Detailed Instructions for the Critical Month Tab

Maybe you decided that you have so much surplus energy you don’t need to worry about the worst case days, weeks, and month.  We hope that is indeed the case, because that means you are wisely matching your capacity to your needs.

But if you’re trying to avoid overspending, and are getting fairly close to the wire in terms of your capacity and your needs, it probably makes sense to do a day by day very careful analysis of the month in which you go down to your minimum energy storage point.

Here’s the information you need to plug the appropriate data into this page, and how to understand the results that are presented to you.

 Critical Month Tab Inputs (the orange values in the grey cells)

Avg Sunlight Hours Ideally, you should find a source that provides you with sunlight hours per day for your worst month for several years, so you can get a feeling for how much variation there is from one year to the next for this month.
If you can’t find this, we suggest you take the information you do have for the critical month and then reduce it by, oh, about 20%, and more than that depending on how conservative you want to be.
Note that the value you end up with will probably be different to the value you used on the Constants tab.  The Constants tab data is more general/normal, this tab is more for ‘worst case’ projections.
Put the number into the box, and it will copy over for every day of the month.
Daylight minutes Go to a website like and find out how many minutes of daylight there are for each day of the month, and put the value for each day into this row.
Avg daylight minutes See the total sunlight hours at the end of the row of sunlight hour data above?  You want to enter a value here that will create close to the same number at the end of the adjusted sunlight hours row, immediately below.
We suggest you start with a number sort of halfway between the maximum and minimum number of minutes you just entered, then increase or decrease until you get it close to exact.
Weather factor Okay, this is where you start to plug in some ‘worst case’ values.  This weather factor adjusts the daily sunlight hours.  If you put in 0.1, it will drop the hours down to one tenth their value.  If you put in 2.0, it will double them, and so on.
We suggest you plan for some bad weather, right at the time of month with the shortest days.  We suggest you consider ‘bad weather’ to be something like a weather factor of perhaps 0.3 – 0.5, and we suggest you have this ‘winter storm’ last several days.  This will create the major ‘stress test’ for the month.
The rest of the month you can do pretty much anything you like with, but you’ll need to add better than 1.0 factors on some days so as to balance out the worse than 1.0 factor days.  Adjust this as needed, so as to get the total of the next row – net sunlight hours – to end up close to the totals of the other two sunlight hours.
Solar Array Power (kW) Enter the theoretical power production rate of your solar array here.  This will probably be the same as you entered on the Constants tab.
Efficiency (%) This too will probably be the same as you entered on your Constants tab.
Power consumed (kWhr) This also will probably be the same as you entered on your Constants tab.
Roundtrip Power storage efficiency Another thing that will probably be the same as you entered on your Constants tab.
Minimum/wasted power (kWh) And another thing you should copy over from your Constants tab.
Daily storage loss (%) More data to copy over.
Daily storage loss (kWh) One last thing to copy over.
Power reservoir Enter here the amount of power that was stored at the beginning of the month, from the Daily tab.


Analyzing the Results

The key thing you want to look at here is the bottom line – the balance of power in the Power Reservoir line throughout the month.  If it goes down to zero, you’ve got problems and you’ll need either more panels or more storage.

You will quickly see that the results on this tab are different from the results on the Daily tab.  Trust the results on this tab more than you do the Daily tab results.

If there’s a huge anomaly in results, that either means you’re used very different data in your two tabs, or there’s a logic bug in the spreadsheet.  If you’re sure the big apparent error isn’t due to your data, please let us know so we can check things at our end and fix anything we find wrong.

You could also try changing some of your ‘weather assumptions’ by adjusting the weather factor numbers.  Give yourself ‘bad weather’ (ie low weather factor values) when your energy storage is at its lowest, and see if you can make yourself run out of energy.

You can also look at another thing.  When you have zeroed out your energy reserve, how much power is coming in from the panels each day for you to live on?  This is the value shown on the line ‘kWh of electricity generated’.

Another way of looking at it is to see what the difference is between the power you would like to have and the power you actually have.  This is the value shown on the line ‘Surplus/(deficit)’.

The good news, such as it may be, is that even on your worst day, you still have some electrical power being generated.  Not much, but probably enough to keep some lights on, and keep essential electronics charged up.

For more information, please return to the main page explaining the Solar Energy Calculator spreadsheet.

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