# Calculate the Size of Your Solar System from Kill A Watt Data

Yesterday I put up a blog on using a Kill A Watt to determine your power usage. Kill A Watt can determine the Size of a Generator/Solar System  Now I know that many of you want to use this information to figure out how big a solar system or generator you need to run your necessary appliances.

A while back I ran a post on   This morning I received a comment asking how to calculate the size solar system you need from the information gathered by the Kill A Watt. So this morning I will take you though the calculations based on the information that was sent me. I am using the information shown in the second post.

Here is the information that I was given, “I used a kill a watt on my frig. 24 hrs. 2.57amp, 180w, 311va, 4.32kwh, 700 watt startup. How does this translate to solar panels and batteries needed for off grid all the time.

In this case we know that in 24 hours the refrigerator used 4.32 kwh (kilowatt hours) which equals 4320 watts (4.32 x 1000 = watts-hours).  Now multiply your watt-hours by 3 to allow for bad weather. This equals 12960 watt hours.  Now because you should not discharge your batteries below 50% multiply this number by two.  This means you need 25920 watt hours. This will give you total battery capacity your system needs to store to run for three days.

Now let’s work on the number of 12-volt batteries you will need.  To determine this we will need to find amp hours.  Amp hours are used because this is the standard by which batteries are rated.  Now divide your watt-hours 25920 by 12 (the voltage of the battery).  This shows that you need 2160 amp hours of storage.  Now divide the amp hours by the amp hour rating of your batteries.  For example if your batteries are rated at 300 amp hours, you would divide 2160 by 300 which shows that you would need a minimum of 7 300-amp hour batteries.

Next, we will determine how many solar panels you will need.  So if you have 200 watt panels you would divide the daily watt-hours (4320) by the wattage of your solar panels, times the number of hours of sun.  Now if you get 6 hours of good sun a day you would then multiply the panel size by the number of hours of sun.  This is 200 times 6 which equals 1200 watts that one panel will produce in one day.  Now divide 4320 the watt-hours by 1200 which means that you would get more than enough power with 4 panels.

So for this solar system you would need 7 batteries and 4 200 watt panels.  If you add an extra battery you would be able to run a couple of lights with no problem.  The refrigerator used in this example must be an older one because its power requirements seem to be on the heavy side.

If you decided to use a generator to run this refrigerator you would need minimum of a 1000 watt generator.  The start up surge is only 700 watts and the draw when running is 180 watts, so a 1000 watt generator would work, but I would suggest a 2000 watt to allow you extra capacity.

A small solar generator using a 12-volt battery pack might be a good project to learn more about how solar power works. I have detailed instructions for that project in this article.

Hope you can understand this.

Howard

### 7 thoughts on “Calculate the Size of Your Solar System from Kill A Watt Data”

1. gonewiththewind