Power (Watts)=Voltage (E) X Current (I)
This is for purely resistive loads. When you get into inductive loads then it is a whole new ball game. But for the most part most home applications will still follow this formula.
So in you case 10W/12V = 0.833 amps
One string of 70 (7 watt) lights 7X70= 490 watts 490 watts/110 volts= 4.45 amps
One 100W light bulb 100/110 = 0.9 amps
Four Halogen 50W bulbs 200/110 = 1.8 amps
Most fires occur because extension cords are overloaded. Most extension cords are not rated for more than a 10 amps. They use 16 or 18 gauge wire or even smaller. Check your load tables
Wire Current max at 110V
gauge
22 5A
20 7.5A
18 10A
16 13A
14 17A
12 23A
With extension cords comes another danger. Frayed wires. Most extension cords are in poor shape. They are already poorly made, with poor quality connectors. Then you add your load to it, walk on it, the kids run their toys over it, it is hidden under a carpet. It's no wonder there are fires. It is actually more of a miracle that there are not more.
Now we have another factor involved here and that is the length of the wire. This adds resistance. Resistance adds extra load. All of that turns into heat. Which needs to dissipate. Too much heat equals fire.
Years ago I worked for Square D. The first thing one of my co-workers showed me was a suitcase (for trade shows) with a transformer in it and switch a receptacle and a bunch of breakers. The transformer was connected to a 110 volts and the output voltage of the transformer was very low(2 volts). The current output of the transformer was around 200 amps. The display was intended to show how fast different breakers trip. By inserting a paper clip into the outlet (dead short) the switch was turned on. With every single breaker manufacturer on the market the paper clip caught fire. All but with the Square D breakers. With theirs you could plug the paper clip in the outlet while the power was turned on. Now granted not everyone will do this and the test was biased toward their breakers, but the intent of the display was to show what can happen with a frayed extension cord. Once you get into very few wires and a high load it will draw a lot of extra current. Enough to start a fire, as the paper clip demonstrated. Needless to say I went home and switched out my breakers.