As a pragmatic centralist environmentalist, hope this helps

People should keep their common sense about them on this one.

Well you saved energy (perhaps some cash also) by only boiling 2 cups worth as most kettles are thirsty 2000W appliances, and the 66% (steam) and 9% (national grid transmissions)lost at the central thermal power station generating the mains juice initially.

Cleaning and pumping water does comsume energy but less energy per volume than electrically boiling water via the mains/kettle. Yes perhaps research should confirm what seems common sense

Other choices

Put excess water into a filter jug to reuse later
Put excess water on house or pot plants

What else could you have done to reuse the excess water, conveniently? Don’t think we have to live bizarre extreme green lifestyles to use water and energy mindfully or wisely. Although if the hydrogen revolution comes about one day, people could drink water coming from the car exhausts in the future!