The point is most people have a bunch of free space on their hard drive they're not using anyway. I've got a few hundred free GB on my desktop, and a couple more TB on my home server. The physical hardware for those drives is sunk cost, and I'm already paying for the electricity to run those machines all day. It doesn't cost me anything extra to merely have some data sitting on the unused space on my drives, so why not make some money off of it?

On the other hand, if I wanted to create a dedicated machine just to farm filecoins, that _would_ cost me extra for hardware and electricity.

This is why decentralized storage has an advantage in this regard. Unlike mining, storing files incurs almost no extra costs for the average home user, but does incur costs for dedicated storage systems.

It costs you the bandwidth (it's not a long term cold storage) and extra processing power (so electricity cost) for running filecoin. This may well be lower cost than the gain from hosting, but it's still non-zero.

Bandwidth is effectively free for many users (no data cap), and the amount of processing power required for something simple like file storage is pretty trivial. You're right that it's non-zero, but I'd imagine it's probably on the order of pennies a month.

Is it really? Even just extra 10W over a month is just under 7 kWh and might approach a dollar[1]. Spinning 3.5" HDDs from time to time is especially costly and they often spin down when unused for a long time. Load spike might also make CPU freq scale up and consume more power. Same with bandwidth, you'll incur ISP wrath or saturate it (or keep wasting most of that free space), even 80 Mbps[1] is 'just' 36 GB / hour and under 1TB per 24h. And that's without including overhead from Filecoin, TCP/UDP and IP themselves.

It sounds lose-lose-lose situation: keep wasting your space or saturate your bandwidth or waste power on keeping HDDs spinning all the time due to accessing them every few minutes. It may vary with your usage patterns (if these disks are already always spinning and you have REALLY good bandwidth or two separate lines) but still, seems barely worth the effort.

And why couldn't it be optimized in a purpose built machine? Cheap Tibetan hydro and free cooling (altitude), low power CPUs, fast internet, fast switches and routers, 10 gigabit lan, many HDDs and SSDs (dug up from the Western trash piles even, it's not like failing once a month is a big deal). Chinese already have ASIC for bitcoin and this seems easier to make than that.

[1] I have no idea if I'm generous or stingy on USA power prices and network speed.

I guess the exact costs would depend on usage patterns, frequency of file read/writes, etc.

It's worth noting though that Sia (another decentralized storage network) is estimating their current storage costs at $2/TB-Month. That's half the price of Amazon Glacier. Whether that's the result of users farming from their home PCs or giant Chinese storage farms though I have no clue.

If you don't have data caps, be happy. But, they're still very common and even if you don't have them in your contract, you may get penalties applied after some threshold.

The cost can be also in concurrent access. Unless you apply a heavy QoS on your network, serving filecoin network may degrade your other tasks.

Now if you don't have caps and want to keep it that way, I wouldn't be excited about filecoin. Unless you're actually paying for guaranteed bandwidth and it's part of your contract, your ISP is overcommitting. The moment people actually start relying on that fact, the ISP can do one of: raise prices and buy more pipes, lower the speed, reintroduce caps. The first one is a limited resource that takes time to apply. The other two are quick and known solutions.

You could make the same argument about AirBnB. If it's profitable to rent out a room in your house, it's profitable to open a hotel. But people are still renting out AirBnB's because their homes are a sunk cost.