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Sizing Your Solar Power System

You know you want to install solar, but how much solar do you want to install? There are many factors to consider when making decisions about system size.

The system size will be dependent on how much energy you want to generate. If you are doing a grid-tied system, you will start by looking at your electric bills. You should add up the last 12 months of electric bills so you have your annual usage. If you only use one month or one season, you can end up way off because your electric needs can change dramatically from summer to winter. 

Sample Electric Bill
Sample Electric Bill – look for the kWh!

Once you know your annual usage, you should ask yourself some questions. If you have solar on your roof and you’re no longer paying crazy high prices for every kilowatt hour (kwh), what might you do differently? Would you turn the air conditioning setting down a couple of degrees so you never break a sweat? Would you buy an electric car? Do you have any other plans like installing a swimming pool?

If you answered yes, then you will want to add some kwh to your current usage to account for the additional air conditioning run time, the electric car, the swimming pool or whatever else you might dream up that will use extra electricity. 

The next big question is how much of this projected energy usage do you want the solar to offset. This will depend on your electric rates and how your electric provider deals with net metering. If your local utility company gives you full retail credit for every kwh that you feed into their system then you should consider a system that generates close to 100% of your projected usage. Going over 100% is generally not beneficial as most utilities will not pay much for the excess power generated over the course of a year. 

Sample electric bill before and after solar

If you are sizing an off-grid system, you will have to do a little more work to get your projected usage.  You will need to make a list of everything that will use electricity. Then you have to determine how many watts each of those items will use and how many hours you plan to run them. You can multiply the watts by the hours to get watt hours. Divide the watt hours by 1,000 and that will give you the kilowatt hours (kwh).

Now you know what you need to generate but that still doesn’t exactly tell you what size system to install.  Because there are so many factors (like location, weather and orientation) that affect solar panel output, it is best to use an online calculator to determine system size. PV Watts (at is a free tool that can help. 

Because location is important, PV Watts will ask for your zip code. It already has weather data for your zip code factored in so if you live in a rainy area, that will be considered when it tells you how much a system will output. 

For orientation, PV watts will ask you for tilt and Azimuth. Tilt is the angle the solar panels will be from horizontal. If you are installing on your roof, then it will be whatever angle your roof is. If you are doing a ground mount, you have some flexibility but be careful because the steeper the angle, the higher the back end of the rack must be. If it is too high, you may need extra structural support. Azimuth is the number on the compass that corresponds to the direction the solar panels will be facing. You can get this using an actual compass, a smartphone with a compass app or Google Earth. 

Solar Tilt and Azimuth
Angles to consider for solar panels

Don’t forget to also factor in shade because that can make a big difference on your output and may cause you to need a larger system. Typically, you want to install your solar panels where there is no shade, but that isn’t always possible. Shade in the early morning or late evening won’t affect much but shade in the middle of the day (10am – 2pm) will significantly reduce your system output. 

So all that work will provide you with the amount of solar watts you need to install. The next thing you need to do is choose the right size inverter. Grid-tie inverters are generally sized a little lower than the DC wattage of the solar panels. For example, if you decide you need 8,000 watts (or 8 kw) of solar, you would generally choose a 7,600 watt inverter. The reason for this is that your 8 kw of solar will not ever really produce the full 8 kw. The rating on the solar panels is based on all conditions (like temperature and sun angle) being perfect which doesn’t really happen in the real world. 

But, you do have to be careful about going too small on your inverter. If you attach a 12 kw system to a 7.6 kw inverter, the inverter will clip the output. For example, if conditions are really good, that 12 kw system might be outputting 10 kw but your inverter will only output 7.6 kw so you aren’t getting the full amount of power you could be. If that happens every day the overall energy loss can be significant.

The sizing for battery-based off grid inverters is different. The size for those is based on your loads and has nothing to do with how many solar panels you have. To size the off grid inverter you need to add up the watts of everything you might have turned on at one time. Don’t forget to add appliances that cycle on and off automatically like the refrigerator and air conditioner. If you add up all the loads and it is 6,000 watts, then you need an inverter that is at least 6,000 watts. Going too small means your inverter will overload and shut off when you try to run too many appliances at one time. When it doubt, get the larger option.

So that is the basics of system sizing. If it all seems too complicated, don’t despair, the professionals at can do all this work for you and provide you with a solar package that is just the right size for what you need.   

Author: Harold Tan

I believe clean, renewable energy is key to the evolution of society as a whole. Solar powers our planet, why not harness it to power humanity? Let's power our homes, our work, and our vehicles with solar energy. It begins with raising awareness and encouraging those around us to go green.

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1 Comment

  1. Off grid systems should be designed to recharge the 24 hour day expected use of the battery pack on a hot summer day within 5 hours. Anything less will age the battery pack unacceptably.

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