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How Many Batteries Do I Need for My Solar Power System

When you are considering installing a solar electrical system, you need to think about what your needs are and how much power does that involve.  One solar panel putting out 18Volts is not going to get you very far when you need to supply a cabin on the farm or supplement your home power supply.  To put it into perspective, 5 Compact Florescent Globes (CFL) of 15W, will draw  75W per hour, run them for 5 hours and that means 375Watts you need to provide either directly or from batteries.  If you are on the Grid, no problem, flick the switch. But, if you are on Stand-Alone, then you need to design your system to provide your power.

Are you ready to take the step to energy freedom and get your Handyman Guide to Building a DIY Solar Panel

The main place to start is your batteries.  When the sun is shining, your solar panels convert light into electricity and charge your batteries.  At night or when the sun does not shine (cloudy days), you draw from your batteries.  The size of your battery array depends on two things :
1. How much power you use when everything is on.
2. How long you run your appliances on your batteries.

Now, there is a couple of other details that you may not come across so often.  Batteries do not discharge in a "linear" way.  To explain, a 200Amp Hour battery should theoretically provide say 50Amps over 20 Hours, but this is not so due to its non-linear discharge.  Add to the fact that batteries are affected by cold, so if you live in snowy climates, make sure you factor in bigger battery arrays. The more slowly energy is taken from a battery, the longer it will last.

Ok then, lets do some calculations to work out how many batteries you need.


  • Make a list of all the electrical appliances you intend to use
  • Get the wattage of each item from the items product label
    (If the label shows only Volts & Amps, Then, Volts x Amps = Wattage (per hour of usage).
  • For each item, calculate the number of Watts it uses (per hour)
  • Estimate the number of hours you will use that item
  • Multiply by the hours of usage with the hourly Wattage for its consumption (per day)
  • Add up all the items power usage in Watts for your daily usage.

Just to give you an idea of what you are looking at :

  • 5 x 15W Compact Florescent Globes = 75W per hour. Run for 5 hours = 375Watts/Day
  • 1 x 75Watt TV for 3 hours day =   225Watts/Day
  • 1 x 50 Watt Laptop, 3 hours/Day = 150 Watts/Day
  • Total Daily usage = 750 Watts/Day.  


  • Using over 3 days =  2250  Watt Hours (with no recharging)
    Now also remember that you should not discharge you batteries any lower than 40% capacity or you will begin to damage them, we will work on a 50% discharge to keep them safe.
  • We need 2250 Watt Hours at 50% discharge = 4500 Watt hours.

So you will need batteries that will supply 4500Watt Hours for you.
To work out you batter rank, or how many and how big your batteries we need to work on what is termed Amp Hours.  Batteries are usually quoted in Amp-Hours, so we need to do a small calculation. the formula is easy,   AMPS = WATTS divided by VOLTS

OK, lets say you are buying 6Volt batteries rated at 225Amp/Hours, how many do we need?

  • Each 6Volt battery at 225Amps provides 1350Watt Hours.
  • Our usage of 4500Watts divided by 1350Watt/Hours = 3.3, so we need 4 Batteries.
  • That means you would have have a “bank” of 4500 watts of power in your four batteries.

Something else you would need to consider is how to connect them to match your inverter.  You could now connect your batteries in series to get a 24 Volt output or 2 x 2 in parallel  to push out 12 Volts.  

So you can see that when designing a DIY solar system there are many details to consider.  You really are advised to get hold of a comprehensive guide to explain all  the details in putting together your solar panel and supply system.

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