We boil a full kettle once in the morning and put what we don’t need in a flask. Can easily reheat a cup full in the microwave for a cuppa later in the day. I’ve started turning the washing machine down from 40o to 30o. Pre plan cooking and fill the oven when you’re switching it on. eg. Pop in some potatoes to bake which can be reheated in the microwave for dinner next day, or roast veg such as carrots or cauliflower rather than boiling them so you’re only using one heat sauce. And NEVER buy ice cream from a kiosk in the summer, find the nearest convenience store and buy a box of them from the freezer department.
When making anything with mince spaghetti Bol / chilli / cottage pie etc etc cut down on the mince and use lentils instead. Wish I’d have known this when my kids were little. Makes no difference to the flavour and the mince goes twice as far.