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What Size Solar System Do You Need?

If you are in the market for solar, you’re probably thrilled with the idea of zeroing out your energy bill. But you can’t just tack a few solar panels on your roof and expect to reduce your bill to nil. 

That’s why many customers ask us: what size solar system do I need? The size of your solar system comes down to how much energy you use on a daily basis and how many solar panels it takes to produce that amount of energy. 

Some people are surprised to learn that the size of their home is not the main factor in determining the size of their solar system. It’s all about your energy needs.

It’s a common misconception: the size of your home is not the deciding factor in the size of your solar system.

A retired couple that lives in a 3,500 square foot home is likely to use less electricity than a family of four that lives in a 2,500 square foot home. In this case, the family of four would need more solar power to cover their energy demand. 

When we are approached by a customer wondering what size solar system they need, we always ask how much electricity they use. You could live in a mansion in Beverly Hills yet your average electricity usage might be less than a suburban family of four.

If you think “my house is small, so I only need a panel or two,” you could be wrong. Same goes for those with larger homes who fear they’ll need a massive, expensive solar system. 

Calculating your average daily energy usage is the determining factor in how much solar you need. 

To calculate daily energy usage, refer to your past 12 months of energy bills. If you don’t have them handy, log in to your PG&E, SMUD, or other energy provider’s account to find your statements online.

Let’s assume that you used 10,000 kilowatt hours (kWh) of electricity in the past 12 months, or an average of 833 kWh per month. Divide 833 by 30 to calculate your average daily energy usage. 833 divided by 30 equals 27 kWh. You can even take the calculation one step further by finding the average energy you use during the summer months, the time of year in which you use the most energy. This value will give you the maximum average energy you use, so you won’t have to deal with energy bills when your usage is more than normal. 

But let’s return to our assumed average daily energy usage of 27 kWh. What size solar system do you need in that case? There are a number of factors that come into play. For one, you want to make sure the system you purchase is capable of producing at least 27 kWh of electricity per day. But that’s not all: how much power is generated by your panels depends on where you live and how much shade your home or business gets, since solar panels are rated on the number of kilowatts of energy they produce given 24/7 direct sunlight. Obviously, more shade will knock down the amount of energy your panels can produce. 

Solar System Size Chart

The chart above shows what a typical day might look like. The yellow bars represent the energy produced by your solar panels. Notice that at midday, when the sun is directly overhead, energy production peaks. As the afternoon progresses and the sun goes down, total energy production decreases. While this is just an example, it’s a good way to visualize solar energy production throughout the day.

Say you’re looking at a solar system that is rated to produce 5 kilowatts (kW) under ideal conditions. In our example, divide 27 kWh by 5 kW and get 5.4 hours, or the amount of time a 5 kW system charging at maximum capacity would need to charge up. If you get 6 hours of sunlight in one day, and your panels produce at peak efficiency, then a 5 kW solar system is large enough to cover your energy needs.

The more intense the sunlight, and the longer you receive direct sunlight, the fewer solar panels you need. But if your roof is partially shaded for much of the day, or much of the roof is at an angle where it does not receive direct sunlight for much of the day, then you will need more panels to compensate.

Compare solar models to find the exact number of solar panels that you need. 

Once you figure out the system size you need, you can actually determine the exact number of solar panels that are equal to this energy output. 

There are many solar manufacturers with different models. Each solar system model is rated differently. At Ilum Solar, we sell only premium solar systems from reputable manufacturers such as SunPower and REC. One of our most popular solar models is the REC Alpha 370, a system with a rated power of 370 watts. Divide the system output that you need—in our example, 5,000 watts—by 370 to get 14. In other words, you will need 14 REC Alpha 370 panels in order to generate the desired energy output we previously established. 

Go one step further and you can calculate the space that 14 REC Alpha 370 panels would occupy on your roof.

REC Alpha 370 panels measure 5.6 feet long and 3.3 feet wide, or roughly 18.5 square feet. Multiply 14 by 18.5 to arrive at 259 square feet. 14 REC Alpha Panels occupy 259 square feet of space.

Calculating the size of your solar system is not all that difficult. Once you establish the average daily energy that you use, you can pretty quickly work out what size solar system you need in order to generate this power.

Can the right size solar system reduce your energy bill to zero?

Energy prices are only increasing—rising more than 40% since 1995. The good news? You can completely avoid rising energy costs with solar! 

It’s become very clear that solar-sourced energy costs less than traditional energy from the grid. But for some, significant savings is not enough. You might want a zero dollar monthly energy bill, and think solar is the way to achieve it. 

Solar will reduce your energy bill. However, only in those months with particularly high energy production relative to energy demand will you see your bill reach zero. Even with the right size solar system, most months your energy bill won’t quite be zero. 

That’s not to say your monthly energy costs won’t significantly decreasethey will.

As we mentioned above, peak solar production can only occur with direct sunlight. Sure, if your solar panels had direct sunlight for 8 hours a day, you should expect no monthly energy costs. The reality is that you might get an hour or two of direct sunlight per day, which means an hour or two of peak production.

Most homeowners use electricity during the evening after work. Lights, kitchen appliances, TV, laundry machine, and air conditioning might be running at the same time. But because it is now 6pm and your panels are no longer receiving direct sunlight, you might have to tap into the energy grid to meet your energy demand. That’s why your monthly bill is not zero, even though your solar system is sized correctly. 

You can cover at least some of your evening time energy demand to shave off more money from your energy bill with a battery storage system, which is a great way to store and use the excess energy that your panels generate. 

The bottom line: solar will decrease your monthly energy costs. How much depends upon your system’s production versus your energy demand. Installing the right size system will get you as close to zero dollar bill as possible. 

Determining the size of your solar system is the first step in the solar buying process. While you’re shopping around for a solar company and looking for system quotes, you can make more informed decisions if you calculate what size solar system you need. Once you engage with a solar company, they will also run some calculations to figure out how much solar you need, to ensure you get the perfect system for your home or business. 

Have more questions? Contact Ilum Solar! Our staff of engineers can help you determine the size of your solar system and answer any other questions you may have.

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