The only disadvantage is that Nicehash has a 0.001 BTC minimum payout compared to freebitco.in 0.0003 and if you're already claiming the faucet and have referral income that does not really take long.
On the question of electricity consumption is there any way to calculate how much extra power is being used if you set the mining to use only 10% or 20% etc.? If the computer is being used all day how much extra cost am i incurring by having this in the background?
You are right about the higher minimum payouts of Nicehash compared to FreeBitco.in, it's even higher (0.01 BTC) if you are using an external wallet. That is why it will take a while (several months at least) to reach the minimum payout amount if you are only mining with your processor (CPU) using Nicehash. But if you also have a modern desktop mid-range or better graphics card (GPU), you will earn bitcoins much faster with Nicehash, because it can utilize your GPU, too. I don't recommend mining on laptops.
Regarding the power consumption, you can use a software like
HW Info to check how many Watts your CPU is using when idle and when mining and then do the calculations to see if mining is worth it for you.