AC/DC wattage conversion
When it comes to cables delivering power, then it is the current that is the most important. Or rather, easiest to calculate with. This is because power across a component is resistance times current squared. 

Your household-cable is rated for 6 A AC. This value is in RMS (Root mean square) value. So no conversion is required, it's equal to 6 A DC. 

So no. You cannot use one of your cables to supply all that current. You can however put several cables in parallel, namely \$\frac{21 A}{6 A}=3.5 \rightarrow 4\$. If I were you I would add a couple more to add some margin, so say 5 or 6. 

You can do it with at least 4 of those household-cables in parallel. It would certainly work. Though I'd recommend getting some other cable rated for 21 A, because it will look more professional + take less space because you won't have as much plastic around the cable.