Should I trust this solar spreadsheet?

Thankfully "Steve" did not protect the sheet, so you can see what the calculation/formulas say.

There are a couple of issues with nomenclature... kW per hour and Watts per Hour--Are just kW and W (both are already rates). kW consumption -- really just kWatts (already a rate of consumption).

And cell A7 (minutes per day) is not used for anything useful as cell B7 is simply =A7/A7 (will always equal "1"--Unless "zero" were entered).

And the equations are not what I would use... For example, let us take the numbers entered from your saved file:

• 3 Days reserved power (I would suggest 2 days as "optimum")
• 12 volt battery bank
• 27.24 kWH / 280 Hours = 0.0973 kWatt (rate) = 97.3 Watt (rate of consumption)
• 97.3 Watts * 24 hours per  day = 2,335 Watt*Hours per day = 2.335 kWH per day

I am a little confused needing a 584 AH @ 12 volt battery for 1 day and 2,292 AH @ 12 volts for 20% discharge for 3 days of storage. 

For solar, 5 hours per day of sun (pretty sunny/summer usage for most areas).

Now, go through my "nominal" calculations... Use 3 days storage, 50% maximum discharge, 85% efficient inverter...

• 2,335 WH per day * 1/0.85 AC inverter eff * 1/12 volt battery bank * 3 days of storage * 1/0.50 max discharge = 1,374 AH @ 12 volts
• 2,335 WH per day * 1/0.85 AC inverter eff * 1/12 volt battery bank * 2 rule of thumb days of storage * 1/0.50 max discharge = 916 AH @ 12 volts

And since it is >~800 AH battery bank, I would be suggesting a 24 volt battery bank minimum; 458 AH @ 24 volt battery bank configuration...

Using their 5 hours and 2,335 WH per day for charging... Taking all of the losses into account:

• 2,335 WH per day * 1/0.52 end to end AC system eff * 1/5.0 hours of sun = 898 Watt array minimum

And then a 10% rate of charge for the battery bank nominal (rule of thumb: recommended for full time off grid system):

• 1,374 AH battery bank (spreadsheet) * 14.5 volts charging * 1/0.77 panel+controller derate * 0.10 rate of charge = 2,587 Watt array nominal
• 916 AH battery bank (my rule of thumb) * 14.5 volts charging * 1/0.77 panel+controller derate * 0.10 rate of charge = 1,725 Watt array nominal

And the spreadsheet recommended 3.1 panels * 150 Watt panels = 465 Watt array

The spreadsheet "ignores" 81% solar panel derating, 95% solar charger efficiency (MPPT), and 80% efficiency for flooded cell lead acid batteries. And ignores the 5% to 13% typical solar charging current for an FLA battery bank (5% minimum for weekend/summer evening usage; 10%+ for full time off grid usage). And ignores the 85% efficiency for the AC inverter (for AC systems).

The reason I like to not use a spreadsheet is because there is a narrative on how and why we do things. Just plugging numbers in without understanding why--Makes it sort of difficult to figure out the optimum system (and how conservation vs fuel choices--i.e., electric coffee maker vs propane) for your needs (generating power is generally more costly than conserving power usage).

My calculations reduce the size of the battery bank a fair amount. And the calculated size of the the solar array is way off--No losses accounted for (52% end to end system efficiency)... And does not take into account the 5% to 13% (typical) rate of charge for a battery bank (large battery banks are not "free"--They need a larger base solar array to have a proper rate of charge for Lead Acid batteries).

It would certainly be easy to update the formulas in the spreadsheet, and even put some conditionals in (i.e., >800 AH battery bank, go next higher bank voltage).

However, then I would worry that much of the "understanding" would be lost again (why certain things are done/suggested, etc.).

And my/our rules of thumbs are just that... The are for the "typical" cabin or home use (generally harvest energy during the day, and use energy evening and nighttime). If your needs are different, then the calculations and results will be different too.

-Bill

The formulas are simple... It would be easy to sub my formulas (or whatever you prefer) into Steve's spreadsheet.

I agree, it saves a bit of typing (and I have never been known for my long posts  )...

I try to type my math out as a "near English sentence"--So that you can read and understand each parameter's contribution to the end result.

Spreadsheets tend to bury that. And sometimes, spreadsheets have been known to have errors creep in and do some bad things to the data.

-Bill