# Solar based battery systems

N

#### Nayon Kontrol Systems

Hi everyone,

Does anyone how to compute for the number of solar panels size and capacity
of batteries and inverter/charger?
Say example I have continiuos system load of 40watts, how many solar
panels at specific rating per panel, number and sizes of batteries and
charger?

In addition if i would need the system to sustain 5 days autonomous battery
operation (meaning no sunlight for the next 5 days) how would you compute
for this?

Can someone help me for compute for these configuration?

Thank you

Nayon Kontrol Systems

E

#### Eligio Palcich

What is the voltage requirement for your system?

J

#### Johan Bengtsson

Well, if you want 40W AC, your inverter needs to handle that. It is probably marked a number of VA instead of W however and it could be the same (if you have resistive load) but otherwise VA is greater than W. You have to check what your equipment need....

Ok, for the next step, you have a continous load of 40W, this will be drawn from batteries for 5 days - that is your battery requirement. Most batteries are marked with a voltage and a Ah rating
V*Ah will give you Wh.
for 40W and 5 days operation you need 40*24*5=4800Wh
To this you have to add losses: in the inverter, voltage drops (both inside and outside the batteries) and all other losses. I am rounding this to 5.2kWh for the calculations to go on, you have to find out your need....

Ok, if you use a 12V battery 60Ah ("normal" car battery, at least where I live) you get 12*60=720Wh per battery, this means you need 5200/720=7.22 batteries. That means 8 batteries.

How much does the charger and solar panels need to deliver? Well they need to be able to peek deliver more than your average continous load takes, that much is clear. The charging that is above current use is what is really charging the batteries and the question is how long it is ok to need to recharge your batteries. (Note the sun does in most places not shine on the night, you need to charge what the night takes in the day, and then some more). You normally need to put in more energy in a battery in you can take out, for lead type batteries that is about 20%, for other types you need to check.

And at last, even if the day is cloudy normal solar panels will give some energy, not full power but neither zero.

/Johan Bengtsson

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B

#### Briam Johnson

Campbell Scientific has a pretty good application note on sizing batteries and solar cells. Some of the information is particular to their equipment, but there's some pretty good general information as well.

The application note can be found at the following address:

ftp://ftp.campbellsci.com/pub/outgoing/apnotes/pow-sup.pdf

Briam Johnson