If your neighborhood is like many others today, you’re noticing a lot more solar panels going up on your neighbors’ rooftops. Residential solar PV is booming. If you’re starting to envision money-saving clean energy solar panels on your roof, you’re probably wondering, “How many solar panels will I need?” Here you’ll answer that question and learn about just how much power you could be generating for your household from a rooftop solar PV system.

Size Up Your Solar Situation

The exact number of panels and rooftop square footage required for your solar PV system will vary depending on your specific household’s energy consumption level and solar goals. The table below, however, will give you a rough estimate of what to expect.

The above numbers are based on the following factors:

3 ft. x 5 ft. panels producing 250W each

$.12/kWh electricity rate

1700 energy production factor*

* The energy production factor is an industry estimate for sizing purposes that 1kW of solar will produce approximately 1,700kWh/year. For more information on the energy production factor, see below.

Determine Your Solar Goals

Some customers are only interested in offsetting part of their energy consumption. That’s called a “peak saver”. A peak saver means you’re shaving off [a percentage] of your peak time energy costs. That’s the part of the day, from maybe noon to seven or one to eight, when your electricity costs most – and you are offsetting that portion of your bill.

Gary R. Held
Gary Held

As mentioned, the table above is designed to offer only a quick ballpark estimate of the number of solar panels a residential solar PV system might use. Determining the size of any specific system will take a little more figuring based on several factors. First and foremost, it’s important to determine the amount of energy the household uses. Once that it known, the consumer must decide the specific energy goals he or she has in mind for their solar PV system:

  • FULL POWER

    Some consumers want their system to generate enough power to meet 100 percent of their energy needs. This may include using electricity provided by their local utility during evenings or other times when the panels are not producing, and placing excess power back into the utility’s grid at times when the panels are generating more electricity than the household is consuming.

  • PARTIAL POWER

    For other consumers, the goal is to use solar PV to partially compensate for their entire household energy consumption. An individual may be interested in installing a system, for example, that generates enough power during the day to allow him or her to avoid paying higher “peak time” rates for utility generated electricity.

  • CLEAN POWER

    Then there are those consumers with an interest in lessening their impact on the environment by decreasing their dependence on non-renewable energy sources.

The point is that for every individual consumer with an interest in solar power, there is likely an array size to fit their unique needs and goals.

Here’s a look as some of the most important factors to consider when sizing up a residential solar PV system:

How Much Energy Does Your Household Use?

The first misconception is that people think that the size of their house and the number of bedrooms make a difference. That doesn’t matter at all. The most accurate way to determine how many solar panels you need is to determine how many kilowatt-hours you used in the last 12 months. We’re able to pinpoint right down to within literally hundreds of kilowatt-hours a year how much [power] we can offset if you tell us how much you used last year.

Gary R. Held
Gary Held

Think of it as the first rule of residential solar PV: The more energy your household uses, the more energy that needs to be generated by your solar PV system. And the more energy required from your solar PV system, the more solar panels you are going to need. Note here that there is no mention of the total square footage of your home or the total number of rooms. That’s because it’s not about the size of the house, it’s about the amount of energy used inside it.

So, how do you know how much total energy your household is using? You could figure it out the hard way. That is, make a list of every appliance and item consuming energy in your home, determine how much electricity each one consumes on a chosen basis (daily, monthly, yearly, etc.), and then add all of that consumption up. That’s your household energy usage.

Another way to assess your energy use is to check your utility bill, or more precisely, check the last 12 months of your utility bills. Somewhere on your bill, you are going to find a number for the total amount of your electricity consumption for the billing period, typically one month. Look for the total number of kilowatt-hours (kWh) consumed. That’s the number you want. Then do the same for the 11 months prior to your current bill. Add those 12 numbers together and you have your total household energy consumption for the past year.

By the way, there’s one other, even easier, way to determine your total energy usage for the past year: call your utility company and ask them.

What if you want to add solar PV to a brand new home, i.e. one without an energy usage history? In that case, you are going to have to come up with an estimate.

Off-Grid or Grid-tied

Off-grid or grid-tied – that is the question. Whether it is nobler to disconnect completely from one’s local utility and take a stand for full and total self-sufficiency, or to remain tied to that utility, so as to benefit from both one’s newly acquired renewable energy system and one’s older and dependable (and ever more expensive) power source. The question for readers of this guide is: “Does being off-grid or grid-tied affect the size of my solar array and, therefore, the number of solar panels I need?”

The answer is: “It depends.” Let’s look at both possibilities:

On-grid

For a consumer whose solar PV system will be connected to his or her local utility grid, the question of solar array size is much more dependent on the specific energy goals of that consumer:

  • If the consumer intends for his or her system to generate 100 percent of the energy for their household, then that means a full-sized solar array with the number of panels to match.
  • If the consumer’s goal is to offset a small portion of the household’s energy needs, then a smaller array with fewer panels will suffice.
Off-grid

If an off-grid solar consumer needs a constant and reliable power stream at all times, day and night, that will require:

  • A solar PV system that provides enough electricity for 100 percent of the household’s needs.
  • An on-site backup system, such as a fossil-fuel powered generator or a solar PV-connected battery system for the times (night, bad weather, cloudy days, etc.) that the PV system is not generating electricity. And that means enough solar panels for 100 percent of the household’s power needs.

Insolation:
Know Your Energy Production Factor

If you’ve done any amount of reading on the subject of solar energy, you have likely run into the term “solar insolation.” Solar insolation, strictly speaking, refers to the amount of solar radiation received over time by a given surface area. To be precise, “solar insolation” is a bit redundant, since the word “insolation” is a shortening of the term, “incident solar radiation.” To avoid confusion, simply think of solar insolation as the amount of sunlight shining on a particular location during a particular time.

What does solar insolation mean to someone trying to size a residential solar PV system? Solar insolation for a specific location is factored in when calculating the size of a solar PV array. For the purpose of this calculation, solar insolation is expressed in terms of kWh/kW-year. Take a look at the map below, which indicates kWh/kW-year figures for the continental United States.

map

Source: NREL

The number associated with a consumer’s location, known as the “energy production factor,” will be used in this calculation to determine the total kilowatt-hours per year that a PV system will generate:

kW (from the PV system) x kWh/kW-year (energy production factor) = kWh/year

Now, take the kWh/year figure from the calculation and apply it to the total number of kilowatt-hours per year for your household (as discussed above) to determine the size (in kilowatts) of your solar array.

So, what does solar array size mean when it comes to number of solar panels needed?

How Many Panels Can Fit?

Now that you know the number of kilowatts your solar array needs to generate, it’s time to figure out the number of solar panels it will take to meet that need. Here’s the next specific question that requires an answer: “How many panels can I actually fit on my roof?”

There are a couple factors to consider:

Direction of the roof

For those living in the Northern Hemisphere (like all of us in the U.S.) the ideal roof direction for solar PV is south. The reason is that a south-facing roof is exposed to the maximum potential sunlight.

Shade

But, the total amount of roof space really isn’t the issue; it’s the total amount of shadeless roof space where panels can efficiently make use of the sunlight hitting them.

For the sake of this discussion, let’s make two assumptions. The first assumption is that the home has a nice south-facing section of roof that can be used for locating solar panels. The second assumption is the solar panel size. Currently, the average size of a solar panel is 3 feet by 5 feet, or 15 square feet. With these two assumptions in place, the calculation to determine the number of panels that can fit on the roof is pretty straightforward: total available roof space (in square feet) divided by 15 (the square footage of a single 3 ft. x 5 ft. panel) gives you the total number of panels. For example, if the total available roof space is 375 sq. ft., the calculation would be:

375 ÷ 15 = 25 (total number of panels)

Remember that this is only an estimate of the actual number of panels that will fit on a roof based on the assumptions given. Panel sizes do vary and an additional small amount of space around the entire solar panel arrangement will be required.

What Size Panels Will You Need?

It the case of a small roof area, you will want to use a bigger watt panel. It happens all the time.

Gary R. Held
Gary Held

You’ve figured out the amount of power you need to generate and you’ve figured out the number of panels you can fit on your roof. But there’s a problem. How do you know that the power generated by all of the panels will be sufficient to cover your energy needs? The answer is: panel rating.

“Panel rating” refers to the amount of power generated by an individual solar panel. Each panel on the market has a rating indicating the amount of electricity it can produce. Panel ratings vary by model, but typically range, in watts, from the upper 100’s to the mid-300’s.

Now, here’s how to calculate the power rating for the panels in a specific solar array. Let’s assume a 6kW (6000 watts) total system generation power and a roof area that can accommodate a maximum of 25 panels (at 3ft. x 5ft. each). The calculation will be:

6000 ÷ 25 = 240 (watts per panel)

In this example, each panel would have a rating of 240W or higher. Bear in mind that this example assumes using the maximum number of panels that will fit on the roof, which is not always necessary. For example, with a 6kW system requirement, an option might be to use panels with a rating of 300, in which case a total of only 20 panels would be required.

In short, it’s important to remember that solar panels designed and built in a variety of sizes and ratings, which in turn allows the consumer some leeway when it comes to the total number of panels used in his or her solar array.