Best shunt resistor for power meter application?
The key is finding one with low enough resistance that it doesn't have to dissipate much power.

Two of these in parallel would do the job: http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&amp;name=TMC5-.10-ND

With two in parallel, the resistance will be 0.05 ohms, so at 20 A, you'll have to dissipate 20 * 20 * 0.05 = 20 W. They're rated for 5 W continuous, or 10 W for the two of them. As long as you don't have to maintain 20 A, they'd work fine.

I'll see if I can dig up something better.

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Edit: here's a better solution.

Try one of these: http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&amp;name=MR50.01FTR-ND

The resistance is 0.01 ohms, so at 20 A, you'll dissipate 20 * 20 * 0.01 = 4 W. The part is rated for 5 W continuous, so you're safe.