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Energy Audit and sizing tool issue and question.

RVLiFe

Solar Enthusiast
Joined
Jun 14, 2021
Messages
234
I noticed an issue with the spreadsheet that I wanted to bring to attention.

First, Cell P33 is supposed to add all of the DC Watt hours together (SUM(P6:32)) but instead refers only to P8:32 witch misses the first two cells in that column. Minor change but it can have a big difference if your first two items that you are trying to do an audit on are DC loads.

Secondly, I have a question regarding the logic behind the "Required Total daily Solar output (Sunny)" calculation in the Solar Array Sizing section. Can anyone explain why the formula adds both the batteries "Daily Storage Requirement" and the" min Battery capacity W-Hrs" together? Isn't this like doubling the capacity needed to recharge the battery? Also, the formula then multiplies the answer by the Max discharge from full CHARGE. No real need to do this.

Example: If the spreadsheet says that my Daily Storage Requirement is 1897 Watt Hours Per Day and I set a 85% Max discharge from full CHARGE, the min Battery capacity W-Hrs shows as 2231.2 W-hrs. My solar panel array only needs to put 1897 Watts back into the battery to fill it. I suppose a theoretical 2231.2 Watts to be on the safe side. Why would the spreadsheet need to add both values together and base the Required Total daily Solar output (Sunny) off of this? Then why multiply by 85%(or whatever User Input was entered)?

I understand if someone tells me that it is doubling the capacity to factor in being under load but for a lot of people, they use their lights and other loads after the sun goes down. At least that is when I use my lights. If I wanted to be certain I have the capacity, that is when I would change the number of days of storage capacity in the spreadsheet, not the amount of PV Panels needed. I can run some lights off of the PV panels without the battery but I can't run my inverter and heavy loads off of them. That's why you need to make sure you have the battery capacity first and then compute the amount of PV panels needed to put the entire capacity back into the batteries in the number of days that you are looking for. Am I wrong?
 

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You may want to post your comment on the resources page so @FilterGuy can fix it.

 
You may want to post your comment on the resources page so @FilterGuy can fix it.

Not sure if it should actually be posted as a resource.
 
Why shouldn't the energy audit spreadsheet be a resource? It already is. There are already comments on that resource pointing out issues. Post your comment there too.
 
Why shouldn't the energy audit spreadsheet be a resource? It already is. There are already comments on that resource pointing out issues. Post your comment there too.
Sorry if I'm missing something. It is and should be a resource. My comment should not be a resource. Also, you can only post a resource on the resource page, not a comment.
 
Sorry, I'm not suggesting that your comment be a resource. Click the resource link I posted to take you to the energy audit resource page. Then go to the Reviews tab of that resource and post a review including your comment about the issue. Or simply click the "Leave a Rating" button on the resource's page.
 
First, Cell P33 is supposed to add all of the DC Watt hours together (SUM(P6:32)) but instead refers only to P8:32 witch misses the first two cells in that column. Minor change but it can have a big difference if your first two items that you are trying to do an audit on are DC loads.
Good Catch. THank you. I will correct it.

Secondly, I have a question regarding the logic behind the "Required Total daily Solar output (Sunny)" calculation in the Solar Array Sizing section. Can anyone explain why the formula adds both the batteries "Daily Storage Requirement" and the" min Battery capacity W-Hrs" together? Isn't this like doubling the capacity needed to recharge the battery? Also, the formula then multiplies the answer by the Max discharge from full CHARGE. No real need to do this.

Example: If the spreadsheet says that my Daily Storage Requirement is 1897 Watt Hours Per Day and I set a 85% Max discharge from full CHARGE, the min Battery capacity W-Hrs shows as 2231.2 W-hrs. My solar panel array only needs to put 1897 Watts back into the battery to fill it. I suppose a theoretical 2231.2 Watts to be on the safe side. Why would the spreadsheet need to add both values together and base the Required Total daily Solar output (Sunny) off of this? Then why multiply by 85%(or whatever User Input was entered)?

I understand if someone tells me that it is doubling the capacity to factor in being under load but for a lot of people, they use their lights and other loads after the sun goes down. At least that is when I use my lights. If I wanted to be certain I have the capacity, that is when I would change the number of days of storage capacity in the spreadsheet, not the amount of PV Panels needed. I can run some lights off of the PV panels without the battery but I can't run my inverter and heavy loads off of them. That's why you need to make sure you have the battery capacity first and then compute the amount of PV panels needed to put the entire capacity back into the batteries in the number of days that you are looking for. Am I wrong?
All interesting points. I will think them through.

Yes, the spreadsheet takes a conservative approach and estimates system sizing on the high side. This is fine for the smaller systems the spreadsheet was originally designed for but perhaps it needs to be adjusted for larger systems. I'll play with it some and see what changes could/should be done. At a minimum, I should make some notes about it being conservative.
 
Secondly, I have a question regarding the logic behind the "Required Total daily Solar output (Sunny)" calculation in the Solar Array Sizing section. Can anyone explain why the formula adds both the batteries "Daily Storage Requirement" and the" min Battery capacity W-Hrs" together? Isn't this like doubling the capacity needed to recharge the battery? Also, the formula then multiplies the answer by the Max discharge from full CHARGE. No real need to do this.

The worst-case need is that the batteries have been drained to empty and the solar has to charge it up in the number of days specified by the user. Each sunny day the solar has to produce enough to cover that day's usage plus enough to get the batteries recharged to 100% in the specified number of days. (NOTE: The spreadsheet takes a conservative assumption that there will be no production on cloudy days. In reality, there will be some production on cloudy days... but I have not tried to build in that nuance.)

So, the formula needs to be:
One-day's-usage + Extra-to-charge-batteries-to-full-in-the-given number of days.

Expanding on that, we can change it to
One-day's-Usage + (usable-battery-capacity / number-of-sunny-days-to-recharge-while-under-load)

Usable battery capacity is the total capacity * the % limit we are willing to allow for discharge, so, we can expand the formula to this:
One-day's-Usage + ((Total battery capacity * Max-discharge-from-full) / number-of-sunny-days-to-recharge-while-under-load)

Just as an aside, if we remove the max discharge factor, the solar output calculated would go up.

The formula in the sheet is

=IF((R28=0),"",(R17+ R21*R18/R28))

The IF statement is just to prevent divide-by-0-errors when the sheet is not complete. The 'real' calculation is this:

R17+ R21*R18/R28
For clarity, I could change it to this: R17+ ((R21*R18)/R28) The extra parentheses are redundant but it makes the order of execution clearer.

Replacing the Cell numbers with names of the cell variables we get this:

Daily-storage-requirement + ((min-Battery-capacity-W-Hrs * Max-discharge-from-full-CHARGE) / #-sunny-days-to-recharge-while-under-load)

The 'Daily-Storage-requirement' is the same as 'One-day's-usage' so that turns into:

One-day's-usage + ((min-Battery-capacity-W-Hrs * Max-discharge-from-full-CHARGE) / #-sunny-days-to-recharge-while-under-load)

The formula seems correct to me.... or am I missing something?
 
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BTW: Once we sort things out on this thread, I will update the resource and respond to the review.

Thanks for the help.
 
The worst-case need is that the batteries have been drained to empty and the solar has to charge it up in the number of days specified by the user. Each sunny day the solar has to produce enough to cover that day's usage plus enough to get the batteries recharged to 100% in the specified number of days. (NOTE: The spreadsheet takes a conservative assumption that there will be no production on cloudy days. In reality, there will be some production on cloudy days... but I have not tried to build in that nuance.)

So, the formula needs to be:
One-day's-usage + Extra-to-charge-batteries-to-full-in-the-given number of days.

Expanding on that, we can change it to
One-day's-Usage + (usable-battery-capacity / number-of-sunny-days-to-recharge-while-under-load)

Usable battery capacity is the total capacity * the % limit we are willing to allow for discharge, so, we can expand the formula to this:
One-day's-Usage + ((Total battery capacity * Max-discharge-from-full) / number-of-sunny-days-to-recharge-while-under-load)

Just as an aside, if we remove the max discharge factor, the solar output calculated would go up.

The formula in the sheet is

=IF((R28=0),"",(R17+ R21*R18/R28))

The IF statement is just to prevent divide-by-0-errors when the sheet is not complete. The 'real' calculation is this:

R17+ R21*R18/R28
For clarity, I could change it to this: R17+ ((R21*R18)/R28) The extra parentheses are redundant but it makes the order of execution clearer.

Replacing the Cell numbers with names of the cell variables we get this:

Daily-storage-requirement + ((min-Battery-capacity-W-Hrs * Max-discharge-from-full-CHARGE) / #-sunny-days-to-recharge-while-under-load)

The 'Daily-Storage-requirement' is the same as 'One-day's-usage' so that turns into:

One-day's-usage + ((min-Battery-capacity-W-Hrs * Max-discharge-from-full-CHARGE) / #-sunny-days-to-recharge-while-under-load)

The formula seems correct to me.... or am I missing something?
In my previous post, I incorrectly stated that there was an error in cell P33. It's actually cell O33 that omits the first two columns in its formula.
I totally understand the formula as you entered it but I don't agree with the logic. It's Okay, I will change it to suit my needs but I do wish to share my perspective.
Set your number of days for your Daily Storage Requirement to 1. I don't think it's necessary to take cell R21 and multiply that by R18 in your above formula. This will only give you the same value that you already computed in Cell R17. If you really want to make it simple and for clarity, just use the formula R17+R17*R19/R28. I know it's a moot point really but I'm trying to show that by doing this, you are basically stating that you want the solar panel equation to factor in two days of storage and not one (as was entered in cell R19). Again, I understand that some people might assume that they want the panels to charge a previous days usage plus their current usage but then they should input that they want two days of storage in their Battery Storage Requirement should they not? If people started using this spreadsheet with a dead battery every time then yes, your formula might make more sense, although, I don't think you can get be "under load" if you have a dead battery. But most people start with a full battery and one 24 hour period of usage only requires one day to charge up with the minimum solar cell wattage that is computed. I don't care how you slice it, if 1500 Watts is used from a full battery, it only needs 1500 Watts to charge back up. You can't put more power in by adding additional solar panels even if you tried. If after one day of usage I still want to draw from the battery under load, then it would be best to factor in two days of storage and not one.

Example: If I use a trolling motor or an RV, or anything really, and I use it for one day, I only need enough solar to charge that battery up in the number of days that I input into cell R28. Now, if I plan on using that trolling motor or RV for a second day (No matter if it's while under load or after load), I need to input 2 days of storage into field R19. If I put 1 day into cell R28(# of sunny days...), the computed cell Wattage should be enough to provide the total Watt usage if I entered the Insolation # correctly (Assuming that we will always have sunny days). But you adding yet another days worth of storage use for me in your formula is not necessary. Yes, it is nice to oversize the solar panels but is this necessary to do this in this way? Shouldn't that be determined by the person purchasing the panels? I just feel that the spreadsheet over sizes the actual watts needed by a factor of one day.

To extrapolate your formula even further:
One-day's-usage + ((min-Battery-capacity-W-Hrs * Max-discharge-from-full-CHARGE) / #-sunny-days-to-recharge-while-under-load)
is the same as:
One-day's-usage + (One-day's-usage / #-sunny-days-to-recharge-while-under-load)
Which is two days of usage / #-sunny-days-to-recharge-while-under-load or (2*(One-days-usage))/ #-sunny days-to-recharge-while-under-load
Not sure if this is really what you intended.
If you want two days of usage, enter two days of usage in cell R19.

I totally see your perspective if you are saying that 400 Watts of solar panels doesn't really give you a full 400 Watts. Other things to consider would be solar panel losses due to temperature, dirt, charge controller losses, cable losses, battery losses, etc. In that case, I think it would be important to maybe factor in a margin of these losses to maybe 20-25% when computing the minimum solar cell wattage that is needed. I still stand my by formula of Required total daily Solar output calculation but I can see why a person might want to increase the solar size by 20-25% to account for these losses. I believe this is a more suitable approach and gives a more accurate and realistic value of solar panel size and retains the Required Total daily Solar output cell.

Like I said, I already changed it on my end for my purposes ((R17*R19)/R28) and taking into account typical solar panel losses. Even in your explanation, you admit to wanting more than one day of storage while under load. So that's when you should have recognized that you truly need more storage requirements and not necessarily more panel requirements.

I do want to thank you for creating this spreadsheet and making it available. It helped me immensely in computing my battery and solar requirements. It is nice to over panel your system and that I can agree with. It does help on those cloudy days.
 
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I added an attachment so it is easier to see the changes I was referring to. I feel this gives the user the ability to set their own variables and oversize or not oversize their panels as they see fit based off of the number of W-Hrs they actually need and the solar losses that they want to take into account. I believe that this is how I've seen most companies adjust for solar PV losses. Simply adding an additional days worth of W-hrs to the solar array does work but could be a little aggressive. Inverter, Battery and now Solar have input fields that take the equipment into consideration. Please use if you wish and discard if you don't.
 

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That's a good addition. I had been thinking about something similar (I was thinking more along the lines of efficiency instead of loss...but that is just the inverse of the same thing.

The other thing I was thinking was if I could incorporate notes about using the 'normal' conditions wattage that some panel manufacturers provide (As opposed to the "Standard Test Conditions" that are used for standardized rating).... The problem with that is that not all panels provide that.

I should probably add the 'loss factor' or 'efficiency' and perhaps provide notes about 'normal conditions' vs 'standard condition' test value. (They can always put 0 in for loss factor if they use 'normal condition' values.)
But most people start with a full battery and one 24 hour period of usage only requires one day to charge up with the minimum solar cell wattage that is computed. I don't care how you slice it, if 1500 Watts is used from a full battery, it only needs 1500 Watts to charge back up. You can't put more power in by adding additional solar panels even if you tried. If after one day of usage I still want to draw from the battery under load, then it would be best to factor in two days of storage and not one.
I get what you are saying, but I was looking at it like this: If there had been a long series of cloudy days and the batteries were empty, what is it going to take to recharge from empty in the given number of days. I will have to think about how to better guide them on what this is all trying to do.

Originally I considered only publishing the survey part of the sheet and not attempting to project system sizing because of all of these nuances. Simple things like translating the total wattage of all the loads to inverter size can way oversize the projected inverter size because the user rarely runs all the loads at once. Furthermore, no matter how careful a user is, the input won't be perfect.... and those imperfections can multiply as the sizing calculations are done.

I believe one of the most important things about the spreadsheet is going through the effort of figuring out the watts and watt-hours of all the loads. Once that exercise is complete the user has a far better understanding of the energy patterns for their particular situation. (And many users get surprised at what the real energy hog are)
 
That's a good addition. I had been thinking about something similar (I was thinking more along the lines of efficiency instead of loss...but that is just the inverse of the same thing.

The other thing I was thinking was if I could incorporate notes about using the 'normal' conditions wattage that some panel manufacturers provide (As opposed to the "Standard Test Conditions" that are used for standardized rating).... The problem with that is that not all panels provide that.

I should probably add the 'loss factor' or 'efficiency' and perhaps provide notes about 'normal conditions' vs 'standard condition' test value. (They can always put 0 in for loss factor if they use 'normal condition' values.)

I get what you are saying, but I was looking at it like this: If there had been a long series of cloudy days and the batteries were empty, what is it going to take to recharge from empty in the given number of days. I will have to think about how to better guide them on what this is all trying to do.

Originally I considered only publishing the survey part of the sheet and not attempting to project system sizing because of all of these nuances. Simple things like translating the total wattage of all the loads to inverter size can way oversize the projected inverter size because the user rarely runs all the loads at once. Furthermore, no matter how careful a user is, the input won't be perfect.... and those imperfections can multiply as the sizing calculations are done.

I believe one of the most important things about the spreadsheet is going through the effort of figuring out the watts and watt-hours of all the loads. Once that exercise is complete the user has a far better understanding of the energy patterns for their particular situation. (And many users get surprised at what the real energy hog are)
Your spreadsheet really does help a person understand everything with more clarity for sure. I totally understand the aspect of having a dead battery and wanting to factor that in. As long as # of days of storage capacity is larger or equal to the # of sunny days to charge, It shouldn't be an issue and I think people will play with the spreadsheet and play with those numbers and understand it. Hypothetically, even if a battery is dead and I start putting 1500 total Watt-Hrs into a battery in a one day duration and I pull the same 1500 Watt-Hrs out in a one day duration, I should get exactly 1 days worth out of the battery. Of course the battery will still be dead but I got my 1 day use out of it that I put in the spread sheet. A 75% discharged battery would still be at 75% discharge if all things are equal. If I increase the # of days of storage capacity in the spreadsheet to 2 and leave the # of sunny days to charge at 1(requires more PV panels) and I'm dealing with a dead battery, at the end of day 1 it should theoretically charge the battery to 50% and if the sun goes away, I should still get 2 days of power out of that battery (One day of charging and one day of battery reserve).
When you start adding additional days to the # of sunny days to charge, that is where things get dicey but no matter how many days you set it at, you will get the # of days of storage as long as every day of charging is sunny. It might not be full but it will meet the criteria that you entered if all things are equal. You certainly don't want to take longer to charge than discharge but I hope people are smarter than that (I actually added conditional formatting to highlight this as being an issue). It's all about understanding your systems limitations and I think this spreadsheet helps people see that. You can't predict the weather and that is where over-paneling comes into play. Your spreadsheet isn't going to help anyone if they don't get sun for prolonged days anyway and their batteries get exhausted. This is why anyone with solar has a generator and/or an alternate way to charge and why a lot of us have that automated to take over and charge during those no sun periods as well. You can certainly leave a note for people to understand the importance of over paneling or even having more storage but that is up to the user to understand and you have provided them with a great tool to help them. Thank you!
 
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So thinking more on this, my exercise and solution never takes cloudy days into consideration and only sizes the panels to put in the minimum that it would need to put back what was taken out under perfect weather conditions. Your solution would put back that amount as well as an additional days worth of storage requirements ensuring that you would always have a full battery. I can see your perspective on this and am curious if that is what people would think as minimum requirements? I can see where this could get confusing. Your formula is simple and elegant and always ensures that you have enough PV to double the minimum requirements to get you to a full battery under clear skies and with being under load (2XDaily Storage Requirement). Mine simply calculates enough PV as required to get you the batteries capacity back and does not take being under load in consideration(1XDaily Storage Requirement). I am curious as to what people think is the best solution. I see so many websites that size the panels to the batteries capacity and that's it. That is why I did it this way. I think of that as the minimum.
There are many ways to address this and they all have some form of adding additional PV panels. I have seen people just simply increase the # of panels. I've also seen people use their areas winter Insolation # as a base and thus insuring that they meet the minimum during those months but then in the summer months their panels will provide additional output for those extended periods of time.
This is import to think about for the beginner. What are your thoughts on this? Personally, I think that a system should be designed to meet the absolute minimum that they need during the lowest Insolation # times of the year. As long as they are expecting to use their system during that time period. Then, as the months pass and the duration of sunshine goes up, they should always have more than enough PV to meet their needs. This is something that I don't really hear many people talk about. I just see people adding more panels to their system because its the cheapest solution. What does the industry say is the correct way of sizing your panels?

One last thing to consider. Your battery capacity is based off of a 24 hour time period I would imagine. Solar PV output is based off of 4-5 hours. I'm sure it would come close to filling the battery in that time frame(time frame=#of days to recharge) even with being under some kind of a load. I'm done ranting now! :)
 
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