How To Estimate Your Solar Panel Needs: An In-Depth Guide

While it’s great to have a solar power system installed successfully, it is important to first calculate household consumption. This will be the target for calculating solar panel output. This will help you determine how many and what type of panels you need for your home or business. Why is this important?

Systems that are too large typically generate excess energy that won’t be of much value to you – except perhaps in the form of solar credits that may offset energy costs. A system that’s too small, however, will translate to large energy costs, meaning your solar panels have not been optimized to your specific needs.

Making an accurate estimation will help you maximize your solar investment by investing in just the right amount of panels you need for your solar panel installation. The average energy usage of a 2500 square foot home in the US is approximately 11,000 kWh per year. This figure will vary depending on the actual size of your home, your typical energy consumption, and your location.

Generally, you need the following information to determine how many solar panels you will need:

Your current energy usage in kilowatt-hours (kWh)

Future electrical appliance additions

The climate in your location

The amount of sunlight your home or business is exposed to

Your solar panel size in wattage.

Calculating solar panel output and determining the average size of solar panels needed for your property is best left to a professional solar provider. However, you must also understand the principles behind solar power, solar panel output calculation, and how a solar panel works in general.

person making solar panel calculations

Solar Panels and How They Generate Solar Power

Solar panels comprise varying configurations of small photovoltaic cells typically made of semi-conductive materials like silicon, cadmium, tellurium, glass, and metal. These cells convert sunlight into electricity by creating an electric field when sunlight hits the panels. The electricity then travels through a conductive wire to an inverter, where it is transformed from DC to AC.

The AC electricity is then transported to the property’s breaker box or electric panel, which in turn distributes it throughout the home or business, as needed. Surplus electricity generated by the panels flows through a meter and goes into the utility electrical grid, serving as solar credits.

With the passing of California’s Net Energy Metering (NEM) 3.0 legislation, homeowners will likely need to add a battery to their solar systems to avoid selling energy to the electrical grid. Even with federal solar tax credits at 30% until 2032, NEM 3.0’s implications on Californians’ energy bill savings still give homeowners in the area more reason to go solar sooner to get grandfathered into NEM 2.0.

How to Calculate Solar Panel Output

To calculate the amount of solar power you need, you must first determine your property’s average energy requirements. You can do this by referring to previous utility bills. Analyzing 12 months' worth of utility bills is generally a good rule of thumb, as these will give you a rough estimate of your energy requirements. You should also consider future electric appliance additions, such as new HVAC, Electric Vehicle, and the like.

Then, calculate the required monthly solar panel output by dividing your total energy consumption by 12. This number is an estimate of how much energy you’ll need your solar panels to generate every month. 

You’ll also have to take your location into account when determining how many panels you’ll need and whether you’ll need a backup solar battery system. For instance, residents in states with longer winters will likely need more solar panels to generate the same amount of power as those in sunnier and warmer areas.

Knowing how many peak hours of sunlight your location receives is also crucial, as this will factor in while calculating solar panel output. Californians, for example, enjoy 5.2 average daily peak sun hours compared to those in Alaska, who only receive roughly 2.5 hours of peak sun.

man holding a small solar panel

How to Calculate the Required Number of Solar Panels

To calculate solar panel size as well as the total number of panels required, you’ll need to first find the theoretical solar power output by dividing daily kWh (based on previous energy bills) by your location’s irradiance value (peak sun hours). Average house sizes vary from state to state, but overall, the average house size in the US is around 2,500 square feet. This roughly translates to an annual energy usage of 11,000 kWh per year, or 30 kWh daily. This is equivalent to 30,000 watt-hours.

Let’s use average values for the US in the sample solar panel output calculation:

Theoretical solar power output = 30,000 watt-hours / 5.2 peak sun hours 

Theoretical solar power output = 5,769 watts

Based on this computation, the average solar panel output for a standard US home is 5,769 watts. Once you’ve acquired the theoretical solar production needed for your property, you’ll need to adjust the theoretical solar kWh by a loss factor of 1.4 and divide the value by the individual solar panel’s watts rating. 

The average size of a solar panel is around 400 watts, so to acquire the number of solar panels needed, divide the above value (theoretical solar power output) by solar panel size in watts:

5,769 watts / 400 watts = 14.42

Rounded down = 14 solar panels 

Taking into account the standard percentage of losses for all photovoltaic systems (23%), we can get the value of adjusted solar output by multiplying the solar panel’s average output by the average loss factor of 1.4:

Adjusted solar output = 5,769 watts x 1.4 loss factor 

Adjusted solar output = 8,076 watts (or 8.076 kW)

Using the watt value for standard solar panel size and the adjusted solar output value, we can finally compute the actual number of solar panels needed:

Actual number of solar panels needed = 8,076 watts / 400 watts 

Actual number of solar panels needed = 20 solar panels

In terms of dimension, the standard size of a solar panel is around 5 feet by 3 feet for residential systems and 6.5 feet by 3 feet for commercial systems. Depending on the brand, the panels may range between 40 to 50 pounds each. While the values may vary from brand to brand, solar panels remain standard in size and weight.

solar panels on the roof

What Affects Solar Panel Output Efficiency?

The output efficiency of solar panels is affected by many different factors, such as:

The quality of the materials used in the panel construction: High-quality materials such as monocrystalline silicon or gallium arsenide are more efficient than lower-quality options like polycrystalline silicon.

The age of the panel: Solar panels will lose efficiency over time as they accumulate dirt, dust, and other debris that prevents sunlight from reaching the solar cells. So it's essential to keep them clean and well-maintained for optimal performance.

The location of your property: If your home or office is located in a place with a hot and humid environment, the solar panels may lose efficiency as the heat can damage the materials used to build them.  On the other hand, solar panels installed in locations with colder climates will generally have higher output efficiency since the cold temperature improves panel performance.

To get an estimate of the number of panels you’ll need, as well as the size and type of panels required to power your property, book a solar consultation with AMECO or use our free Estimate Calculator.