If you mean electronic ballast for halides such as e-ballast then I believe the savings from reduced current demands are "chump change" unless you are using many 400 watters and pay high rates for KWH per your utility company.
E-ballast do use slightly less current and are much cooler. This will heat your house less resulting in less room AC. But the lamp will be just as hot.
In my case 3 400 watt halides only cost about $30 a month to run at .07/KWH. The real cost in the power bill is the AC required to cool the house from the heat generated by these lights.
