A possibly stupid question

[uranian]

Having spent the past week getting my first 2 mining rigs up and running, I've read in the CGMiner readme files that the figure I should  ultimately be paying attention to is the WU, rather than the kh/s, despite the fact that everyone seems to obsess over this. I've been researching Block Erupters a bit, and I see 3 of them running at 1gh/s, but producing a WU of 13.8/minute, in contrast to the 435 WU/minute I'm getting from an HD7970 (yes it's a crappy version of this card, unfortunately). I understand that USB block erupters are using loads less power than a GPU, but given that the auction linked below is for a similar cost to the 7970 I bought while producing 23 WU/min rather than 435 WU/min, I'm wondering if I'm either missing something (my electricity costs to run the 7970 are 100 times that of the USB sticks, but still less than a dollar a day), or if Block Erupters are a waste of money.

http://www.ebay.co.uk/itm/5-x-USB-Bitcoin-ASIC-Miner-Block-Erupter-IN-HAND-Ships-Immediately-asicminer-/190942176648?pt=UK_Computing_FlashDrives_SM&hash=item2c750a6588

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[PincoPluto]

Having spent the past week getting my first 2 mining rigs up and running, I've read in the CGMiner readme files that the figure I should  ultimately be paying attention to is the WU, rather than the kh/s, despite the fact that everyone seems to obsess over this. I've been researching Block Erupters a bit, and I see 3 of them running at 1gh/s, but producing a WU of 13.8/minute, in contrast to the 435 WU/minute I'm getting from an HD7970 (yes it's a crappy version of this card, unfortunately). I understand that USB block erupters are using loads less power than a GPU, but given that the auction linked below is for a similar cost to the 7970 I bought while producing 23 WU/min rather than 435 WU/min, I'm wondering if I'm either missing something (my electricity costs to run the 7970 are 100 times that of the USB sticks, but still less than a dollar a day), or if Block Erupters are a waste of money.

http://www.ebay.co.uk/itm/5-x-USB-Bitcoin-ASIC-Miner-Block-Erupter-IN-HAND-Ships-Immediately-asicminer-/190942176648?pt=UK_Computing_FlashDrives_SM&hash=item2c750a6588

I don't know much of mining for now but they warned me about mining with a graphic card, you warm it basically for nothing, i got a hub with 10 ports and would like to start with 10 block erupter

[-ck]

Sounds like you're comparing bitcoin mining on the erupters to sshyt mining on the GPUs which are not comparable.

[Xyver]

Block Eruptors are wayyyyy more power efficient then GPU's.

Graphics cards are ~200 watts for 300-700 megahashes.

Block eruptors are 2.5 watts for 335 megahashes.  They're basically a graphics card equivalent, yet they take 2.5 watts instead of ~200.

[uranian]

Sounds like you're comparing bitcoin mining on the erupters to sshyt mining on the GPUs which are not comparable.

does the 0.9 ratio between kh/s and WU applies to BTC mining too, or is that just for scrypt? ah i l know, i shall just go RTFM.

related, am i correct in understanding that the WU is the figure i should be paying attention to? i can get my 7970 hashing faster but producing less WU, so i'm thinking that it's better to have a lower hash rate with a higher WU.

[uranian]

aren't manuals useful? 

Q: What should my Work Utility (WU) be?
A: Work utility is the product of hashrate * luck and only stabilises over a
very long period of time. Assuming all your work is valid work, bitcoin mining
should produce a work utility of approximately 1 per 71.6MH. This means at
5GH you should have a WU of 5000 / 71.6 or ~ 69. You cannot make your machine
do "better WU" than this - it is luck related. However you can make it much
worse if your machine produces a lot of hardware errors producing invalid work.