IDACORP's net income increased $6.1 million for 2019 compared with 2018, primarily due to higher net income at Idaho Power and IFS.
Idaho Power's customer growth of 2.5 percent added $18.8 million to Idaho Power's operating income compared with 2018. Lower sales volumes on a per-customer basis decreased operating income by $21.4 million in 2019 compared with 2018, primarily due to lower irrigation sales. Greater precipitation and more moderate spring and summer temperatures in Idaho Power's service area led agricultural irrigation customers to use 12 percent less energy per customer to operate irrigation pumps during 2019 compared with 2018. To a lesser extent, sales volumes on a per-customer basis in 2019 were negatively affected by lower per-customer commercial and industrial sales.
The net decrease in retail revenues per MWh reduced operating income by $2.8 million in 2019 compared with 2018. As provided by the settlement stipulation approved by the IPUC in 2018 related to income tax reform, retail revenues per MWh in 2019 were reduced by $7.4 million of non-cash accruals for future amortization related to regulatory deferrals that would otherwise be a future liability of Idaho customers, compared with a $1.5 million revenue reduction in 2018. In 2018, a corresponding $4.0 million of non-cash accruals were recorded as other O&M expense for the amortization of specified deferrals. The decrease in retail revenues per MWh from these non-cash accruals was partially offset by changes in the customer sales mix, as volumes sold to residential customers in 2019 made up a greater portion of the customer sales mix compared with 2018. Residential customers generally pay a higher per-MWh rate than other customers.