cost. Payback = installed cost / annual saving.- Apply the formulaEstimated heat pump running cost = current annual cost / 0.92 / COP x electricity rate. Annual saving = current annual cost - heat pump running cost. Payback = installed cost / annual saving.1.8 yearsEstimated running cost $156.52/year; annual saving $1,643.48.