I would advise you to get a 5 AH SLA battery and recharge it when you get to work. It will support your 40W very well for an hour, but won't last 2 hours. If you want 2 hours straight run-time without recharging, you'll need at least a 7AH SLA battery.

The formula is basically this:

Watts = Volts X Amps or Amps = Watts / Volts

So for every hour ("Amp/hr") of battery power you need, divide the total watts you are using (40 watts) by the voltage (12.)

40 / 12 = 3.33 or 3.3 Amp/hr. So you would need at least a 3.3 AH battery for each hour of run-time you want.

BUT--

There is a problem. The smaller the battery, the less it can carry higher wattages. For example, a battery that supports 20W for an hour might not support 40W for half an hour; it might run for 20 - 25 minutes. So you have to use "overkill" when selecting an SLA battery for your needs.

40W / 12V = 3.3 AH
2 hours X 3.3 AH = 6.6 AH
"Overkill factor" -- minimum of 7 AH (or even more.)

Remember that SLA batteries, while cheap and reliable, don't like to be completely drained. It shortens their life. ALSO--always buy a fuse for safety reasons, even a 12V battery can start a fire. You can get details from your local battery store.

William, if you don't like the idea of such a large battery, and you must have 2 consecutive hours of run-time, you can put separate switches on each of your 20W lights. That way, you can use 20W most of the time, and use the second 20W as a "high-beam" for special needs. Even with a larger battery, two switches will allow you to conserve power as you need to.