# Batteries ratio to panels



## budgetprepp-n (Apr 7, 2013)

I have had some people ask me how to figure how many panels or watts they need to match there battery pac.
I don't thing you can do that at least not where I live. In the summer like now I can power everything in the 
house even the refrigerator (not the well pump or washer/dryer) And still have juice left over. But in the winter
it's totally different story. I like to keep my batteries charged up high. In the winter I unhook about half of the 
batteries to keep them up. We just don't get enough sunlight with all the gloom and short days. 

So in the summer I could use more batteries to hold all that extra power or less panels

But in the winter I need more panels or less batteries

So how you going to set a ratio for that? 

I even been thinking of getting a grid tie in inverter for the summer months. Do you need to notify the
electric if you plan on spinning the meter backwards? -- would they know?


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## 8301 (Nov 29, 2014)

Answer as simple as I can make it.

Take your panel watt rating and multiply it by .75. So a 100 watt panel will put out about 75 watts in strong sunshine with a controller. Assuming a 12v battery... 75 (watts) / 12v = about 6 amps feeding to the battery. 

Let's assume you're charging a 100 amp/reserve car battery that is half drained (50% SOC) (state of charge). So the battery needs another 50 amp/hours worth of charge to be topped off. So... 50 (amp/hrs) divided by 6 (amps) = 8 hours worth of strong sunshine to top the battery off. 

This is a very simplified answer but I wouldn't be surprised if it took over 10 hrs of typical sunshine to top off the battery.


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## dwight55 (Nov 9, 2012)

Due to the unpredictability of the sunshine, . . . there is no way to "absolutely" match the battery and panel capabilities.

If it were possible (we're talking $$$, . . . not mechanics) the ideal solution is to start out saying "I need X amount of electricity per given day in my house. Then make up the battery bank that will take care of that need level.

The last thing to do, . . . this is where the $$$ comes in, . . . buy enough panels so that even in a bad day in the winter, . . . there is still enough to so it.

What I'm going to mess around with this fall, . . . a "single wire alternator" that's used on cars. I'm gonna try to rig some sort of windmill thingamadoo, . . . that will power it. It is far more powerful than a whole hand full of panels, . . . and works even at night if the wind is blowing. Plus, . . . I understand that most, if not all, have a built in voltage regulator, so you don't have to worry about frying your batteries.

Anyway, . . . it's gonna be fun "experimenting".

May God bless,
Dwight


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## dwight55 (Nov 9, 2012)

Due to the unpredictability of the sunshine, . . . there is no way to "absolutely" match the battery and panel capabilities.

If it were possible (we're talking $$$, . . . not mechanics) the ideal solution is to start out saying "I need X amount of electricity per given day in my house. Then make up the battery bank that will take care of that need level.

The last thing to do, . . . this is where the $$$ comes in, . . . buy enough panels so that even in a bad day in the winter, . . . there is still enough to charge it up.

What I'm going to mess around with this fall, . . . a "single wire alternator" that's used on cars. I'm gonna try to rig some sort of windmill thingamadoo, . . . that will power it. It is far more powerful than a whole hand full of panels, . . . and works even at night if the wind is blowing. Plus, . . . I understand that most, if not all, have a built in voltage regulator, so you don't have to worry about frying your batteries.

Anyway, . . . it's gonna be fun "experimenting".

May God bless,
Dwight


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## Maine-Marine (Mar 7, 2014)

In my humble opinion...

the mind set should be... how many batteries do I need to run what I want to run for x non-sunny days (based on an average of your local climate) AND how many panels do I need to charge those batteries to make sure i can continue to run the things I want to run.


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## 8301 (Nov 29, 2014)

budgetprepp-n said:


> I even been thinking of getting a grid tie in inverter for the summer months. Do you need to notify the
> electric if you plan on spinning the meter backwards? -- would they know?


Most modern meters don't run backwards anymore and it's a big no-no to back feed the grid without their knowledge. Instead you have to contact the local power company and have them install a dual (or two way) meter. Some electric companies are reasonably price, others eat you up with fees. You can use a grid interactive inverter that will pull off the grid when needed and use your solar when you've got it without feeding power back to the grid. It's a seamless transition between your solar energy and the grid feeding your house. You will sell no power back to the grid but you'll save a lot in monthly fees, installation costs, and in many areas no building permit is required.

That's what I did for a while but currently 98% of the time I disengage the grid from most of the house circuits and run totally off grid. I only hook to the grid if we get 3 days of winter bad weather.


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