It doesn’t sound like much, but boiling too much water when you cook is a waste. It’s not just the water, after all. It’s the energy it takes to heat it.
You aren’t always going to be certain of the exact amount you need, of course. Most people I know aren’t going to measure the water they put in the tea kettle when boiling water for tea. Then again, it’s more efficient to boil the water in the cup you’re going to use in the microwave if one cup of tea is all you’re doing. Just watch out for sudden boiling from superheated water.
You may also want to consider steaming vegetables rather than boiling them. First of all, steaming is generally better for keeping nutrients in the vegetables, rather than releasing them into the water. I think they taste much better that way too.
In addition to only boiling what you need, remember to put a lid on the pot when you’re boiling water. It will come to a boil sooner, saving both energy and time.