Does anyone know a way of calculating the amount of heating I need to maintain an average temperature in terms of kWh of heating per 24 hours? Ideally one taking into account weather conditions.

I have a pretty big Home Assistant setup which includes switches for individually controlling all the (electric) heaters in my home. I'm also using an electricity supplier that changes the amount they charge every 30 minutes to reflect supply and demand. Given these rates are published at least 24 hours in advance I can currently choose a number of hours to run the heaters per day and have an automation automatically select the cheapest periods. I'm paying less per kWh for heating than I would if I was using a gas boiler. Plus, it's all from renewables, so working out that number of hours is the next step.