Walmart sells a rack that will hold 19 GPU’s to build a mining rig. You might have considered putting two PCIe cards in a computer, but what does it really mean to put 19 video cards in a single computer?
First off you need a motherboard with 19 GPU slots. Actually, that is not entirely true, because the mining cards do not need the full x16 interface. So a professional will start with an Asus B250 that has 18 x1 slots and one full x16 slot. The x16 interface can be used in a standard case as a standard video adapter, but the remaining 18 x1 connectors are not meant to be used in a standard case. They are used with an x1 extension cable to the mining cards.
So what we are doing is using a ribbon cable to carry the PCIe x1 signals throughout the cage of video cards. Just a note, if each video card costs $1,000 then you can calculate the investment in just video cards.
But we are not done yet, because we need a power supply that is powerful enough and has enough power connectors to power 19 gpu’s. The B250 has three 24 pin power slots, so if you buy some less expensive video cards, they can get all their power from the ribbon cable going to the motherboard. We still have the problem of consuming something in the range of 75 watts up to 175 watts per card.
Since power supplies typically have a single 24 pin connector, it would seem that the intent is to use three power supplies. So three nice power supplies might be able to run twelve to 15 video cards. The fact is, if you were going to attempt this, you should not try to run the maximum number of cards. In fact you should start by running four or more cards, and learn as you go.
But suppose you can get a dozen high end cards mining bitcoin. You could run Linux or Windows, there are mining applications for either system. The GTX 1070 uses 150 watts and a lot of hashing speed. They also have at least one 6 pin, and sometimes two 8 pin power connectors, so that could make it challenging to run more than six cards on three power supplies.
But the reason we get excited about building a rig like this is to make money. So first we have to point out that nobody is mining bitcoin with Nvidia cards, there are more profitable approaches for this type of setup. So assuming you choose your currency wisely, each 1070 can mine more than $50 of currency per month. Now that you are seeing the dollar signs, put off looking at your electricity bill as long as possible. That is because each power supply can use up to $100 worth of electricity each month.
Normally, the power supply in a computer runs at a small percentage of it’s rating. Plus the computer goes to sleep more hours than it is awake. In mining digital currency, the power supplies are going to run full throttle 24 hours each day. So the cost of electricity roughly equals the mining revenue. And as the video cards and power supplies wear out under heavy usage, nothing is earned towards paying for replacements, much less paying off the initial investment.
Of course, the calculation are pretty rough. A slight rise in the price of digital currencies makes the project more profitable. Choosing the currency and finding less expensive electricity is a major component of the cost. The cost of electricity in Florida or New York City can be double what is payed in parts of Washington State.