26 USC 136 - Energy conservation subsidies provided by public utilities

(a) Exclusion 
Gross income shall not include the value of any subsidy provided (directly or indirectly) by a public utility to a customer for the purchase or installation of any energy conservation measure.
(b) Denial of double benefit 
Notwithstanding any other provision of this subtitle, no deduction or credit shall be allowed for, or by reason of, any expenditure to the extent of the amount excluded under subsection (a) for any subsidy which was provided with respect to such expenditure. The adjusted basis of any property shall be reduced by the amount excluded under subsection (a) which was provided with respect to such property.
(c) Energy conservation measure 

(1) In general 
For purposes of this section, the term energy conservation measure means any installation or modification primarily designed to reduce consumption of electricity or natural gas or to improve the management of energy demand with respect to a dwelling unit.

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(2) Other definitions 
For purposes of this subsection
(A) Dwelling unit 
The term dwelling unit has the meaning given such term by section 280A (f)(1).
(B) Public utility 
The term public utility means a person engaged in the sale of electricity or natural gas to residential, commercial, or industrial customers for use by such customers. For purposes of the preceding sentence, the term person includes the Federal Government, a State or local government or any political subdivision thereof, or any instrumentality of any of the foregoing.
(d) Exception 
This section shall not apply to any payment to or from a qualified cogeneration facility or qualifying small power production facility pursuant to section 210 of the Public Utility Regulatory Policy Act of 1978.