need assistance on portable battery/ivert

I do a lot of rock hounding and was looking at a short wave UV light for field specimen use. Here is what I was looking at:

http://www.ultraviolet-tools.com/ultraviolet-lamps/36-watt-shortwave-field-lamp/prod_6.html

I'm not sure what the total continuous usage is in the field with the battery and inverter (sales dept. has not gotten back to me) - but I was hoping someone here a little bit more savvy in calculating power draw might be able to help. Here is the link to the battery:

http://www.ultraviolet-tools.com/lamp-accessories/dc-12-volt-battery-pack-ac/dc-inverter/prod_9.html

For a 100 bucks for the battery and inverter, I was wondering if I might be able to get it cheaper by putting something together myself? Inverter/batt and pack from outdoor store to carry both inverter and battery around. Might be cheaper than 100 bucks?

If so, could anyone recommend a battery/inverter setup that would be practical for carrying around in the field and yield the highest continuous usage?

Essentially any inverter large enough - ie, 50W though I'd add more reserve, & 150W or maybe 90W are probably the smallest you can get. (Aldi sell a 600W inverter for AUD$60.) And battery - an AGM with suitable reserve, the bigger the better. I'd suggest a 7AH (cheap; common) if big enough else a buggy type 12AH or 15AH (also if big enough). Recharging can be from the vehicle's electrical system (when charging, ie engine running), or a dc-dc battery charger, or an AC charger powered by the inverter. There are also 12V lithium batteries which will be far lighter than AGMs, but they can be expensive. PS - it may be worth checking that the UV lamp tolerates normal step-wave (aka modified sinewave)rather than pure sinewave inverters, but I doubt that would be a problem these days. And you're sure a UV torch is not a better solution? (I don't think your linked UV uses LEDs.)