Lol You are a joker with your Watts.
You really think that that is an issue?
The importance is in other things, with
a high frequency change of the chain you:
save your cassette
AND chainrings
AND derailleur pulley wheels
AND keep changing gears on a smooth level
AND limit the possibility of a broken chain (ever seen the damage of a broken chain?)
Buying a bunch of chains in once is the best way to go, you will get reduction in some way and or save on shipping
This especially counts for the more expensive drivetrains. Cheap drivetrains is maybe less a problem because less money is involved in replacement.
However there is more than budget only, look to the sum up here above.
Keep in mind. When you just go on with your chain, then when you need to change the chain for what ever reason,
a new chain doesn't work properly on a fairly used drivetrain. So in this example the chain you need to give up but the rest of the drivetrain is fairly used but not done yet. What will you do? A new chain will not run smooth because the fit is not right.
Now we come to another known method!The breaking in method:Buy 3 chains with a new sprocket and use it in the following way:
use chain nr 1 for 1000 km (orwhatever) and take off the chain and preserve it
use chain nr 2 for 1000 km (orwhatever) and take off the chain and preserve it
use chain nr 3 and use it up to the almost max wearlevel, take it off, throw away
Now use nr1 again till almost max wearlevel, take it off, throw away
Finally take nr 2 again till almost max wearlevel, take it off, throw away
Then its the time to buy 3 new chains and a new sprocket
Why this method?A new chain on used sprockets doesn't work very smooth
A slightly used chain has no problems with a slightly used sprocket
In this method chain nr 1 and nr2 are prestretched and will not cause smooth problems.
Recommended is the High frequency change method.
A good alternative is the Breaking in method.
All other methods are unintelligent and has too many possible problems and should be avoid.