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Subject How many batteries for six Siemens 75 Watt solar panels in New York?
Author Sascha

I was wondering if you could give me some advice.
We have 6 75watt seimens solar panels, I would like to set them
up as 3 sets of 24 volt and use the solarboost 50 charge controller to
drop it to 12volt
and charge a set of batteries for an inverter.

My question is, With that amount of solar panels, charger and the fact
that 3 hours
a day is our worst case scenario for light a day ( close to New York New
York ).
What size of batteries would I be able to charge daily and use.

This is for sort of a long term science project for my scouts. I am
interested in the
sealed AGM batteries for the safety factor, I just don't know the size
or how many
could be charged in a day.
we already have a 12volt 2000watt inverter.

Thank you for your time


Hi Steven,
Your approaching the topic of sizing your batteries from the opposite direction we're used to. Usually, one figures out how much power you need in reserves in your battery bank to supply energy to your appliances. But I believe we can help, in any case.

1st off we need to determine how many amp-hours your solar panels will produce in the winter time (the worst case scenario you refer to). Using the Solar Boost charge controller is certainly a good choice to maximum the power you get out of your panels. Ok, so if we look at the specifications for the Siemens SP75 solar panel on Alternative Energy Store's website:

we see that the SP75 panel has a rated current of 4.4 amps. To figure out the number of amp-hours your configuration of panels would produce we would multiply:

(3 strings of panels) x (4.4 amps) x (3 Sun-hours for NY) = 39.6 amp-hours

Now, we could stop here and say you need ~39.6 amp-hour batteries and assume you were going to discharge your batteries all the way. However, as you probably know that would be a terrible idea as discharging your batteries all the way would kill them very quickly. In the worst case scenario, we want to discharge them no lower than 50%. And we probably want to take into account that you may have a few overcast/raining/snowy days where your panels are producing very little. Let's assume that you could have up to three days like this in a row. So in other words, by the end of the 3rd overcast day your battery bank should not be discharged by more than 50%.

So, what this means is that we need to size your battery bank so that it can discharge 3 days x 39.6 amp-hours = 118.8 amp-hours and be only half (50%) discharged. For it to be only 50% discharged we then multiply 118.8 amp-hours x 2 = 237.6 amp-hours.

In conclusion, what you need is to select a battery that has roughly a 237 amp-hour rating. You will then need to put enough of them in series to make 24V. For instance, you could choose the Trojan T-105 which has a 225 Amp-Hour rating (pretty close to 237). This is a 6 volt battery, so you will need to use 4 of them in series to make 24 volts. (Remember, that putting batteries does not increase their amp-hour rating, but instead their voltage. Putting batteries in parallel will increase their amp-hour capacity but the voltage remains the same.)

The Trojan T-105 is not a sealed AGM battery, but here are some that are sealed and have roughly the same amp-hour capacity and are 12 volt (so you'd need only 2 for 24V):

Concorde PVX-2580L, 12V, 255 amp-hrs (AGM, sealed type)

MK Battery, 12V, 225 Amp-Hrs (8G8D, Sealed Gel Cell type)

Universal Battery, 12V, 225 amp-hrs (UB8D Sealed Gel Battery)

One other note: I've slightly simplified this calculation. For instance, I did not take into account the effects of temperature on batteries. Generally, the lower the temperature is the poorer the storage capacity of batteries are. Batteries at near freezing temperatures will have about half their normal rated capacity (at ~80 degrees F).

Hope that helps!

kind regards,

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