Pharmacan is right as usual.

Here's the thing to remember with any light requiring a ballast----the ballast accepts a certain voltage from source---to make it easy, we'll use 120 volts as our example. In your 70 watt light, that ballast takes the 120 volts from the outlet it's plugged into, and the current (expressed in Amps) and steps it UP. Thats the function of the transformer inside the HID ballasts. Then the starter, also in the ballast housing, gives the bulb much larger "shot" of current than is available at the outlet it originally came from, to get the bulb started----you still with me here?

Now for the "juicy" part--hehe ....If you have a transformer that is engineered to run enough output juice to run a 70 watt bulb, and a starter that is only hot enough to start a 70 watt bulb, your 150 will not even come on. Why?----Because it takes way more juice to start AND run your 150, than the 70 watt ballast assembly puts out.
At most, you'll get a lot of humming, and if you happen to somehow get lucky, your bulb will flicker---attempting to start, but never will get past "start mode".

My two cents. I'm no electrician, but thats MY understanding of HID lighting.
ProGroWannabe Reviewed by ProGroWannabe on . lighting questions if i have a 70watt hps and i put in a 175watt hid bulb should it work if not what kind of ballast would i need Rating: 5