As the description says. I don't currently posses any medium base 100 watt lamps so I'm temporarily substituting a 150 watt lamp in for a few days or so.

Will the lamp output 100 watts of light?

I wouldn’t have thought so, with the 150 watt discharge tube being bigger, it will probably just under run without a full colour?

I wouldn’t have thought so, with the 150 watt discharge tube being bigger, it will probably just under run without a full colour?

The American 150 and 100 watt lamp arc tubes are basically the same size so I'm almost feeling like perhaps electrically they may be atleast compatible enough to avoid damaging the lamp or ballast... Not sure though..

You have two 150W HPS lamps: low voltage and high voltage. With low voltage 150w HPS lamp, the lamp will be underdriven, and with high voltage 150W HPS lamp, the ballast will overheat and fail.

Assuming lamps of same voltage group, the lamp will be underpowered, and put out low CRI light. It may run at lower voltage too so the ballast will get somewhat hotter - You can try to see if its acceptable, or to compare with running the correct lamp

Dor - Please explain why you think so ? The high voltage lamp will most likely only glow on the ignitor output, stressing the ignitor, but not overheating the ballast

The lamp I'm run in is a 55V lamp on 55V gear. It seems to warm up slower than usual and settles on a color that appears to be fully warmed up. I currently don't have time to set up a test rig to compare the 150 watt lamp side by side with a 100 watt lamp and a 150 watt lamp driven on correct gears.

I was wondering this about using 100w lamps on 70w gear, both 55v

Or 70w HPS on 70w MH ballast which worked fine before

70 watt HPS on a 70 watt MH ballast I believe is very hard on the lamp. Either that or it causes the ballast to overheat.

The 150w lamp on 100w control gear will just under run. I know of many street light lanterns in the UK intended for 100w lamps that have been accidentally fitted with 150w lamps. They have just been running happily for years and years (although under run) and the gear nor lamps has never failed. The lamps have been in what must be 15 years now. The 150w last years and years anyway and even longer on the 100w it seems.

The 150w lamp on 100w control gear will just under run. I know of many street light lanterns in the UK intended for 100w lamps that have been accidentally fitted with 150w lamps. They have just been running happily for years and years (although under run) and the gear nor lamps has never failed. The lamps have been in what must be 15 years now. The 150w last years and years anyway and even longer on the 100w it seems.

I second that. We do as well have several lanterns with a power dim switch. It simply switch down the power of the lamp from 150W to 100W by connecting it automatically to the other tap of the ballast.

70 watt HPS on a 70 watt MH ballast I believe is very hard on the lamp. Either that or it causes the ballast to overheat.

Why should that be hard on the lamp?
Lower current means lower efficacy, but lower stress.
Higher OCV? That only makes the arc itself more stable, so in fact less stress on the electrodes during reignition. The thermal runaway problem is not there, because the lamp is just underdriven (my guess ~1.1A instead of the rated 1.5A)
Ignition voltage? Once it is greater than the lamp needs, it is limited by the lamp anyway (as it is supposed to be the case).

What may suffer is the ballast: Seing a 55V (or even below that, because the lamp will never warm up) means there is higher current than designed (my guess was ~1.1A instead of the designed 0.95A), so higher heat dissipation (15% higher current means 30% more heat). But because there is actually less heat from the lamp, the system may still be OK.

So the lamp would be perfectly fine (although not performing), only the ballast reliability could be questioned...

I tried it and it stabilized at 43v 2.4a instead of 2.1a 60v with a 100w bulb. If you have a well used lamp that should work better, I have a 50w lamp that cycles on a 50w ballast but works normally on a 35w ballast.

That's a good idea.. Never thought of that!