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You can set the usage in BOINC to just a % of your CPU for example. I had a wall power measure device, running my PC at idle with the screen off used about 200W, at full tilt (no GPU) running BOINC 100% was 250W, the marginal variance was small.
Of course I do have solar panels, which means in daytime its really net zero impact, but 50Watts isnt much.

Wish you would reconsider. I run a desk top and lap top commuter do work for World Community Grid they are trying to help humanity. My lap top is a cheap HP with a AMD ^ processor and is about 5 years old. The desk top is nothing fancy either. Any help you could apply would make a difference. Stop worrying about those GPU rigs the projects do not support those type of computers. Plus they do not take away anything from your regular power. Just don't run them 24,7 you would be still be contributing to a good cause.

I've given a lot before and I will consider it.