You'll be pulling about 1.6 amps from the battery, you'll get a bit less than 1 1/2 hours runtime (probably less than 1 1/4 hours before it starts dimming noticeably) is that enough runtime for you?
If you spend just a little more you can replace that halogen bulb with one or two LEDs and get up to triple the runtime for the same ammount of light. Although there's a bit more work in building it.
If you're keen on going halogen you'll get a little more efficincy with a 12v 10w setup.
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"Surely one can love his own country without becoming hopelessly lost in an all-consuming flame of narrow-minded nationalism" - Fred Birchmore