I’m trying to answer a question that most solar calculators dodge: How much do you generate on bad days compared to good?
Nearly every online calculator reports average production without breaking down what you can get on cloudy days vs sunny. This is great if you are designing grid scale solar, or maximizing production for grid tie with net arbitrage. But it's nearly useless for off grid capable homesteads.
The basic question is based on a tradeoff: How often do you want to run a generator?
Lets put some numbers to it. Say you use 30kwhr per day of electricity.
The easy answer you get is you need a solar array capable of generating on average 30kwhrs of power. Say a 10kw array, south facing. Put it at a 45 degree angle to maximize production in winter. And generate, an average of 40kwhr per day.
Which sounds great until you get 4 overcast days in a row, the panels only generate 10kwhr on those days, and you are stuck running the generator every night several times a year.
But what if you only want to run the generator in emergencies? Then you need to base your system sizing based off the bad days, not the good, and then you need to figure out how to flatten out the difference as much as possible.
East/West arrays that capture sun for shorter periods on sunny days are suddenly your allies for maximizing generation on cloudy days, as well as providing more hours of solar power.
Vertical mounted panels are wildly inefficient, but suddenly come in clutch for getting you a few hundred watts on a bad day, and are less likely to bust from a hail storm.
But the actual data is sparse, with most of it being averaged to uselessness for me. So, I created a trailer mounted array so that I can test things like east/west facing arrays in sun and clouds, vertical arrays, etc.
So far I have confirmed that I can get about 10-15% of nameplate output with a vertical south facing array in overcast conditions. While this is just rough data right now, it's already hinting at a 10kw array producing 1-2kw on a cloudy winters day, which puts a huge dent in my energy needs. If I get good data from east/west testing it may show that a 10kw east and 10kw west array can produce 30kwhrs of power when overcast. If so that will confirm my design.
More to come!
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