# How Many Kwh Does A 400 Watt Solar Panel Produce Per Hour?

Solar panels are devices that convert sunlight into electricity. They are made up of solar cells which contain photovoltaic materials capable of converting light photons into an electric current. This electric current is measured in units of watts. Watts measure the rate of energy production or consumption.

To determine how much energy a solar panel produces over time, we need to look at kilowatt-hours (kWh). A kilowatt-hour represents the amount of electricity consumed or produced over 1 hour at a constant rate of 1,000 watts. By calculating the kWh, we can estimate the total energy output of a solar panel during a specific time period.

## Power Rating of Solar Panels

Solar panels are rated by their maximum power output in watts. This rating is determined under standardized test conditions at a temperature of 25°C and with irradiance of 1000 W/m2 perpendicular to the panel. Under these test conditions, a 400 watt solar panel can produce up to 400 watts of power.

The power rating given for a solar panel is its direct current (DC) power output, not alternating current (AC) power which is what comes out of a wall outlet. An inverter is required to convert the DC output of a solar panel to usable AC power.

## Factors That Affect Solar Panel Output

There are several factors that affect how much energy a solar panel can produce:

**Solar Irradiation** – The amount of sunlight that reaches the solar panel. This depends on the panel’s geographic location, local weather conditions, and time of day. Areas that receive more annual sunlight will produce more energy. Cloudy days will cause less production. A solar panel will produce the most energy when pointed directly at the sun.

**Temperature** – Solar panels become less efficient as temperatures increase. Cooler days will see higher energy production than hot days. Solar panels in hot climates may need to be cooled to operate at peak efficiency.

**Angle of the Sun** – The angle at which sunlight hits the solar panel affects production. When the sun is directly overhead at noon, the energy production will be maximized. Morning and evening sun angles will result in less energy. Proper solar panel tilt and orientation can maximize energy capture.

## Calculating kWh from Watts

kWh (kilowatt-hour) is a unit that measures energy, while watts measure power. To calculate how many kWh a solar panel generates, you need to multiply its power rating in watts by the number of hours you want to measure.

For example, if a solar panel is rated at 400 watts, and you want to know how much energy it produces in one hour, the calculation would be:

400 watts x 1 hour = 400 watt-hours

Since 1 kilowatt = 1000 watts, we can convert 400 watt-hours to kWh:

400 watt-hours x (1 kW / 1000 watts) = 0.4 kWh

Therefore, a 400 watt solar panel will produce 0.4 kWh in one hour. This simple calculation allows you to estimate the energy production of a solar panel over any time period, by multiplying its watt rating by the number of hours.

## Solar Irradiation Estimates

Solar irradiation is a measure of the amount of sunlight energy received at a location on the Earth’s surface. It is measured in peak sun hours, which refers to equivalent hours per day when solar irradiance is 1,000 watts per square meter.

Solar irradiation varies significantly by geographic location and season. Areas closer to the equator generally receive more sunlight. In the continental United States, the Southwest region has the highest average solar irradiation, with about 6 peak sun hours per day annually. The Pacific Northwest averages around 4 peak sun hours per day annually.

Seasonally, solar irradiation is highest in the summer months and lowest in the winter months. For example, Phoenix, AZ averages around 7 peak sun hours per day in June and around 5 peak sun hours per day in December. Portland, OR averages 5 peak sun hours in August and 2 in December.

Knowing the average peak sun hours for a location makes it possible to estimate the potential solar energy production. Photovoltaic solar panel power output is directly proportional to solar irradiation. With estimated peak sun hours, you can calculate approximate kilowatt-hours (kWh) generation per day based on solar panel wattage.

## kWh Estimate for 400W Panel

The key factor for estimating kWh production is the solar irradiation for a given location. This represents the amount of peak sun hours per day the panels will receive. The global average is around 4-5 peak sun hours per day.

With an average of 5 peak sun hours per day, a 400W solar panel can produce about 2000 Wh or 2 kWh per day (400W x 5 hours = 2000 Wh = 2 kWh). This assumes ideal conditions with the panels facing direct sunlight during all peak sunlight hours. The actual production may vary depending on panel orientation, shading, weather, and other factors. But in general, a 400W panel in an average location would produce approximately 2 kWh per day.

## Real-World Considerations

The maximum kWh estimate assumes perfect sun conditions year-round. However, factors like cloud cover and weather will reduce the real-world energy output of a solar panel below its maximum rating. On average, solar panels in the United States operate at around 75% of their rated capacity when accounting for real-world conditions. This is known as a “derate factor.”

To estimate the actual annual kWh production of a solar panel, the rated kWh can be multiplied by a derate factor, usually between 0.7 and 0.85 depending on location. For example, with a 0.75 derate factor, the 400W solar panel in this example would produce around 1,095 kWh per year (1,460 kWh x 0.75). The derate factor adjusts the maximum output to more closely reflect real-world energy production.

## Increasing Energy Production

There are a few ways to maximize the energy production from solar panels beyond just having an optimally-oriented fixed mount:

**Using tracking mounts to follow sun**

One method is to use a solar tracker mount so the panels move throughout the day to directly face the sun. This exposes the solar panels to more direct sunlight over more hours of the day. Dual-axis trackers that follow the sun’s movement across the sky and from east to west can increase daily energy production by 30-40% compared to fixed mounts. However, trackers add cost, maintenance, and points of failure.

**Combining panels into arrays**

Connecting solar panels together into an array allows the system to produce more total power. For example, combining two 400 watt panels into an 800 watt array will double the energy output. When space allows, adding more panels is an effective way to increase solar production from your system. Just be sure your inverter can handle the increased wattage.

## Conclusion

In summary, a 400W solar panel can generate up to 1.5kWh – 2kWh of electricity per day, under optimal conditions. The actual output of a solar panel in real-world installations depends on several factors, notably the strength and duration of sunlight at the installation location, ambient temperature, and the panel’s orientation to the sun. Assuming average sun hours per day of around 4-6 hours for many regions, a 400W solar panel could be expected to produce 1.6 – 2.4kWh per day. With estimated monthly energy generation between 48-72kWh, a single 400W solar panel can make a significant contribution to household electricity needs.

While solar panel output relies on environmental conditions, various strategies can maximize energy production. Careful selection of solar panel location, angle, and free airflow can enable a 400W panel to generate closer to the theoretical maximum. Overall, a 400W solar panel paired with batteries for energy storage has the capability to provide a meaningful amount of clean, renewable electricity.

## References

[1] National Renewable Energy Laboratory (NREL). “Photovoltaic System Performance Monitoring – Inclinometers.” https://www.nrel.gov/pv/system-performance.html

[2] EnergySage. “How many kilowatt hours (kWh) does a solar panel produce?” https://news.energysage.com/how-much-electricity-does-a-solar-panel-produce/

[3] Energy.gov. “Solar Performance and Efficiency.” https://www.energy.gov/eere/solar/solar-performance-and-efficiency

[4] PV Magazine. “How to calculate the annual energy output of PV systems.” https://www.pv-magazine.com/2018/02/20/how-to-calculate-the-annual-energy-output-of-pv-systems/

[5] PVWatts Calculator. https://pvwatts.nrel.gov/