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Let's say I have four computers, each mining bitcoins at 5 Mhash/s. Is this the same as mining bitcoins on one computer with a speed of 20 Mhash/s?
Does mining with multiple computers add up?
Answers
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Yes, and that'll net you around 0.01 USD worth of BTC per year (as of January 2016), assuming your power is free.
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Yes, the hashrate adds up.
But GPU mining ended in 2013. The tiny bitdust you will earn will never reach a mining pool's minimum cash out limit. So you will never see any bitcoin. This will be a pure loss consisting of spent electricity and worn-out hardware.
You could buy a USB stick for mining, for 10 USD. It would mine at a loss due to electricity costs. Even if you have free electricity it would never earn back the 10 USD. So this would also be a loss.
The only way to mine now is with modern stand-alone miners. They have a much higher hashrate and efficiency and they use a tiny built-in computer instead of connecting to your PC. You'll want to get one that runs 1 TH/s (1000 GH/s) or faster. You will not want to pay more than 0.30 USD per GH/s for a miner with power supply. You will need cheap electricity.
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Yes. More generally though:
x mining machines with an average hashrate of y hashes per second will have the same chance of finding an arbitrarily low blockhash in a period of time as one mining machine with a hashrate of xy hashes per second in the same length of time.
1Alternately, you can sell your keyboards, and that'll net you more money. You can still use a computer with just a mouse. – Nick ODell – 2016-01-26T22:03:28.417
1.01 seems generous for 20 Mhash/sec – Mark S. – 2016-01-27T00:19:31.797