This is something that used to happen to older photovoltaic installations.
What used to happen with the old magnetic meters is when there is a flow of electrons out of the house, it moved the magnetic dial in the opposite direction reducing the KWH reading.
Today, all domestic installations have an export meter installed alongside. There is now a legal requirement on all feed-in tariff applications for the recipient to inform the Energy Company if they notice the meter turning backwards.
Hope this helps.