Solar Calculator

Q: What is the difference between grid-tied and off-grid solar?

A grid-tied system connects to your utility power grid. Extra energy you make goes to the grid, and you pull power from the grid at night. An off-grid system uses batteries to store power and is not connected to the grid at all. Grid-tied systems cost less and don't need batteries. Off-grid systems work for cabins, RVs, or places with no grid access.

Q: What system size do I need for my home?

Click the Estimate from my bill button after entering your monthly electric bill. The calculator turns your bill into yearly usage at the U.S. average residential electricity price, which was 18.19 cents per kWh for January through July 2026. 6 It then divides that usage by your local solar resource to suggest a system size in kilowatts. In 2022, the average U.S. residential electricity customer bought 10,791 kWh. 3 For its analyses, NREL uses an average home solar system size of 7.15 kilowatts, within a 3 to 11 kilowatt range. 2

Solar Calculator

Introduction

This free solar calculator helps you figure out how much energy a solar panel system can make for your home or business. It works for two types of setups: grid-tied solar systems that connect to your local power grid and off-grid solar systems that run on batteries alone.

In the grid-tied mode, you enter your monthly electric bill, your location, and a few details about your panels. The calculator then estimates your yearly energy output in kilowatt-hours, your cost savings, and your environmental impact. It also shows monthly production numbers, a step-by-step breakdown of the math, and key performance metrics like capacity factor and specific yield.

In the off-grid mode, you enter your daily power needs and battery specs. The tool tells you exactly how many solar panels and batteries you need, how to wire them, and gives you a visual diagram of your panel layout.

All results update instantly as you change your inputs. Whether you want to size a rooftop solar system, compare panel types, or plan a fully independent off-grid setup, this calculator gives you clear estimates from standard solar-geometry formulas and the sky clearness you choose.

How to Use Our Solar Calculator

Enter details about your location, electricity use, and solar setup below. The calculator will estimate your annual energy output, system size, battery needs, and environmental impact. Choose Grid-Tied Solar if your home or business is connected to the power grid, or Off-Grid Sizing if you need a standalone system with batteries.

Grid-Tied Solar Mode

Output Units: Pick US Customary or Metric to change how results like CO₂ and distance are shown.

Property Type: Select Residential for homes, Commercial for businesses, or Non-Profit for churches, schools, and similar buildings.

Average Monthly Electricity Bill: Type in the dollar amount you usually pay each month for electricity. This helps estimate how large your solar system should be.

Location Preset: Pick a city close to you from the dropdown list. This fills in your latitude and sky clearness automatically. Choose "Custom" if your city is not listed.

Latitude: Enter your location's latitude in degrees. This is filled in when you pick a location preset, but you can type your own value.

Longitude: Enter your location's longitude in degrees. It is kept with your inputs but does not change the estimate, which depends on latitude and sky clearness.

Local Sky Clearness: Choose how sunny your area typically is. Pick "Very Sunny" for desert climates and "Cloudy" for rainy or overcast regions.

System Size (DC kW): Enter the size of your solar panel system in kilowatts. Click Estimate from my bill to have the calculator suggest a size based on your electricity bill. For a more detailed look at individual panel sizing and specifications, you can also try our solar panel calculator.

Module Type: Pick the kind of solar panel. Standard is most common. Premium panels produce more power. Thin Film panels cost less but produce less.

Array Type: Choose how your panels are mounted. Roof Mount is the most common for homes. Tracking systems follow the sun and produce more energy.

Tilt: Enter the angle of your panels in degrees. 0° is flat and 90° is vertical. Click Suggest Optimal Tilt & Azimuth to get the best angle for your latitude. For the highest yearly output, panels in the Northern Hemisphere are pointed due south and tilted at an angle equal to the local latitude.¹

Azimuth: Enter the direction your panels face in degrees; 180° is due south. For the highest yearly output, panels in the Northern Hemisphere face due south.¹

System Losses: Enter the total percentage of energy lost to wiring, shading, dirt, and other factors. Click Break down losses to set each loss type on its own. Wiring losses in particular depend on cable length and gauge. Our voltage drop calculator can help you size conductors to minimize those losses.

Soiling Loss: Enter the percentage of energy lost due to dirt, dust, and debris on your panels.

DC to AC Size Ratio: Enter the ratio of your panel wattage to your inverter wattage. A typical value is 1.10 to 1.25.

Inverter Efficiency: Enter the efficiency of your inverter as a percentage. Most modern inverters are between 95% and 98%.

Albedo: Enter how much light the ground near your panels reflects. Grass is about 0.2, concrete is about 0.3, and snow is about 0.8.

Ground Coverage Ratio: This field appears when you select a tracking array type. Enter the ratio of panel area to total ground area.

Bifacial Panels: Turn this on if your panels collect sunlight from both sides. Then enter the extra gain percentage, which is typically 5% to 12%.

Off-Grid Sizing Mode

Total DC Load: Enter the total wattage of all devices and equipment you plan to power.

Hours Equipment Runs Per Day: Enter how many hours per day your equipment will run, from 0 to 24.

System Voltage: Pick the voltage for your battery bank. Common choices are 12V for small systems, 24V for mid-size, and 48V for larger setups.

Days of Backup Power Required: Enter how many days you want your batteries to last without any sun.

Battery Amp Rating: Enter the amp-hour (Ah) rating of one battery. This is usually printed on the battery label.

Region Preset: Pick the region closest to you. This fills in the peak sun hours value. Choose "Custom" to type your own.

Peak Sun Hours Per Day: Enter the average number of peak sun hours your location gets each day. This is set automatically when you pick a region.

Solar Panel Watt Rating: Enter the wattage of one solar panel you plan to use. The calculator will tell you how many panels you need in total.

Solar Panel Calculator: Grid-Tied & Off-Grid System Sizing

A solar panel system turns sunlight into electricity for your home or business. This solar calculator helps you figure out how much energy a solar system can produce and how many panels you need. It works for two types of setups: grid-tied and off-grid.

Grid-Tied Solar Systems

A grid-tied system stays connected to your local power grid. When your panels make more electricity than you use, the extra power goes back to the grid. When the sun is down, you pull power from the grid as normal. Most homes and businesses use this type of system because it costs less and does not need batteries.²

The grid-tied side of this calculator estimates how many kilowatt-hours (kWh) your system will produce each year. It uses your location, roof angle, panel type, and system size to run the math. It also shows your capacity factor, specific yield, and performance ratio, three numbers that tell you how well your system performs. You can even see how much CO₂ your system offsets each year.

Off-Grid Solar Systems

An off-grid system is not connected to the power grid at all. It stores energy in batteries so you have power at night or on cloudy days. This setup is common for cabins, RVs, and remote buildings where grid power is not available.

The off-grid side of this calculator tells you exactly how many batteries and solar panels you need. You enter your daily power usage, pick your battery size, and choose how many backup days you want. The calculator then sizes your full system, including how to wire panels in series and parallel.

Key Terms to Know

Peak Sun Hours (PSH) is the number of hours per day that sunlight is strong enough for full panel output. A location with 5 peak sun hours gets strong, direct sunlight for about 5 hours a day. Sunnier areas like Arizona have more peak sun hours than cloudy areas like Seattle.

System losses are the small amounts of energy lost to things like dirty panels, wiring, shading, and inverter conversion. Every solar system has some loss. A typical total loss is around 14%, meaning your panels deliver about 86% of their rated power in real-world conditions.

Depth of Discharge (DoD) is how much of a battery's stored energy you actually use. Most lead-acid batteries should only be drained to 50% to last longer. This calculator uses a 50% DoD to protect battery life.

Formulas used

Plane-of-Array Irradiance (Isotropic Tilt Model)

H_{POA} = H_b \, R_b \cos\gamma + H_d \frac{1+\cos\beta}{2} + GHI \cdot \rho \frac{1-\cos\beta}{2}

Annual AC Energy Output

E_{AC} = P_{dc} \times H_{POA} \times D

Combined Derate Factor

D = G_{array} \times M_{mod} \times B_{bifacial} \times (1 - L) \times \eta_{inv} \times C_{clip}

Capacity Factor

CF = \frac{E_{AC}}{P_{dc} \times 8760} \times 100\%

Off-Grid Corrected Daily Demand

E_c = P_{load} \times h \times 1.2

Off-Grid Required Battery Amp-Hours

Ah_{req} = \frac{\frac{E_c}{V_{sys}} \times d}{DoD}

Off-Grid Total Solar Panels Required

N_{panels} = \left\lceil \frac{E_c}{P_{panel} \times \frac{PSH}{1.55} \times N_{ser}} \right\rceil \times N_{ser}

Frequently asked questions

What is the difference between grid-tied and off-grid solar?

A grid-tied system connects to your utility power grid. Extra energy you make goes to the grid, and you pull power from the grid at night. An off-grid system uses batteries to store power and is not connected to the grid at all. Grid-tied systems cost less and don't need batteries. Off-grid systems work for cabins, RVs, or places with no grid access.

What system size do I need for my home?

Click the Estimate from my bill button after entering your monthly electric bill. The calculator turns your bill into yearly usage at the U.S. average residential electricity price, which was 18.19 cents per kWh for January through July 2026.⁶ It then divides that usage by your local solar resource to suggest a system size in kilowatts. In 2022, the average U.S. residential electricity customer bought 10,791 kWh.³ For its analyses, NREL uses an average home solar system size of 7.15 kilowatts, within a 3 to 11 kilowatt range.²

What does the clearness index mean?

The clearness index measures how much sunlight passes through the atmosphere to reach the ground. A value of 0.62 means very sunny, clear skies like a desert. A value of 0.40 means cloudy or coastal weather. Pick the option that best matches your area's typical weather.

What tilt angle should I set my panels to?

For the highest yearly output, panels in the Northern Hemisphere are pointed due south and tilted at an angle equal to the local latitude.¹ Click the Suggest Optimal Tilt & Azimuth button and the calculator will set this for you. If your panels are on a roof, use the angle your roof already has. Solar panels typically perform best on south-facing roofs with a slope between 15 and 40 degrees.²

Why is azimuth set to 180 degrees?

An azimuth of 180° means the panels face due south. In the Northern Hemisphere, south-facing panels get the most sunlight over the course of a day.¹ If you live in the Southern Hemisphere, the calculator will suggest 0° (due north) when you click the optimize button.

What are system losses and why do they matter?

System losses are small amounts of energy lost to things like dirty panels, wiring resistance, shading, snow, and inverter conversion. The default value is 14.5%, which is typical. You can click Break down losses to adjust each category on its own. Lower losses mean more energy reaches your home.

What is the DC to AC size ratio?

This is the ratio of your total panel wattage (DC) to your inverter capacity (AC). A ratio of 1.2 means you have 20% more panel power than your inverter can handle at once. This is normal and helps the inverter run at full power more often during the day. Most installers use a ratio between 1.10 and 1.25.

What are bifacial solar panels?

Bifacial panels collect sunlight from both the front and the back side. Light that bounces off the ground or roof hits the back of the panel and makes extra electricity. The gain is usually 5% to 12% more output. Turn on the Bifacial Panels toggle if you plan to use this type of panel.

What is albedo and how does it affect my results?

Albedo is how much light the ground reflects back up toward your panels. A white roof or snow reflects a lot of light (0.6 to 0.8), while dark grass reflects very little (0.2). Higher albedo means slightly more energy hits your panels, especially at steeper tilt angles.

Why is a 1.2 battery loss factor applied?

Batteries lose some energy during charging and discharging. The 1.2 factor adds 20% extra capacity to make sure you actually get the energy you need after these real-world losses. Without it, your battery bank would be slightly undersized.

What does 50% depth of discharge mean?

Depth of discharge (DoD) is how much of a battery's total energy you use before recharging. At 50% DoD, you only drain the battery halfway. This makes lead-acid batteries last much longer. The calculator uses 50% DoD as a safe default for all off-grid sizing.

What are peak sun hours?

Peak sun hours (PSH) is the number of hours per day when sunlight is strong enough to equal full-power output from your panels. It is not the same as total daylight hours. Phoenix, AZ gets about 6 peak sun hours, while Seattle, WA gets about 3.5. The calculator uses this number to size your solar array.

Why are peak sun hours divided by 1.55?

The 1.55 weather derating factor accounts for cloudy days, rain, and seasonal changes that reduce actual sunlight below the average. Dividing by 1.55 makes the off-grid system large enough to keep your batteries charged even during bad weather stretches.

What is capacity factor?

Capacity factor is the ratio of the energy a system actually produces over a period to the energy it could have produced at continuous full power over the same period.⁴ A 7 kW system could theoretically make 61,320 kWh in a year if it ran at full power 24/7. If it actually makes 10,700 kWh, its capacity factor is about 17.4%. Solar systems typically have capacity factors between 15% and 25%.

What is specific yield?

Specific yield is the total energy produced per kilowatt of installed panel capacity. It is measured in kWh per kWp per year. A specific yield of 1,500 means each kilowatt of panels produces 1,500 kWh in a year. Higher numbers mean your location and setup are more productive.

What is performance ratio?

Performance ratio (PR) compares your actual energy output to what the panels would produce if there were no losses at all. A PR of 80% means you keep 80% of the available solar energy after all losses. It helps you judge how well your system is designed regardless of location.

How is CO₂ offset calculated?

The calculator multiplies your annual energy output in kWh by 6.72 × 10⁻⁴ metric tons of CO₂ per kWh, the EPA's U.S. national marginal emission factor, which EPA uses to convert electricity avoided by renewable energy into avoided CO₂.⁵ For the distance equivalent it uses 3.93 × 10⁻⁴ metric tons of CO₂e per mile for a typical passenger vehicle, and for trees 0.060 metric tons of CO₂ per urban tree planted, per year.⁵ The result shows how much CO₂ your solar system prevents from entering the atmosphere by replacing grid electricity.

Should I pick 12V, 24V, or 48V for my off-grid system?

Use 12V for very small systems under 1,000 watts. Use 24V for mid-size systems between 1,000 and 3,000 watts. Use 48V for larger systems above 3,000 watts. Higher voltages reduce the current in your wires, which means less energy loss and thinner cables.

What module type should I choose?

Standard panels are monocrystalline and the most common choice. Premium panels have higher efficiency and a lower temperature coefficient, so they make more power in the same space. Thin Film panels cost less but also produce less energy per square foot. Most homeowners choose Standard or Premium.

How many backup days should I plan for off-grid?

Most off-grid systems plan for 2 to 3 days of backup. This covers a typical stretch of cloudy weather. If you live in an area with long cloudy periods or harsh winters, consider 4 to 5 days. More backup days means more batteries and higher cost.

What is ground coverage ratio?

Ground coverage ratio (GCR) is the area of your panels divided by the total ground area they sit on. It only appears when you select a tracking array type. A lower GCR means more space between rows, which reduces shading between panels. A typical value is 0.3 to 0.5.

Sources

Solar Photovoltaic System Design Basics. U.S. Department of Energy, Solar Energy Technologies Office. Mounting Structures; Inverters. Accessed September 29, 2026.

Homeowner's Guide to Going Solar. U.S. Department of Energy, Solar Energy Technologies Office. Accessed September 29, 2026.

How much electricity does an American home use? U.S. Energy Information Administration. FAQ 97. Accessed September 29, 2026.

Glossary: Capacity factor. U.S. Energy Information Administration. Accessed September 29, 2026.

Greenhouse Gas Equivalencies Calculator - Calculations and References. U.S. Environmental Protection Agency. Electricity reductions (kilowatt-hours); Miles driven; Urban tree seedlings. Accessed September 29, 2026.

Electric Power Monthly, Table 5.6.B: Average Price of Electricity to Ultimate Customers by End-Use Sector, by State, Year-to-Date through July 2026 and 2025. U.S. Energy Information Administration. 2026;U.S. Total, Residential. Accessed September 29, 2026.

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