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.