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Does anyone know a way of calculating the energy needed to heat a home/room? (feddit.uk)

submitted 10 months ago by [email protected] to c/[email protected]

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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.

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[–] [email protected] 4 points 10 months ago* (last edited 10 months ago) (1 children)

I might be able to help you get an approximation.
Though quite honestly, you'd probably save time and effort by just using a thermostat in each room, and setting HA not to power the heat on when pricing is >75% day average, since you know ahead of time, and it'll probably save you about as much.

Overall, you'll need to calculate the heat loss per delta T per room.
You'll likely have to do this by best estimate, then perhaps a bit of real-life experimentation with a 1KW heater, or similar.

For each room, make a list of the external walls, their composition, and the depth of each layer of material depth.*
Look up the rough WMK (watts of heat lost per square meter (at 1m thick), per degree difference from the outside to the inside) for each material.
Take those values, and use them to make an estimate of the room loss.
So for example, a room with 1 external wall of 10m2, made of 215mm brick (0.7WMK), and 50mm of fibreglass insulation ( 0.038WMK).
1÷(((1÷0.7)×0.215)+(1÷0.038)×0.05) = 0.61 WMK.
Multiplied by 10m2, 6.1Watts per degree difference.
So if it's 0 outside, and 20 inside, 120W lost by that wall.

There are a lot of other factors that you can bring in (air tightness, rsi, thermal gain). But this will give you a low ballpark number you can start working off for each room.
So long as you're supplying more than the room is losing, it will gradually heat.
If you want to work out how much it will take to warm all the objects/walls, that's a whole other calculation. But if you're maintaining temps, it doesn't matter so much.

Oh, and a thin sheet of glass will lose about 5W/m2/k. Scary!

* I'm ignoring internal walls, as I'm assuming you'd plan to keep the house roughly the same temperature. And it would get complicated.

[–] [email protected] 2 points 9 months ago

Thanks, I already suspected I would need to get Excel involved and this confirm it! The window thing you mentioned is very real - my place has single-pane 2×3m windows everywhere; their insulative properties are basically negligible.

Once I've got a reasonable set of estimates going I'll probably push the calculations into a Helper to produce daily numbers automagically. If it works reasonably I'll post an update on here. Thanks again!