Are energy efficiency requirements effective? Yes. Are they efficient? Not necessarily. As with anything, one can overinvest in energy efficiency and waste ratepayers’ money. I think California has wasted money on conservation through their stupid “Loading Order” policy. Any competent economist understands that the order in which you add resources does not matter so long as you add the right amounts of each type of resource.
Politicians are not competent to decide how much energy efficiency is optimal.