D. Upon receipt of the certificate from the local building department or the Department of Waste Management the local assessing officer shall, if such local ordinance is in effect, proceed to determine the value of such qualifying solar
energy equipment, facilities or devices or certified recycling equipment, facilities, or devices. The exemption provided by this section shall be determined by applying the local tax rate to the value of such equipment, facilities or devices and subtracting such amount, wholly or partially, either (i) from the total real property tax due on the real property to which such equipment, facilities, or devices are attached or (ii) if such equipment, facilities, or devices are taxable as machinery and tools under § 58.1-3507, from the total machinery and tools tax due on such equipment, facilities, or devices, at the election of the taxpayer. This exemption shall be effective beginning in the next succeeding tax year, and shall be permitted for a term of not less than five years. In the event the locality assesses real estate pursuant to § 58.1-3292, the exemption shall be first effective when such real estate is first assessed, but not prior to the date of such application for exemption.
I believe the regulation allows you to deduct the tax rate of the solar equipment from the value of the property thus lowering your real estate tax.