I drink a lot of hot drinks and am aware the majority of our electric bill is that. 
So, anyone who understands this stuff, what’s the cheapest way to boil a kettle?
So far I’ve looked / thought at;
One cup dispenser, had one. It was shit, gave out varying amounts of water at differing temperatures. However, newer fancier ones may be better.
Using a thermos, but your still boiling the same amount of water, just at once - does this work out cheaper?
Insulated kettle, see above!
An urn. Not quick at boiling, uses lower wattage to keep it warm. Would that work out cheaper than boiling the kettle 4/5 times?
Microwave?
Gas hob??
We rent so a hot water tap is probably out due to outlay, instillation and being pissed off at leaving it behind.
Thank you knowledgable people!!