Decarbonising Welsh council buildings alone has been estimated to cost at least £2.3 billion.
The original thinking was that if government-backed bodies invested in electric vehicles, charge points, renewable power and heating, it would become easier and cheaper for the public to follow.
A spending watchdog has strongly criticised the lack of progress by Welsh public sector organisations in cutting planet-warming emissions.
The watchdog found limited progress in understanding how much it would cost to reach net zero in the near or longer term.
Where organisations had estimated longer-term costs, some described them as unaffordable.
It is meant to give a shared direction of travel and an expectation that all bodies contribute to reducing emissions.
That ambition does not require every public body to reach net zero individually by 2030.
Some public bodies said the lack of a firm target had added uncertainty about how much priority to give carbon reduction. Audit Wales said public bodies were struggling to invest in decarbonisation because of overall financial pressures.