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In questions like this one and in websites everywhere it's noted that SSDs use much less power than HDDs... usually citing ~2 W vs ~6 W for HDDs, my question (and sorry if its stupid just need to be exact) is that per hour? Need to calculate costs savings in power consumption if we switched all our workstations to SSDs

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up vote 20 down vote accepted

If your drive consumes 2 Watts for 1 hour it would have consumed 2 Watt-hours of energy.

A Watt is merely a measure of power use. It's derived by multiplying voltage and current draw. A drive that runs at 12 V and draws 100 mA of current would consume 1.2 Watts of power.

To restate - if you ran that drive for an hour, you'd consume 1.2 Watt-hours of power.

In your case, 2 Watts vs 6 Watts, your cost would be 1/3 for an SSD vs HD. Calculations of time are unnecessary.

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Gotcha, thanks! – Hershizer33 Apr 30 '13 at 15:33
Technically, power is the rate at which energy is transfered. A watt-hour is a measurement of energy: the amount of energy used in an hour at the rate of one watt is one watt-hour. Also note that many computer parts are labeled with the maximum power they may draw; actual power usage may be less at most times. – depquid Apr 30 '13 at 16:42
Also, keep in mind that residential electricity is usually billed in kilowatt hours. Using 6 watts for 24 hours/day, 365 days/year will be about 52.5kWH. At my power company's current residential rates, that's about $4, so an SSD isn't going to pay for itself in energy costs anytime soon. – depquid Apr 30 '13 at 16:54
@depquid, right, but for laptops on batteries with ~50Wh capacity a 4 watt difference in power consumption may be quite a significant factor. – ulidtko Apr 30 '13 at 20:50
@ulidtko Yes, but the OP was about power cost savings, not battery depletion. I just realized that it says, "all our workstations," which implies a business environment. Since electricity for businesses may be billed for both power (demand) and energy used, and many rate plans are available, calculating business electricity costs for HDDs vs. SSDs is going to be much more complicated. – depquid Apr 30 '13 at 21:36

TLDR: Its not a cost-effective way to reduce your energy use, except on a laptop.

One watt is equal to one joule (a unit of energy) per second. Watts measure the rate of energy transfer (power). You're billed, most likely, by amount of energy consumed, probably in kilowatt-hours (kWh). That's equal to 1000 watts used for one hour (hence the name).

You don't actually need to figure that out yourself, Google will do the math for you. (That's 1 watt, used constantly, for one year—24x7. You can multiply by "(40 hours/week)", etc. as needed, Google calculator is pretty good at this kind of stuff.)

Now, the next problem you'll face is that neither HDD nor SSD are constant-power devices. Both use more power when actually reading or writing than when sitting idle. And hard disks that are idle for a bit will spin down, and use almost no power. Further, SSDs are generally faster, so faced with the many workloads, will get back to idle sooner than HDDs. So you'd really have to measure to get a good number, as Brad Patton says.

But, as an upper limit, let's take that 6W figure, and ask how much it actually costs to use it 40 hours per week, all year—assume that a SSD uses no power. Google gives 13 kWh. Even if you're paying a fairly high rate, say 30¢/kWh, that's under $4/yr. At over $100/SSD, even with a 0% discount rate, the payback period well exceeds the lifespan of the SSD.

It's different on laptops. For example, my laptop for example runs on about 6.5W total. So saving even a fraction of a watt increases battery life noticeably.

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To calculate cost savings you should really get a Kill-a-watt type device and measure the workstation for a day or more with a HD and with a SSD. Since usage will vary quite a bit over time and that will be the best representation of the actual savings. Also this completely ignores the incredible performance benefits of SSD drives.

See Jeff Atwood's blog post Why Estimate when you can measure for more information.

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You state 2w vs 6w SSD and HDD, that's peak power useage, if you are reading web pages, the SSD will be idle alot of the time, whereas the HDD will spin. The SSD longevity is 1/4 longer than average HDD i've read, which relats to transport and production energy, and it uses less raw materials.

At 50% idle time, the comparison is 1w vs 6w.

for a laptop of 20 watts with 3 hours of battery, you would have 1/5th more battery life. it's about 30 minutes extra.

hdd ssd

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E=PT (Energy = Power * Time ) so see what Power is. It's Energy over Time.

P=IV (Power = Current * Voltage) FYI in case you need to calculate.

Using E=PT (which shows nicely what power is) Power(Watts) = E/T = Energy(Joules) / Time(Seconds)

Power(Watts) is Joules per second.

There is such a thing as a watt hour and a kilowatt hour.

a watt hour is a unit of Energy, an alternative to Joules. Not a unit of power. It's how much Energy that is one watt(joule per second) over an hour. One watt hour is 3600 Joules. So, it's a bigger unit.

If you included in your post, the electricity cost, then somebody might be able to tell you. Otherwise perhaps somebody can tell you an electricity cost to use as an example

I suppose you could get an energy monitor and plug a drive in so it gets its power from the wall/mains socket. Or just use the rated power that the hard drive shows you. Then find out what the rate of charge is for electricity.

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When trying to cost justify SSD drive conversions, the BIGGEST Factor by far for any business to consider is PRODUCTIVITY. Since your employees are your largest expense, their TIME is the most precious commodity you have. SSDs are at LEAST TWICE as fast as hard drives at EVERYTHING, including boot time. In many cases they are 10 times faster or better.

Other considerations as to the POWER calculation, you need to consider the fact that SSDs generate almost no heat at all as opposed to the roasty toasty HD. If your company runs a LOT of drives, the overall effect will subtly lower your cooling costs, probably resulting in more power savings than the drive hardware itself.

Lastly there is the NOISE Factor. Obama's GOOD ideas have made more noise than any SSD ever will. Compare that to the constant whirring and clacking of any HD.

In my opinion, there is NO CONTEST. If you have the budget, go fully SSD. If you are on a shoestring, consider this: install 2 drives in each desktop - one SMALL SSD (8 to 16 gigs will do in most cases) to hold the operating system and program files, and a larger DATA ONLY HD. That will get you most of the performance kick without breaking the bank.

I speak from 30+ years experience with personal computers when I tell you this: I've made countless upgrades in my life to countless systems, and in terms of bang for the buck and immediate performance improvement, none of them hold a candle to the HD to SSD upgrade.

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This does not answer the actual question as asked -- how do you calculate the power savings? – Kevin Panko Nov 16 '14 at 16:33
This is an off-topic rant that, setting aside all the pointless SHOUTING, is aimed at different topics than asked. I wish I had enough rep to downvote – underscore_d Oct 3 '15 at 14:26

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