I have a home security fixture that needs a new HPS lamp. Unfortunately, there are no markings on the fixture or the old lamp to indicate the wattage, and as far as I can tell, the candidate lamps (35W–150W, medium base) all look pretty much alike.

I am hoping to find a solution that does not require accessing or replacing the fixture, as that would require me to buy a new ladder (in which case, I would probably hire an electrician to replace the fixture). I can replace the bulb using the tool that I used to remove the old lamp.

My guess is that a 150W lamp is needed. I say that because Home Depot sells a 150W fixture that looks like mine, the old lamp was very bright, the socket is ceramic, and there is a lot of empty space around the lamp in the fixture. But, I have been led to believe that there could be problems if I pick the wrong wattage.

Can anyone suggest a way to proceed?

The only thing I can think of is to try a 150W lamp and hope for the best. If I do that, will I be able to tell if a lower wattage lamp is needed?

Take a clear picture of the lamp with ruler for scale. It may be possible to do educated guess of the lamp's power from the size of the arc tube inside it

If the old lamp still works (before it heats up and extinguishes), measure the working current it approaches, as late into working as it goes (for example, across the switch terminals with the switch itself in off position). On a simple reactor ballast and with relatively warmed up lamp, this current will clearly tell what lamp you got

If you want to try and hope for the best, up to 150W all US lamps have the same arc voltage, and therefore they will start and at least partially run up on the gear. And then :

 - If the lamp is too big it will not warm up to full power, but remain stuck for long time in the Yellow-ish color of early startup

 - If the lamp is correct it will work fine

 - If the lamp is too small it will warm up quickly (this alone is not a sign of problem, as fresh lamps tend to warm up faster to some extent), but then its light will start changing to Whiter than the normal Orange, and it might extinguish

Both mismatch conditions are bad to the lamp and ballast, but they won't cause damage to the ballast in the short time of a test, and usually the lamp won't immediately burn out from overpowering condition either. However, it is better to try the other ways first so you dont have to return a mismatched lamp to the shop in case your guess was wrong

Looks like an old ElectriPak.  I'm pretty sure that would be 70 watts.

Like Ash said, if you can post a photo of the old lamp and something to compare the size of the tube inside the bulb that would help, but I'm also with Icefoglights, of it being a 70 watt HPS lamp, they are the most common size available to home users, and if your close to a habitat for humanity Re-Store you might even find them for a buck, also if you end up changing out the fixture think about taking it there for a second home...

You may make a jig:
Take a base from a worn lamp (or a CFL), connect wires long enough so you may access them, glue the socket on the end of some plastic tube or a wooden stick (so you may screw it into the fixture). Then connect the other ends of the wires via an A-meter to some socket and to a V-meter.
Then you may try different lamps (start from 150W):
The voltage after it stabilizes should be around 55V (50..65V would be about the range), the current should correspond to the wattage (~3A for 150W, ~1.5A for 70W, ~0.7A for 35W).

Why all that ? Connect the A meter across the light switch when the switch is off

If the old lamp is cycling (which i assume it does, because it is HPS), that means its voltage goes above the normal 55V. And this means, that when it is allready fairly warmed up but not yet extinguished, it is working at fairly close to the rated current. So no need in new lamp for the measurement

Thank you all for the suggestions. I'll start with the photograph, as that is easy and I don't have to wait for nighttime...

Looks like an old ElectriPak.  I'm pretty sure that would be 70 watts.

Yes, it matches this image, even to the warning message on the front and the painted serial number on the arm:

http://www.lighting-gallery.net/gallery/displayimage.php?album=859&pos=119&pid=47067

An additional picture, more zoomed out so the entire lamp gets in the shot may be useful too

looks like a 70 Watt bulb..

Won`t it be printed on the ballast?

I have a home security fixture that needs a new HPS lamp. Unfortunately, there are no markings on the fixture or the old lamp to indicate the wattage, and as far as I can tell, the candidate lamps (35W–150W, medium base) all look pretty much alike.

I am hoping to find a solution that does not require accessing or replacing the fixture, as that would require me to buy a new ladder (in which case, I would probably hire an electrician to replace the fixture). I can replace the bulb using the tool that I used to remove the old lamp.

My guess is that a 150W lamp is needed. I say that because Home Depot sells a 150W fixture that looks like mine, the old lamp was very bright, the socket is ceramic, and there is a lot of empty space around the lamp in the fixture. But, I have been led to believe that there could be problems if I pick the wrong wattage.

Can anyone suggest a way to proceed?

The only thing I can think of is to try a 150W lamp and hope for the best. If I do that, will I be able to tell if a lower wattage lamp is needed?

Those Electripak fixtures to my knowledge only came in 70 Watts.