System Energy Audit and Sizing Spread Sheet

System Energy Audit and Sizing Spread Sheet 2021-01-29

To get the spreadsheet, click on the orange
button at the top of this page.

This sheet is intended to help you plan your system. Fill it out down to the last decimal place, look at the results..... and then make your best guess;)

  • There is now a tab for stationary systems (Only AC Loads) and a tab for Mobile (AC & DC loads)
  • Be sure to look at the "Sample" tab in the workbook.
  • One of the best uses of the sheet is to help you identify where to reduce/conserve. (Energy not used is the cheapest)
  • To get the best sizing, use measured values of wattage for the load calculations. If you use the values shown on the back of the device, it will almost always be higher than actual. The higher-than-actual number then gets fed into the system sizing and the difference can get amplified.
    Note: @Brendan Wood posted a resource with 'typical' wattage/power usage for many different appliances.
    If you don't have any other numbers, you can use these to get a 1st approximation of your projected usage.
  • Figuring out hours of use to put in the spreadsheet:
For things like lights that you manually turn on and off, it is usually good enough to just watch what you do for a couple days to make a reasonable estimate.​
For things like refrigerators that turn on and off by themselves, it is handy to put a Kill-a-watt on it and measure the watt-hours consumed over a set period of time. From that it is possible to calculate the daily usage and the time on per day. Here is an example with random numbers:​
Measured with Kill-a-watt
Watts used while on: 6W​
Watt-hours over a 6 hour period: 18Whr​
Average Watt-hours per hour = 18Whr/6hr = 3Whr/hr​
Watt-hours per day = 3Whr/hr * 24 Hr/day = 72Whr/day​
Time on per day = (72Whr/day)/6W = 12 hrs/day​
Edited to clarify run-time math​
Edited to include cross reference with other resources.​
Updated spreadsheet to handle surge wattage​
Updated the resource to a download file rather than google sheets.​
Corrected Error in "Required Total daily Solar output (Sunny)"​
Corrected Error in " Full Surge load Inverter size"​
The forum now supports Xcell files so it is now a direct download from forum archive.​
1/28/2021 - Released Reb B.​
- Added a tab for mobile systems that have both AC and DC loads​
- Cleaned up Format a bit.​
Note: This resource is a duplicate of the resource of the same name in the 'Beginners' resource Section. This resource is linked to the same download file.
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As usual, FilterGuy does not disappoint, this is a great calculator! Thanks !
Excellent, easy-to-use system calculator ala Excel. Thank you 😊
I have a question, the sheet is multipling the watts by the hours of usage to calculate everything, how ever when I use my kill a watt on my devices I get a totaly different story, for example the desktop computer is using 100 wats so if I use it for one hour the sheet calculates 100 wh but my kill a watt is saying that the computer is using 0.075 kwh so in one hour is using only 75 wh, the fan is actually using 0.04 kwh while is a 60 w am I doing something wrong? Maybe the wattage is the maximum usage in extreme conditions?
The nameplate wattage will always be the max wattage. The actual wattage will always be less. In the case of a laptop, it gets even more complicated. The load for a laptop is going to vary based on a combination of the state of charge of the battery along with the activity on the computer. For a laptop, the actual usage for most people will almost always be a lot less than the name-plate number.

Almost without exception, the Kill-a-watt measured value will be more accurate (and less) than the name-plate value.
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. 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. I don't believe this would be needed to be done if you just started with min Battery capacity W-Hrs in the fist place.

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?

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?
noob says "thank you" - gotta start somewhere!
Very nice and useful spreadsheet
This has been so tremendously helpful for planning my system, thank you! Curious about why "# of days to recharge while under load" is a user input. How am I determining the answer to this question? Shouldn't this spreadsheet be able to tell me that?
>Curious about why "# of days to recharge while under load" is a user input.
> How am I determining the answer to this question?
> Shouldn't this spreadsheet be able to tell me that?
It is asking how quickly the user wants the system to recharge from fully drained and then uses it as an input to determine solar panel sizing. Some users will want just one or two days, others might be willing to wait more days to fully charge.
The alternative is to ask the user for Solar Panel size and then calculate the number of days it will take to recharge, but that would kinda defeat one of the purposes of the spreadsheet.
Very thorough, incredibly detailed, and you spent a lot of time. Might be easier to just plug in information from Electric Bill?
very helpful
Firstly, this spreadsheet is gold, thank you so much for this resource.

Secondly, I'm a bit confused. Shouldn't the "Required Total daily Solar output (Sunny)" - cell L19 calculation which looks at the "# sunny days to recharge (from ZERO) while under load" - cell L18 (as you mention in your definitions, it is the number of sunny days required to recharge the battery from ZERO (or in this case assuming from max discharge) while you are still using power) be referencing the "Battery capacity" - L14 x "max. discharge" - L11 as well as the "Daily Storage Requirement" - cell L10)? You are only considering daily requirements and not replenishing the entire battery in your current formula.
You have: L19 = L10+ L10/L18
Should it not be: L19 = L10+ (L14*L11)/L18

If I'm wrong I may need an explanation why you are not referencing the battery's capacity in a. calculation to full recharge, where a recharge is directly dependent on how much of the battery has been depleted on not only on your daily usage.

Good catch!!! I fixed it yesterday. Sorry it took so long