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

solar systemYesterday 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 How Many Solar Panels and Batteries, For Your Off Power Grid System.  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.


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6 Responses to Calculate the Size of Your Solar System from Kill A Watt Data

  1. michael weiss says:

    Thank you very much.. This explained very well.

  2. gonewiththewind says:

    Your math is correct but…
    If you live in Las Vegas it will work out exactly as you describe. If you live further North or East, not so much. The bad thing about all the alternative energy methods is that you cannot depend on them. Most advocates of PV will recommend doubling or even tripling your system size to compensate for the problems with weather, winter and other unknowns. But even then it is likely that at some point your PV system will let your freezer contents thaw. I like a small amount of PV to provide lights and power laptop computers, recharge phones, etc. These are things that you can control and choose not to use if there is no solar power. I like “small” because the cost of PV is outrageous. A single panel, charge controller and two 12 volt deep cycle batteries is fairly affordable but a house sized system is too expensive to even consider especially when you take the problems into account.

  3. Donna Pope says:

    Very helpful information on solar watts. I have never seen your web site before. Thank you for the help.

  4. Leslie Weir says:

    Your refrigerator example was very timely for me. I am looking to do exactly that, power a refrigerator at my off the grid cottage. I will be buying a brand new frig and when I checked consumption numbers it’s quoted in kw hours per year, in this case it was 344 kw hours per year. I’m trying to decide if it makes sense or not to go solar simply to power my frig. I’m trying to get away from propane. In my case I don’t have to worry about the winter because we’re not there then.

    • admin says:

      The question that enters my mind is will this be a bugout location. If so you should take the winter into account.

  5. pablo garcia says:

    Thanks. I picked up alot of info by you using the frig exsample. in my case, I am off grit, am running a 2015 premer trailer using a new 2016 fuelinjection honda generator(rated at 7000 watts) with a 30 amp breaker built in. In the summer, running the AC , TV, Micro Wave(110 volt) the generator only shows 2400 wattts is being drawed when the ac / tv is being used. The rest runs off propane(Heater, Water Heater,Frig). LTS are 12 volts leds. Using your information, 7 -300 amp batteries, 3- 200watt pannels on 8 hr days, 1-30amp charge controler#121070 NPOWER. I am not sure on the inverter, 5 or 10,000 inverter? Are my numbers correct, can you help me with the correct inverter.

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