Looks to be an AEL 250 watt with a glass lens. I'v alway's liked nemas on long arms.

How can you tell it's a 250 watt?

Not shure it is but looks like it by the size of the lamp and it has a glas lens.

the 175 and 250W lamps are the same size but the 250W fixtures typically had glass refractors.

Treu but I'v noticed with ED shaped lamps the hole at the tip of a 250 watt lamp is sometimes slightly larger than a 175's. But honestly it may be 175 watt. Never truly know without a nema label. Hey since we'r on the nema head subject I would like to e-mail you a pic of one I just installed in my yard it's an AEL 11-S-RN-120-BA and a Utility Metals 6 foot 1-1/4 bracket with underbrace. Let me know what you think. I added the nema label because I though your's looked realy good.

Hmm, I always thought the size of the hole at the tip of ED shaped lamps varied among different lamp companies. I guess one way to tell if the ballast is visible is to look at the ballast itself. A 250w ballast should be noticeably bigger than a 175w.

I doubt this one has a glass refractor as very few, if any, around here do. Also no NEMA label on it (uncommon NEMA's around here as well)

this probably has a FP lens since those look pretty close to glass until they start yellowing (well, the poly ones at least). This is probably 175W. Anyhow, I have a 175W clear Sylvania merc and a 250W /DX sylvania merc and both have the same sized dimple.