OK, two possible scenarios (based on real-life usage):
1) I have a regular PC or laptop running. I use a computer a lot and my PC is running basically for 16 hours a day. I have space storage that I want to use for SIA. Thus, my electricity costs directly caused by Sia are a PC running for 8 hours per day. My PC (old Intel C2D E8400) consumes about 80 watts when on at idle. Let's assume 20 watts in case someone uses his laptop. 8 hours 365 days a year at 20W will give me about 14 dollars per electricity (Europe). If I let Sia use 1TB of storage, from my understanding of Sia, ~340GB can be effectively utilized for (the rest is redundancy). I earn 10.24 USD per year (current price of 2.56 USD/TB/month).
Result - loss of ~4 dollars per year.
2) I will use a dedicated hardware (let's assume something like Raspberry Pi) and 1 TB drive. RPi consumes about 1W of electricity. Let's assume SSD drive which consumes 0.05W in idle. So that means about 1W of power. Yes, then we have the electricity "for free". But your RasPi which is 40 dollars will get paid in more then 3-4 years, not counting the cost of that damn drive. Or you have spare 1TB SSDs for free?
I don't know - try to explain it to me - why is it actually worth for someone to rent its hard drive space. Even if the rewards were 5x that big. Even if I had electricity and the hardware (1TB drive) for free and the prices were x5, I would receive mere 5 USD per month. That is one hour of dishwashing somewhere in the pub like work in my country. And your space has to be 100% utilized which is clearly not the current state of Sia.
Yes - it may make sense for big server players who have hundreds and hundreds of GB spare. But then we are getting closer to centralized storage and not decentralized.
How about 3rd possibility? because 1st one you mentioned won't work at all, you need 99.9%+ uptime to earn good money
So...how about four 8TB disks for 32TB overall (about 220kWh per year plus maybe 200kWh more for Intel Atom-like PC) for about $1000 per year minus electricity?
That would about 1y ROI, but since whole market is low atm, so is sia coin...not too long ago that would be more than $3000 a year
Of course, I took best case scenario, and truth is somewhere in the middle...point of the story is, you can forget using laptop, or anything non-dedicated...you need dedicated
rig, same as with GPU mineable coins
So the only possible way for Sia to work is to be dependent on a large number of individual large-scale (tens of TB at least) providers which will own the hardware? Well, my opinion is that this won't end well. Sia markets that everyone can rent his free hard drive space for Sia customers.
We have concluded that this is not profitable and users have no reason to do that.How are you calculating the Sia rewards? The redundancy is x3 so either only third of space of is used effectively to store user data or the user is charged 3x as much for the data storage. Personally I don't know - do you know how this works?
The redundancy must be stored somewhere. That means not 1000USD per year but one third of that which makes your ROI about 4 year-ish counting the electricity. Still not very good.
If the price of storage will rise to 3x current cost, the cost advantage against let's say Google Drive or Dropbox diminishes. Then it will be almost solely about speed, apps developed on top of the storage and marketing.