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Convert Joule to Wh, when 'Reporting Frequency' is different from 'Hourly'

asked 2020-09-18 18:05:41 -0600

rainbow's avatar

updated 2020-09-20 09:00:01 -0600

I'm running some simulations by asking EnergyPlus (through the IDF) to output Output:Meter,Electricity:HVAC (an output meter which measures the electric energy consumption in Joule).

If I choose an 'hourly' reporting frequency, like this:

Output:Meter,Electricity:HVAC,hourly; !- [J]

I guess that I can obtain Wh from J in this way:

Wh = J / 3600 s,

E.g.: 4 J ==> 0.11111 Wh

But, if I choose a reporting frequency equals to 'timestep', like this:

Output:Meter,Electricity:HVAC,timestep; !- [J]

where 'timestep' = 4 (so there are 4 timesteps per hour, hence I have 15-minutes timesteps):
how can I convert that value of energy measured in Joule (but "sampled" every 15-minutes) to a value measured in Wh?

Is it sufficient to divide it by a factor of 3600, and then divide it by 4 (n°timesteps per hour), to finally obtain an energy value expressed in Wh?

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answered 2020-09-19 17:07:56 -0600

Your equation converting $J$ to $Wh$ is correct for the hourly output from EnergyPlus, which could be rewritten as

$Wh_{hourly} = J_{hourly} * \frac{1 hr}{3600 s}$

If you compare the timestep output to the hourly output, you will see that EnergyPlus is already accounting for less energy consumed each timestep. For example, if the hourly HVAC electricity use was $400 J$, using 4 timesteps per hour would give a timestep output of $100 J$ (assuming constant HVAC consumption). Because of this, you should use the same equation above for converting the $J$ timestep output from EnergyPlus into $Wh$.

$Wh_{timestep} = J_{timestep}* \frac{1 hr}{3600 s}$

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Ok, thanks for your clarity!

rainbow's avatar rainbow  ( 2020-09-21 02:31:51 -0600 )edit

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Asked: 2020-09-18 18:05:41 -0600

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Last updated: Sep 19 '20