When considering a switch to solar, the available space for your system is a key consideration. Solar panels are quite large, with most consumer-grade panels weighing about 40 pounds each and measuring about 5.5 feet long and 3 feet wide. In this guide, we’ll walk you through determining the required solar panel size, estimating energy requirements, and the key terms to know when buying solar panels.
How to determine what solar panel size you need
When choosing solar panels, you first need to figure out how many Kilowatt-hours (kWh) you use per day. To get this figure, review your monthly utility bills to determine your total usage. Then, divide that number by 30 to get your daily average.
For example, if you use 900 kWh per month, your average daily usage would be 30 kWh.
The appliances that drive power demand in your home are the HVAC system, water heaters, EV chargers, and heat pumps. The more demanding appliances you have in your home, the bigger your solar panel array will need to be.
After calculating daily kWh usage, most homeowners find 60-cell panels suit lower to moderate usage or limited roof layouts. On the other hand, 72-cell panels produce more power per panel and fit homes with high electricity demand and ample, unobstructed roof space.
You may also see 66-cell panels, which sit between 60- and 72-cell designs and are often used to balance panel size with higher wattage output. In addition, many modern panels use half-cut cells, which split each cell in two to reduce electrical losses and improve performance in partial shade.
Solar panel sizing details
There’s a lot to keep in mind when sizing solar panels for your system. The table below lays out the key concepts you’ll need to consider.
| Concept | Definition | More details | How it applies |
|---|---|---|---|
| Residential panel size | Standard size for rooftop solar panels on homes. | About 65 in x 39 in, 60 cells, typically 350–450 watts | Home solar systems, pitched roofs |
| Commercial panel size | Larger panels are designed for commercial rooftops or ground-mount systems. | About 78 in x 39 in, 72 or 96 cells, typically 400–600 watts | Flat roofs, solar farms |
| Wattage output | The amount of power a panel produces under standard test conditions (STC). | Measured in watts (W), varies by cell count and efficiency | Energy yield, system design |
| Panel efficiency | How effectively a panel converts sunlight to electricity. | Affects power output relative to size, measured as a percentage | Solar performance, ROI |
| Cell count | The number of photovoltaic cells in a solar panel. | Residential: 60 or 66; Commercial: 72, 96; affects voltage and size | Electrical design, string configuration |
| Weight | The total mass of a solar panel impacts mounting choices. | Typically 40–55 lbs for residential panels | Rooftop load, racking systems |
| Surface area | Physical footprint of the panel on the roof or ground. | Measured in square feet or meters; impacts panel count per project | Space planning, layout design |
| Panel thickness | Depth of a solar panel, including frame. | Generally 1.2–1.6 inches; low-profile designs preferred for aesthetics | Flush mounting, curb appeal |
Does solar panel size make a difference?
Though it’s important to know the average dimensions of a solar panel in square feet, panel size is usually described by the number of cells it has. There are two sizes used for residences: 60-cell and 72-cell panels.
60-cell panel pros and cons
Pros
- More compact
- Fit more easily on smaller roof segments
Cons
- Less average output per panel
- Requires more panels to meet efficiency and output goals
66-cell panel pros and cons
Pros
- More power per panel than 60-cell
- Smaller footprint than most 72-cell panels
Cons
- Less output than 72-cell panels
- Not as flexible as 60-cell panels on tight roof layouts
72-cell panel pros and cons
Pros
- More output per panel than 60 or 66 cell
- Less expensive per watt
Cons
- Can't always fit on smaller roofs
- Heavier and less maneuverable during installation
Half cut panel pros and cons
Pros
- Higher real-world efficiency due to reduced electrical resistance
- Better performance in partial shade and high temperatures
Cons
- Higher upfront cost than full-cell panels
- Slightly more complex panel design without increasing roof-area flexibility
There are benefits and drawbacks to each panel size, and a good solar installer will have recommendations based on your needs and roof space. But factors like output and efficiency are arguably just as important to consider. As you’re researching which solar panels to buy, compile a list of questions you can ask a solar company when you’re getting estimates.
Using a solar panel calculator to estimate energy requirements
The National Laboratory of the Rockies (formerly the National Renewable Energy Laboratory, or NREL) offers the PVWatts calculator on its website to help estimate your daily energy requirements. To get started, enter your street address, ZIP code, or geographic coordinates, and the tool will display the solar resource data for your location.
You'll need to provide six key parameters of your system. These parameters are:
- DC system size
- Module type
- Array type
- System losses
- Tilt angle
- Azimuth angle
Once you've entered this information, the calculator generates monthly and annual electricity production figures and estimates your solar system's economic value using average retail electricity rates.
How to calculate your energy needs for solar
You can estimate your solar energy needs by following a fairly simple formula:
To break it down, you can get the necessary system size, measured in kW, by multiplying the peak sun hours of your area by 1.2 and dividing your daily kWh usage by that value. This accounts for inefficiencies in wiring, inverters, temperature, and shading.
Peak sun hours differ by region
- California: 5 - 6 hours
- Northeast US: 3.5 - 4.5 hours
- Midwest: 4 - 5 hours
For example, if you use an average of 30 kWh per day and live in a location with 5 peak sun hours, you need a system with an output of at least 6 kW. Multiply that number by 1.2 to get the recommended system size of 7.2 kW.
How panel size affects system size
The typical wattage for residential solar panels is 350 to 450 W. When comparing solar panels, it’s helpful to select the most efficient ones, as this will reduce the total panel count you’ll need.
To determine the number of panels you’ll need for an array, you can use another simple formula:
If you need a 7,200W system and the panels you’re considering have a 400W output, you’ll need 18 standard 60-cell panels. You would need fewer 72-cell panels if each panel has a higher wattage rating.
Factoring in solar panel dimensions and roof space availability
When deciding which panel size to purchase, you also need to consider the available roof space. The average panel dimensions are 65” x 39”, which is roughly 17.5 square feet. If your array size is 18 panels, at 17.5 square feet per panel, you’ll need approximately 315 square feet of roof space to install your system.
However, when assessing roof space, you’ll also need to factor in the orientation and tilt of your panels to maximize efficiency. Installing your system on a south-facing part of your roof is best. You want to avoid excessive shading, as that will reduce the panels’ effectiveness. The tilt angle of each panel also affects the year-round output.
When choosing a solar installer for your system, be sure to discuss each of these factors with them. Experienced installers can help you choose the right spot on your roof for maximum output and efficiency.
Bottom line on solar panel size
When choosing the right solar panel size, you need to balance your energy needs with available roof space. By calculating your daily energy consumption and understanding key specifications such as wattage, efficiency, and cell count, you can design a system that meets your household's power needs. Experienced solar installers can recommend panel sizes and help you determine the optimal placement and tilt for maximum system efficiency and output.
