If you're looking only at idle power usage, the difference is around 20 watt compared to an idle pc. That's 0.5KWh. With the currency energy prices in NL (40ct/kwh, subsidized by the gov btw, because it's actually around 70ct now), this will save you €73 per year when idling, and even more when it's in use.
So the macmini essentially for free after a few years compared to your windows machine