Understanding Kilowatts and Kilowatt-Hours

Before you can size a home solar system, you need to understand the two units that describe electricity. The U.S. Energy Information Administration explains that a kilowatt (kW) measures power at a specific moment. One kilowatt equals 1,000 Watts. A kilowatt-hour (kWh) measures energy use over time. One kilowatt-hour is the energy of one kilowatt generated or consumed for one hour.

Short version: A solar system is rated in kilowatts (its size), while your utility bill shows kilowatt-hours (your consumption). Sizing means matching the system’s kW to your annual kWh needs.

This distinction is central to solar system size planning. A solar array rated at 5 kW can produce a certain number of kilowatt-hours per year depending on sunlight, orientation and other factors. Your goal is to find the system kW rating that will generate roughly the number of kilowatt-hours your household uses annually.

Start With Your Annual Electricity Use

The most reliable starting point is your own utility bill history. The EIA reports that the average U.S. household consumes about 10,500 kWh of electricity per year, but it stresses that electricity use in homes varies widely by region and housing type. Apartments in the Northeast tend to use the least, while single-family detached homes in the South often use the most, partly because of higher air conditioning loads.

Look at your bills for a full 12-month period and add up the kilowatt-hours. This total captures seasonal swings: the EIA notes that air conditioning accounts for about 19% of residential site electricity consumption, space heating about 12%, and water heating another 12%. Your own mix will depend on your climate, home size and the appliances you run.

Estimate Local Production Per Kilowatt

Once you know your annual kWh target, you need to know how many kilowatt-hours a single kilowatt of solar will produce at your location. The PVWatts Calculator, operated by the National Laboratory of the Rockies, provides this estimate for any address. It uses 30 years of actual weather data to model the solar radiation a fixed array would receive and the AC energy it would deliver.

PVWatts includes a caution that its predictions contain inherent assumptions and uncertainties. It does not differentiate between PV module technologies except through the inputs you choose. The expected range it shows is based on 30 years of historical weather and is intended to indicate the interannual variability you might see. The laboratory suggests that more sophisticated tools, such as the System Advisor Model, allow for more precise modeling.

To get your local production factor, run a PVWatts estimate with a system size of 1 kW, keeping the default loss, tilt and azimuth settings. The result is an annual kWh figure for one kilowatt of solar at your location. This number is the key to translating your consumption into a system size.

The Core Sizing Formula

With your annual kWh target and your local kWh-per-kW estimate, the calculation is straightforward:

System size in kW = Annual kWh target รท Annual kWh per kW from PVWatts

As a simple arithmetic example with round numbers (not a figure for your area), a household using 10,000 kWh a year at a location where PVWatts estimates 1,000 kWh per kW would work out to about 10 kW. You would round the result to a practical system size and discuss it with installers. This is an estimate, not a final design, but it gives you a solid starting point for conversations.

Factors That Change the Size You Need

Several real-world conditions can push the required system size up or down. The U.S. Department of Energy’s solar design basics explain that to obtain the highest annual energy output, modules in the northern hemisphere are pointed due south and inclined at an angle equal to the local latitude. A roof that faces east or west, or has a shallower or steeper pitch, will produce less energy per installed kilowatt, which may increase the system size needed to hit the same annual target.

Shading from trees or nearby buildings also reduces output. The DOE notes that microinverters attached to each module allow independent operation of each panel, which is useful if some modules might be shaded. Even with such equipment, heavy shade lowers total production and can affect sizing decisions.

Temperature plays a role too. The DOE explains that solar cells generally perform better at low temperatures. Higher temperatures cause a slight increase in current but a much larger decrease in voltage, reducing overall power output. PVWatts and installer estimates account for temperature as part of their production models.

System losses further reduce the energy that reaches your home. PVWatts includes a system losses category that accounts for soiling, wiring, inverter efficiency and other factors. The DOE adds that inverters are expected to need replacement at least once in the 25-year lifetime of a PV array, and their efficiency affects total system output over time.

Degradation is another factor. A 2012 analytical review by Jordan and Kurtz of the National Renewable Energy Laboratory (now the National Laboratory of the Rockies), hosted on OSTI, assembled nearly 2,000 degradation rates from field studies across 40 years. The review found a median degradation rate of 0.5% per year for flat-plate terrestrial modules and systems. This means a system’s output slowly declines over the years, which is worth discussing with your installer when you settle on a size.

Input Source Effect on System Size
Annual household kWh Utility bills; EIA average ~10,500 kWh Higher use increases required kW
Local kWh per kW per year PVWatts (National Lab of the Rockies) Higher production reduces required kW
Orientation and tilt DOE design basics Off-south or steep/flat roofs increase required kW
Shading DOE design basics More shade increases required kW
System losses PVWatts; DOE design basics Higher losses increase required kW
Degradation rate Jordan and Kurtz (NREL, 2012) Median 0.5%/year; discuss with your installer

Net Metering and Utility Rules

Your sizing goal may also be shaped by how your utility credits exported solar energy. The DOE’s Homeowner’s Guide to Going Solar describes net metering as an arrangement where system owners are compensated for solar power generation sent to the grid. It notes that whether your system qualifies depends on policies and practices in your state and electric utility, and that the rules vary widely.

How exported energy is credited affects how much of your annual use it makes sense to cover, so ask about it before settling on a size. The DOE recommends contacting your local electric utility to understand the rates, eligible system sizes and bill credit design in your area.

The DOE lists eligible system size among the basic net metering questions. Because these policies are state-specific and change over time, you should check current rules with your utility and installer rather than relying on any single example.

Ask Installers to Show Their Assumptions

A reputable installer will provide a detailed production estimate and explain the inputs they used. Ask them to walk you through the annual kWh they project, the system size in kW they recommend, and the specific loss factors, degradation rate and weather data they applied. If their numbers differ significantly from your own PVWatts estimate, ask why. Differences often come down to shading analysis, inverter choice or a more refined tilt and azimuth measurement.

The DOE’s guide advises working with a qualified professional who holds a certification to install solar, and it points to the North American Board of Certified Energy Practitioners as the industry-standard certification body. Any work connected to home wiring or the grid must be done by licensed professionals with permits and utility approval.

For a broader look at how system sizing fits into the overall planning process, see our step-by-step home solar planning guide. You can also explore the full system sizing section for more detailed articles, or visit the home solar planning hub to learn about roof suitability, energy use and installation planning.

Frequently asked questions

What is the difference between kW and kWh when sizing solar?

A kilowatt is the system’s power rating at a given moment, while a kilowatt-hour is the energy it produces or you consume over time. The EIA explains that one kWh is one kilowatt generated for one hour. Sizing means choosing a kW rating that will produce enough kWh across the year to match your usage.

Does my roof direction really change the system size I need?

Yes. The DOE states that modules pointed due south and tilted at an angle equal to the local latitude achieve the highest annual energy output. A roof facing east or west will produce less energy per installed kilowatt, so you may need a slightly larger system to meet the same annual target.

Should I oversize my system to account for panel degradation?

The 2012 NREL review by Jordan and Kurtz found a median degradation rate of 0.5% per year across nearly 2,000 field measurements. Whether to allow for this gradual decline in your sizing depends on your budget, roof space and long-term plans. Discuss the trade-off with your installer.

For more on how energy use patterns affect your solar plans, read our guide to household energy use. To understand how system size relates to overall costs, visit cost factors.

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