How are the energy balance items calculated?

I’m trying to figure out how each of the items produced by the energy balance component is calculated, more particularly, from which of the E+ output variable?

May I ask:

  1. if the summary shown below is correct?
  2. is the storage item calculated by summing all the items below it, considering the sign of each value?
  3. how is mechanical ventilation energy calculated?

I assume that mechanical ventilation energy is calculated based on the following code for Read EP Results component. But I’d appreciate if you can advise which particular output variables are involved in its calculation as referred here.



… to update, just found that solar gain through glazing has to be subtracted from glazing energy to get just the heat conduction through glazing surface. So, here is the updated chart. Still appreciate your advice on the calculation of mechanical ventilation energy and energy stored in building materials.



@Grasshope, thanks for doing this! Every now and then I go through a similar exercise, digging through the Read EP Results code trying to figure out how HB calculates some of it’s outputs based on the original E+ outputs.

Once we do get the answer to your questions,please post your completed spreadsheet here! I think it’ll be really useful to all of us.

Hi @SaeranVasanthakumar, the excel file is attached below for your reference.

I added a few more lines to include the E+ output variables related to each of the items on the Read EP Results component.

Appreciate your advice on whether it is correct and complete.


Energy blance _v003.xlsx (12.5 KB)

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Hi Grasshope,
The header of the python script you are showing would suggest to me that ‘mechanical ventilation energy’ is the amount of sensible and latent energy (or lack of it) that was present in the outside air (in relation to the air inside the zone) before it was treated by the air handling system.
I have to admit that i am speculating here though.

Hi Grasshope,

Sorry I didn’t read your post properly. My response did not really answer your question.
Isn’t the mechanical ventilation energy determined by subtracting Supply Air Total Cooling Energy [J] from Zone Total Cooling Energy [J]? That is how I interpeted Chris’s reply to your question on the old forum.
Good that your posting this by the way. I’ve also been puzzled by the EP results component.



Hello @Grasshope and team,

Good question, as always. You are right that many of these outputs have to be calculated from other E+ otuputs since, even though E+ has hundreds of outputs, it’s missing a few critical ones that are needed to make complete energy balances.

  1. Your summary is mostly correct. The window conduction is calculated from the Surface Total Window Heat Gain/Loss energy once solar is subtracted out from this total energy (it looks like you already figured this out on this thread).

  2. The storage term is basically just the remainder after all other energy balance terms have been accounted for. So it’s calculated by computing all of the other gain/loss variables from the E+ outputs and then adding these terms together (since the energy balance terms must sum to 0 in order to obey the first law of thermodynamics). It’s mostly there just to make sure that the energy into the building equals the energy out. For most energy balances, you want this storage term to be relatively small over the long time periods like a month. Otherwise it can be an indication that there’s another important energy balance term that isn’t being accounted for. Along this line of reasoning, the storage term can be useful for accounting for other phenomena that aren’t present in the other terms. For example, if you are looking at the energy balance of a singe zone that is a part of a multi-zone model, the storage term is an indication of the flow to/from your zone to the other zones.

  3. @SamueldeVries has pretty much answered the question about mechanical ventilation heating/cooling. E+ doesn’t give us the energy needed to heat/cool the outdoor ventilation air on its own. However, we can figure this out because we can get the total amount of heating/cooling done by ideal air system (ventilation + zone cooling) and the amount of heating/cooling that the ideal air system has done to just the zone (without ventilation air).

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thank you very much, @SamueldeVries and @chris, for the detailed clarification.

Yes, the mechanical ventilation energy is calculated as the following as you explained:

( Zone Ideal Loads Zone Total Heating Energy
- Zone Ideal Loads Supply Air Total Heating Energy)
( Zone Ideal Loads Zone Total Cooling Energy
- Zone Ideal Loads Supply Air Total Cooling Energy)

The updated excel sheet is attached for everybody’s reference.

May I also ask:

  1. Is the item opaque conduction in the energy balance chart calculated from Surface Average Face Conduction Heat Transfer Energy?
    This is the only item that I’m unable to get the same value as that produced by the Construct Energy Balance component for some reason …

  2. The solar item on the energy balance chart seems to be solar gains through glazing surfaces. What about the solar gains through opaque surfaces, and what is the output variable from which it is calculated?

Appreciate your advice.

EnergyPlus output variables _v010.xlsx (14.3 KB)


@Grasshope ,
To answer your questions:

  1. The opaque conduction is derived from the surface average face conduction heat transfer energy as you have said. Notably, the opaque conduction term is just for opaque exterior surfaces of the model (which the energy balance component figures out by using the HBZones). So, if you connect up a multi-zone model to the HBZones, these terms will be the heat conducting through the exterior envelope of the whole building.

  2. The solar term in the energy balance represents only the solar energy that is transmitted directly through window surfaces into the zone. Any solar energy that gets absorbed by opaque or glazed surfaces and conducts into the zone will appear under the opaque conduction or glazed conduction terms respectively. Let em know if this is clear and if you are able to get the terms to match. The solar term in the energy balance should be the zone total solar energy that comes out of the readEPResult component.


Thank you very much, @chris, for the clarification.

  1. I think the total Opaque Conduction heat gain/loss value I got is the sum of Surface Average Face Conduction Heat Transfer Energy reported for all opaque surfaces including interior floors and ceilings. I may need to find a way to filter the exterior surfaces out to get the building envelope opaque surface conduction heat gain/loss, as you explained.

  2. Do I understand your clarification correctly that the solar gain of a zone is obtained through the following three components?

  • solar radiation -> absorbed by opaque surface -> conduction heat gain for zone;
  • solar radiation -> absorbed by glazing surface -> conduction heat gain for zone;
  • solar radiation -> passing through glazing surface -> direct solar radiation heart gain for zone

I’ll check if there is an output variable on the solar radiation received on each exterior surface.


  1. You are correct about your description here. You can use the surface properties of the HBZone to identify exterior surfaces or you can use information in the .eio file to do so (this file should have a record of all surface properties in the model).

  2. I’m not entirely sure I understand your question here but, assuming that you are asking how EnergyPlus deals with the solar energy that hits the exterior of a zone, you have given a complete description of the portion of that solar energy that can make it to the interior of the zone. It might be worth noting that the solar energy that gets absorbed by opaque or glazed materials can only conduct to the interior of the zone if the combination of sun intensity and outdoor air manage to heat the exterior surface temperature to above the temperature of the zone. Otherwise, the net heat flow will be from the zone towards the exterior. So, if the outdoor air is cold enough, solar energy that gets absorbed by a surface might not be able to conduct to the zone interior. Hopefully, that helps explain why this absorbed solar energy is treated as part of the conduction through the envelope.