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Hi Nerys,
You are correct in your assumptions, you will be dumping wattage across your resistor if you choose to do it that way.
There are ways to get around the loss in the resistor which are
- as suggested already, linking several LEDs together so that the voltage drop of all of them amounts up to say 10 V then the resistor drops a comparatively small amount. Note: changes in your battery voltage (when charging while running the car) will dramatically change the current if you do it this way, however the voltage regulator or a current regulator will fix this issue.
- You can also do zeners but they are just as good as resistors, they will still consume the same amount of power
- The ideal way is to use a LED driver which pulses the power into the LED (sorry im dumbing it down a bit, i can explain in much more detail if anyone wants). These systems can be purchased usually from electronic shops however they are pricey well will likely cost more than the LED however they may be around 80 or 90% efficient which means if your LED uses 1 Watt these systems may take 1.11 Watt from your battery. You can also build these yourself however you will need to make a circuit board and get components so i wouldn't recommend this method for anyone that isn't an electrical engineer or electronics hobbyist.
hope that gives you a bit of a guide,
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