If you skimp on the memory for your system, it’s just going to swap-to and read-from the hard-drive more often which will definitely use much more power.
Also, despite the excellent answers you’ve received, power draw definitely depends on how the memory is used. The refresh cycle you are concerned about only happens about every 10 milliseconds, but the bits also have to be refreshed again after every read, since a read also depletes the capacitors. The typical read latency for RAM is about 5 nanoseconds. That difference is 6 orders of magnitude! That means that simply reading through your whole memory once will use a million times more energy than the refresh ("idle") workload did during that time.
So, if you have more memory than you need, the excess memory you are not using is definitely using a lot less energy than memory you are actually utilizing. (In other words, by about a wide margin, you only pay for what you use. ) Combine this with the disk swapping consideration, and you would likely find that adding RAM appropriate to the workload likely to reduce overall energy usage.
Of course this guideline does not scale to absurd levels, as clearly, if you install 4GB RAM to only run solitaire on windows 95, that extra RAM would be a pure energy waste.
On the other hand, if (as it sounds) you are debating the battery life and utility bill consequences of putting more RAM in your laptop, and you will be multitasking with XP or later, there are any number of factors are going to be far more significant to your consideration. Here are a few:
- The number of apps you run.
- The efficiency of the Antivirus software you use.
- The features of the OS that you enable. (aero, indexing, background services, etc.)
- Backlight level.
Any one of the above factors are going to matter more in the long run than the power used to keep refreshing under-utilized RAM.
1Voltage does not indicate power. (And here's a €-character to copy into your answer.) ;-) – Arjan – 2009-09-12T13:10:39.640