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Currently here it is 9 degrees Farenheit. My home is heated by electric resistance heating (not a heat pump, just plain heating coils). Am I correct in believing that it's more energy efficient to keep our wooden blinds and curtains closed and turn on LED lighting than to open our blinds and curtains for light?

Our windows are double-paned. My local utility says it's better to open the windows than turn on an LED light, but I can't believe that's true under current conditions.

My belief is: Loss of heat due to opening blinds and curtains * electricity needed to replace that heat via resistance > electricity needed to power LED lights (say 8 watts per window).

NOTE: This is when the sun is not shining directly in thru the window. Direct solar radiation can be very heating, as detailed in an answer below.

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  • $\begingroup$ I found the following searching: reddit.com/r/answers/comments/8twpgy/… (which seems to say probably more efficient) $\endgroup$ – Paul Feb 15 at 15:23
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    $\begingroup$ I should perhaps mention that the entire state of Texas is currently subject to rolling blackouts because of this storm, so getting this right could be pretty helpful. $\endgroup$ – Paul Feb 15 at 15:25
  • $\begingroup$ Using sunlight has no carbon emissions compared to LED or candles or arc lights etc $\endgroup$ – Solar Mike Feb 15 at 15:31
  • $\begingroup$ True, but the electricity generation for the electric heat sure has carbon emissions. Although, we get much of our electricity from a nuclear power plant, so I guess there's not as much carbon emissions as if we didn't have that. We have a tiny bit of wind too, but I'm not sure if turbines operate in severe cold and ice. Since this kind of storm happens perhaps only every several decades, I doubt the local wind turbines are set up for it. $\endgroup$ – Paul Feb 15 at 15:33
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    $\begingroup$ hunker.com/12003500/… says to open your blinds in the winter if the sun is streaming thru them. $\endgroup$ – Paul Feb 15 at 16:10
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In order to assess the efficiency you need to compare the heat transfer rate $\dot{Q}$ with and without blinds. There are a few studies comparing the effect of blinds (internal/external/enclosed) on heat transfer. One of them in 2014 Effect of the roller blinds on heat losses through a double-glazing window during heating season in Central Europe, found energy saving of about 33% for internal blinds during the night and 45% for external blinds. (Notice that in order to simplify the calculation they are stating night time).

So in order to calculate the savings you need to be able to calculate the heat transfer of a window.

heat transfer of a window

The actual heat transfer has the following temperature profile:

enter image description here

In this scenario

  • $T_1= -12.5[^oC]$, (9 Fahrneit ) and
  • $T_2= 17.5[^oC]~ \approx 63.5[F]$ (inside temp)

the temperature differense $\Delta T = 30[^oC] $

A typical double glazing window has a U value of $2.8 \left[\frac{W}{m^2 K}\right]$.

Additionally you can estimate that the convective coefficient of transfer is

  • $h_o = 20\left[\frac{W}{m^2 K}\right]$
  • $h_i = 8 \left[\frac{W}{m^2 K}\right]$

Then the thermal resistance of the wall is:

$$R_{total} = R_{out} + R_{windows}+ R_{in}$$ $$R_{total} = \frac{1}{h_o A} + \frac{1}{U_{window} A}+ \frac{1}{h_i A}$$

where A is the window area. This can be simplified to

$$R_{total} = \frac{1}{ A}\left(\frac{1}{h_o } + \frac{1}{U_{window}}+ \frac{1}{h_i}\right)$$

And the total heat tranafer rate is equal given by:

$$\dot{Q} = \frac{T_2-T_1}{R_{total}}$$

$$\dot{Q} = \frac{T_2-T_1}{\frac{1}{ A}\left(\frac{1}{h_o } + \frac{1}{U_{window}}+ \frac{1}{h_i}\right)}$$

$$\frac{\dot{Q}}{ A} = \frac{T_2-T_1}{\left(\frac{1}{h_o } + \frac{1}{U_{window}}+ \frac{1}{h_i}\right)}$$

if you substitute the numbers in you get

$$\frac{\dot{Q}}{ A} \approx = 56 \left[\frac{W}{m^2}\right]$$

energy saving compared to LED

Based on the article that means that during the night you get an energy saving of about $$0.33\cdot 56\left[\frac{W}{m^2}\right]\approx 19\left[\frac{W}{m^2}\right]$$

Assuming you have a window opening of about 1$[m^2]$, the savings are about 1.5 times greater that LED.

Obviously, the savings will be :

  • greater if the window is bigger
  • less as temperature difference get smaller

Radiative heat transfer

The main factor that is of consideration here is the solar radiation. There are three types of radiation:

  • Direct radiation: the direct rays of the sun. (this is zero on a cloudy day).
  • Diffuse radiation: this is always present and its an effect of the presence of atmosphere. In higher latitudes it tends to increase because the sun rays need to "pass through" more atmosphere.
  • reflected (you can neglect this)

If you can get the rays of the sun (direct radiation) into the house, this will always be a significant plus in the heat balance of the house.

Final thoughts

This problem is very instance specific depending on:

  • the location of the house
  • orientation of windows
  • type of blinds, etc.

Therefore, it is impossible to draw a generic conclusion. As such, only trends can be realistically observed.

At a temperature difference of 30$[^oC]$ it makes sense to close the blinds on a cloudy day. However, if you get closer to 15$[^oC]$ then the savings are diminished.

If the windows are positioned in the right angle and light shines into the room, then expect the added solar radiation will improve the heat balance in favour of open blinds.

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  • $\begingroup$ The triple glazed windows are much better at allowing heat energy in and less getting due to infra red - we used those but we did max out on design in terms of passive solar and not many do that. $\endgroup$ – Solar Mike Feb 15 at 20:18
  • $\begingroup$ @SolarMike i have never lived in a house with less that tripleglazed windows. I would posit that chhosing double or tripple is largely based on where you are. I mean if you live at 60 degree north or above its not just about passive solar. $\endgroup$ – joojaa Feb 15 at 23:06
  • $\begingroup$ @NMech what about incomming solar radiation? Ultimately the window leaks even with blinds. $\endgroup$ – joojaa Feb 15 at 23:07
  • $\begingroup$ The scatter radiation information in the answer below is interesting, but I'm going to assume that if I feel like net cold is coming in thru the window when I stand in front of it, that the scatter radiation is not sufficient to overcome the direct loss of heat thru the window. $\endgroup$ – Paul Feb 16 at 1:23
  • $\begingroup$ Great answer, thanks! $\endgroup$ – Paul Feb 16 at 2:14
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The reason your utility company recommends to let the sunshine in is not to save on lighting energy. it is to take advantage of solar radition energy.

a window can let the solar radiation in two ways, direct radiation and scatter radiation.

The calculation of radiation energy has to do with many factors such as the geographical altitude and azimuth of the sun directon. usually 60% of a hypothetical normal ray vector radiates and heats up the interior furniture and walls and floors.

Interestingly the solar radiation energy is independent of the ambient outside temperatures.

The electrical company has calculated the windows heat loss and heat gain and based on combined average seasonal weather offer their advices.

Usually their recommendations is sound.

This is an in-depth article on the solar radiatin energy calculations, heat gains and losses

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    $\begingroup$ This is an important point. The question posits that the sun is not shining directly thru the window, in which case the direct radiation is zero. If I feel cold "coming in" from the window, then presumably this means the scatter radiation is not causing a net heating effect? $\endgroup$ – Paul Feb 16 at 1:21
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    $\begingroup$ @Paul, not necessarily. scattered energy transmitted through the window would warm up the walls and furniture even if the immediate space near the window is cold because of the glazing low surface temperature. If it helps remember the solar radiation has already passed through very cold strata of the atmosphere, with temps in the range of minus 67-75 Fahrenheit. $\endgroup$ – kamran Feb 16 at 1:35
  • $\begingroup$ Interesting. Sounds like conduction vs radiation IIUC. For this to work, the heat generation via radiation must be higher than the heat loss due to conduction. That doesn't seem likely with no direct solar infrared (recall everything in the house is radiating infrared from a 65-degree temp, so the infrared coming in has to be higher than that AND offset the heat loss to conduction). Also, I bet the human body is fairly good at telling whether it is standing next to something that is making me colder or making me warmer. If you have contrary evidence, please share. $\endgroup$ – Paul Feb 17 at 20:06
  • $\begingroup$ I should mention that I re-read my utility's guidance and either they had changed it or I had not read it closely enough, because they did not directly recommend opening blinds and curtains for light when it was cold $\endgroup$ – Paul Feb 17 at 20:10
  • $\begingroup$ Also, if I am standing directly in front of the window, a goodly portion of the radiation coming in the window is falling on me since I'm blocking it, so to speak. $\endgroup$ – Paul Feb 17 at 20:11

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