Basic Solar / Wind Turbine System Sizing

Enter the average daily power use total you calculated from the worksheet:


You will want a solar electric system that will provide at least that many watts in one day so that you can power everything plus you want some extra in case you add an appliance or change to appliances with more watts. A general rule of thumb is to have 30% more than you need.

Your solar panels will only produce power in the daytime and only when the sun is shining. In summer the sun may shine for 8 hours or more each day but in winter you may be lucky to get 3 or 4 hours of good sun. To prevent under sizing of your system we will size it for winter use. That way you will have enough power in winter and more than enough in summer.

To keep it simple let’s assume your total average daily power use is 2000 watts.

You want 30% more than your actual use so you need 2600 watts.

On the map below find the zone you are in and the average hours of sunshine for that zone.

Average hours of sunshine


Zone 1……..6   summer hours                   Zone 4…….4.5 summer hours

Zone 2……..5.5 summer hours                   Zone 5…….4.2 summer hours

Zone 3……..5    summer hours                   Zone 6……..3.5 summer hours

****Subtract about one hour for winter months****


The average hours is based on 365 day year and can vary as much as 25% to 50% from the stated number of hours.

Winter days get much less sun on average than summer days so reduce your zone average by 1 hour and you will have a pretty good approximation of how many hours of good sunlight you will have on a sunny winter day. It your off-grid home is used seasonally and not in winter you can use the actual zone average.

From the example we said you need 2600 watts to run your appliances for one day so divide 2600 watts by the number of hours you will average of good sunshine in winter.

For example I average 4 hours of good sunshine on winter days so 2600 watts / 4 hours = 650 watts needed per hour.

So my solar panels need to be able to produce 650 watts per hour to meet my needs.

Since solar panels produce less in cold weather and there are days when you may not even get the average sunshine I would adjust that figure to 675 watts just to be sure I have enough power.

So now you can use the examples, worksheet and map above to figure out how big of a system you need:

1) Total watts used daily   (from Average Daily total)                                         __2000_____

2) Plus 30% for changes   (multiply total daily watts by .30)                               ___600_____

3) Adjusted watts used daily    (add total watts used and the 30% together)      __2600_____

4) Hours of winter sun shine (from zone map reduced by 1 hour)                      ____4______

5) Watts needed (divide adjusted watts by winter sunshine hours)                     ___650_____

This is the size of solar electric system watts you will need to handle both your winter and summer power uses. You will have more power than you need in summer and enough to run everything you need in winter.

Once again there are a lot of manufactures of solar panels. The solar panels I used are Sharp 225w -24 volts which I bought from Solar Blvd. about two years ago. Which It’s much cheaper to buy panels by the pallet and then keep the extra for future expansion. 225watts was the largest wattage I could get at that time. I just checked online and they are showing panels that are over 300 watts.  Got to love technology.

225 + 225 + 225 = 675 watts

Using three panels in winter months should give me enough power. I also plan on using Wind Turbines as well, which adds power to the system in short burst. With Wind Turbines you must make sure that the power goes somewhere. That’s why some type of load is so important. It’s also hard to figure the amount of power from a turbine you receive through out the day because of the nature of wind.  This figure only represents the amount of watts of solar panels you need and does not tell you how much storage capacity of batteries needed for the system.

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