10-08-2019, 01:33 AM
The first and foremost issue with the question of how much solar do I need will always be enough to produce more power than you use. only then does the ratio of watts to Amp hours comes into play.
The rule of thumb of 1 watt per one amp hour of battery capacity is a rule of thumb that I hear but it is the ratio required to equalize a battery at 77 degrees. Equalizing a fully charged battery does not take that much power.
Now then, what it takes to CHARGE a battery is a different thing all together. Have you ever seen a deeply discharged battery charge to 13.1 volts and hang there even though you have full sun on a 1-1 solar to Ah system? It happens because batteries have resistance to having power flow into or out of them. It isn't like turning on a tap and letting a bucket fill with water. You have to force the charge into them. You have to have enough watts to slap the battery upside the head and say wake the hell up. A 1-1 ratio simply isn't enough to do that. Eventually you will get there but it is going to take longer than you think.
As sterwake mentioned, time of year and location also plays into this. A 1-1 ratio in the dead of winter is actually 0.5 to 1 due to the angle of the sun reducing your peak power to half of what you see in the summer. Now add in the fact that you have less time due to a shorter day and you realize that you are really screwed.
You might think that a oversized system with a 1-1 ratio is the answer because you are not taking enough out of the bank to deeply discharge it. You are right until you get crap weather for days on end and that big bank does get deeply discharged. That's when you need a alternate source of charging to do the bulk and it too had better have the punch to get the job done.
The rule of thumb of 1 watt per one amp hour of battery capacity is a rule of thumb that I hear but it is the ratio required to equalize a battery at 77 degrees. Equalizing a fully charged battery does not take that much power.
Now then, what it takes to CHARGE a battery is a different thing all together. Have you ever seen a deeply discharged battery charge to 13.1 volts and hang there even though you have full sun on a 1-1 solar to Ah system? It happens because batteries have resistance to having power flow into or out of them. It isn't like turning on a tap and letting a bucket fill with water. You have to force the charge into them. You have to have enough watts to slap the battery upside the head and say wake the hell up. A 1-1 ratio simply isn't enough to do that. Eventually you will get there but it is going to take longer than you think.
As sterwake mentioned, time of year and location also plays into this. A 1-1 ratio in the dead of winter is actually 0.5 to 1 due to the angle of the sun reducing your peak power to half of what you see in the summer. Now add in the fact that you have less time due to a shorter day and you realize that you are really screwed.
You might think that a oversized system with a 1-1 ratio is the answer because you are not taking enough out of the bank to deeply discharge it. You are right until you get crap weather for days on end and that big bank does get deeply discharged. That's when you need a alternate source of charging to do the bulk and it too had better have the punch to get the job done.
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