In the previous article we did an energy audit and calculated your solar power needs per day in amp hours.  It's now time to take that figure and use it to estimate the size of the solar panels for your system.  To do this accurately we need four pieces of information and they are:

1. Your daily solar power needs in watt hours;
2. How much solar energy you have available to you in your part of the world each day;
3. How efficient the solar panels are at taking the sunlight and converting it into electrical current;
4. The amount of charge that actually reaches your batteries from the solar panels themselves.

This might sound quite complicated, but as you're going to see it's simple to do if you take it one step at a time.So as an example, I’m going to take the outcome of our energy audit:

##### 1.  Your daily solar power needs in watt hours

From the Solar Sizing Calculator, we found that we needed 48.5 amp hours a day.  To convert this to watt hours we merely have to multiply by the electrical system voltage.

Our system runs at 12 volts so we simply multiply 48.5 x 12 = 582 watt hours per day.

##### 2.  Amount of solar energy available to you each day

Finding out how much solar energy you have available to you in your part of the world on a daily basis is much simpler than you'd think once you know how.  This information has been worked out for us by NASA!

Once you go to the NASA site you'll be able to select your location and after a short sign-up process, retrieve your data for free.  Here's the link to their site where you can find this information:  http://eosweb.larc.nasa.gov/sse/

I've been to the NASA site and found out that during the  UK summer, the average solar energy available is 4.67 kWh/m2/day (kilo watt hours per square meter per day).

This is the sort of number you'll be looking for and it's usually somewhere in a range of between about 1 and 6 kWh/m2/day.

If you'd like more detailed help with this and typical figures for worldwide locations, you can find it all in my FREE Fast Start Toolkit.

##### 3.  Solar panel efficiency

The next item on our list was how efficient the solar panels are at converting the sunlight into electrical current.  We don’t have to worry too much about this because on the whole the solar panels will be 75% efficient.  It varies slightly between different solar panels, but this will give us an accurate result.

##### 4. The amount of charge reaching your batteries

The fourth and final piece of information we need is how much of that electrical current is actually going to reach your batteries and charge them.  It’s generally accepted that if your system’s well designed and working properly, you can assume that 95% of the charge is going to reach your batteries.

##### OK so what do we do with these four bits of information?

When you buy a solar panel, it's rated in watts, so the outcome we're looking for is an amount in watts.

The first thing we have to do is take the daily solar power need in watt hours and divide it by the amount of solar energy available each day.  For our example this means:

582 watt hours / 4.67 = 124.6 (we'll call this 'answer 1')

The second part is to multiply the solar panel efficiency by the amount of charge reaching your batteries (first we'll convert 75% in this way 75/100 = 0.75 and the same for 95/100 = 0.95):

0.75 x 0.95 = 0.7 (we'll call this 'answer 2')

The last thing we do is divide 'answer 1' by 'answer 2' to give us our final answer in watts, like this:

124.6 / 0.7 = 178 watts

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So now you know the theory why not make it even easier for yourself and get FREE instant access to these time-saving, proven tools in my Fast Start Toolkit Includes:

• Solar Panel Sizing Calculator;
• Typical Solar Energy Readings for worldwide locations;