Current Duracell Spec sheet for their MN1604 9V  battery
normal "Alkaline" chemistry i.e. Zinc / Alkaline / Manganese Dioxide
"565 mA.h with 510 ohm [load] to 4.9 V [ending termnal voltage] at
70 F (21C)."

Note that this is the constant resistance load case, where current
tails off as battery voltage drops.

The constant current case, since it would be lower resistance near
end-of-life, would likely yield less A.h.

Also note the temperature caveat.  For really cold (less than
0 F) outdoor stuff, the capacity and especially the drain rate is
much reduced.  Lithiums (such as UltraLife) have much better low temp
performance.

Hope this helps.

------------
Barry King
Engineering Manager
NRG Systems "Measuring the Wind's Energy"
Hinesburg, Vermont, USA
barry@nrgsystems.com
"The witty saying has been deleted due to limited EPROM space"