It came with a bulb in the box, and I need to buy a couple of spares so they're on hand. On the box it says "g24q-1"; that's the pin configuration, I'm sure.

It also says "CF13LD/E/827
lamp code; ;26095
165 mA
Colour: 827
Lumens: 900

So I checked for the "G24q-1" online, and found several.
(I deleted the maker and model number.) I'm particularly interested in what the "3000K" or "3500K" or "4100K" or 2700K" means.

F13DBX/SPX30/4P - NAED 20721 - 13 watt = 4 pin G24q-1 Base-3000K

CF13DD/E/841/ECO - 13 Watt - 4 Pin G24g-1 Base = 4100K

F13TBX/830/A/ECO=NAED 20891 = 13 watt 4 pin GX24q-1 Base - 3000K

PL-C 13W/835/4P/ALTO - NAED 20671 - 13 Watt - 4 Pin G24q-1 Base - 3500K

13W T4 D G24Q-1 5000K 4-PIN Double Tube 4 Pin Base

CF13DD/E/827 - 13 Watt - 4 Pin G24q-1 Base - 2700K

PL13D/E/50 - 13 Watt - 4 Pin G24q-1 Base - 5000K -

What do I buy?

They all *LOOK* the same. I'm not that worried about the base (though I wonder what that "X" is doing in the middle of one of them).

How do I tell which one I need?

The "2700K" and similar numbers are the color temperature. It may seem like a paradox, but the higher the number the "cooler" (more bluish) the light. The 2700K lamp will be very close to incandescent color, and the 5000K lamp more like blue-sky outdoor light.

A common shorthand is to combine the first digit of the CRI (color rendering index, an indication of how well the lamp helps you distinguish colors) and the first two digits of the color temperature.

So, from the number CF13LD/E/827, I'd guess that what you have now is a compact fluorescent 13 watt lamp, 2700K color temperature, 80+ CRI (probably 82). I don't know what the LD and E stand for. The manufacturer's website might explain it, if you're really that interested.

I can't absolutely guarantee it, but the spare you identify as "CF13DD/E/827 - 13 Watt - 4 Pin G24q-1 Base - 2700K" sounds like it should work just fine.

G24 lamps are just starting to get established. A greater variety of spares should be available at home centers in the next couple of years. With luck you won't have to order online next time.

It's interesting that the G24 seems a bit new. The lamp I got had a flange/tab on one side of the base, and it blocked the lamp from fitting into the socket. We had to break it off w/ a pliers. I'm wondering if the light fixture wasn't manufacturered properly to spec. I'd assumed it was the odd-brand lamp that they included w/ the fixture.

But now I'm thinking it's the fixture itself, bcs all the recognizable-name brands show that same tab on their lamp bases.

(and aren't you impressed that I know that "lamp" is the electrician's term for the bulb, and NOT for the fixture? ;-) )

Your statement that you had to break off a tab on the lamp concerns me. What brand is the fixture, and where did you get it?

And the guy from the electrician's was the one who broke the tab off. I pressed him on the idea that it might not be as safe, and he assured me it would be fine. I would never have done that, if left to my own devices; I'd have put the old fixture back and started over.

I was crabby about the stupid light anyway, bcs I thought I'd ordered one w/ a different pin system (for which I already had the expensive bulbs), but when we opened the box, it was different. I didn't want to pay for the electrician to come twice, so I decided not to return the fixture but just go ahead.
Then when he got to the very end and was putting in the bulbs, he discovered that the tab keeps them from fitting. Once he broke that off, they fit very snugly and everything aligned perfectly. They don't get jiggled much, and as I said, they fit well.

I think he and I both assumed that the off-brand bulb was the problem, not the fixture itself. But now that I am finally looking for the replacement bulbs (time to stock up--they haven't burned out, not by a long shot), I see that they all look the same as the photo on the off-brand bulb's box.

(and I don't remember if that tab we took off was reflected in the photo on the box or if it was extra--I may see if I can get up there w/ a ladder to check)

I was looking for the difference between 13w and 18w G24 fixtures, and I've found some information.

http://www.hugeinventions.com/faq/g24lamps.html

This basic FAQ is the basic difference between 2-pin and 4-pin lamps and the nib configurations. It seems like the difference may be in the ballast, but all of my searching for ballast information leads me to wholesalers and importers. I can't tell if there's any problem jamming an 18 watt bulb into a 13 watt fixture. I'm trying to increase the wattage on a fixture I have, and I don't want to replace the whole darn thing just to get more light. That's not very "green" either.

I also found more information than you could possibly need on your lamp labeling.

http://www.energyrating.gov.au/library/pubs/2009-ref-manual-lighting.p df

Go to page 122 of the .pdf, or search the document for "G24", and it will tell you everything about the ANSI and NEMA labeling conventions for fluorescent lamps.