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
If your drive consumes 2 Watts for 1 hour it would have consumed 2 Watthours 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 Watthours 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. 


TLDR: Its not a costeffective 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 kilowatthours (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 constantpower 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. 


To calculate cost savings you should really get a Killawatt 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. 


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. 


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. 

