How much power does an SSD take?

Are you adding or replacing? If you're replacing then it will save you, but if you're just adding the SSD WITH the HDD then I don't think it would save you much. Sorry, I don't know how much exactly it will save you, I just know SSDs don't use very much power.

Very little. Less than a typical mainstream HDD (7200 RPM, SATA II).

SSD uses lesser power than a HDD.
Plus, you save some power because using an SSD, you can complete all the disk bound workload quicker, and then the whole system can come in the idle, power-saving mode quicker.

i was planning to add a SSD in addition to my hard drive since my laptop supports to drives. I was thinking about putting win 7 and other games on the SSD. Do ssds consume power even when idle?

If your worried about battery life, build a desktop.

An SSD uses very minimal power...I think the latest one I bought said it uses 0.2 W when active and .65 mW idle, compared to the typical 10W active a mechanical HDD uses.

So if you are putting your most common applications and files on the SSD you will probably see a longer battery life on your laptop since its using the drive that needs less power more often, but it totally dependent on what files and programs you put where.

every computer part takes some power even when idle. But comparing the idle power usage of HDD vs SSD : SSD wins hand down. An SSD at full load takes about the same or lesser power as an HDD at rest. So at idle it takes much much less.