Power consumption: SSD vs HDD

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.

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.

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.