Solar Array Size Calculator

A solar panel is a single unit containing photovoltaic cells that converts sunlight into electricity, typically producing 250W to 550W. A solar array is a collection of multiple solar panels connected together to form a complete power system. Arrays are sized in kilowatts (kW) and range from 3 kW for small homes to hundreds of kilowatts for commercial buildings. The terms are often used interchangeably, but technically a solar system always consists of multiple panels forming an array.

The right solar array size depends on your monthly electricity consumption, local peak sun hours and system efficiency. A typical US home using 900 kWh per month with 5 peak sun hours needs approximately a 7.5 kW array. Homes in sunnier climates like Arizona need smaller arrays for the same energy output, while homes in cloudy regions like the Pacific Northwest or UK need larger arrays. Use this calculator to find your exact recommended size based on your specific inputs.

The number of panels in a solar array depends on the total array capacity and individual panel wattage. A 7.5 kW array using 400W panels needs 19 panels, while the same array using 300W panels needs 25 panels. Choosing higher wattage panels reduces the panel count and roof space required for the same total array output. Most residential arrays contain between 10 and 40 panels depending on home size and energy needs.

A standard 400W solar panel requires approximately 22 square feet of roof space including mounting clearances. A 7.5 kW array of 19 panels needs roughly 418 square feet of unshaded roof area. Higher wattage panels are more space-efficient: a 500W panel requires about 27.5 square feet but fewer panels are needed overall. Most residential roof areas of 1,500 to 3,000 square feet can accommodate a complete solar array if the pitch, orientation and shading conditions are suitable.

System efficiency accounts for all energy losses between panel output and usable electricity at your appliances. These losses include inverter conversion losses of 3%–8%, DC wiring resistance of 1%–3%, panel temperature derating in hot weather of 5%–10%, soiling and dust accumulation of 2%–5% and mismatch losses between panels of 1%–2%. A combined efficiency of 75%–85% is typical for residential arrays. Lower efficiency means you need a larger array to meet the same energy demand.

Residential solar array costs range from $2.50 to $3.50 per watt fully installed in the US. A 7.5 kW array costs approximately $18,750 to $26,250 before incentives, or $13,125 to $18,375 after the 30% federal Investment Tax Credit. Array cost varies by location, panel brand, inverter type, roof complexity and installer pricing. Ground-mount and flat-roof arrays may cost 10%–20% more than standard pitched-roof installations due to additional mounting hardware.

The average payback period for a residential solar array in the US is 7 to 12 years after the 30% federal tax credit. Homes in high-electricity-rate states like California, Hawaii and Massachusetts often see payback in 5 to 8 years. Homes in low-rate states or with limited sun hours may see payback of 10 to 15 years. After payback, the array continues generating free electricity for 15 to 25 additional years, delivering significant long-term savings.

A single array places all panels on one roof face, ideally south-facing in the Northern Hemisphere, to maximize uniform production throughout the day. A dual array splits panels across two roof faces, typically southeast and southwest, to extend morning and afternoon generation and produce a flatter daily power curve. Dual arrays are useful when roof size limits single-face capacity or when a battery storage system benefits from more spread-out charging. Total annual energy is similar for both configurations with the same total panel count.

Yes, most solar systems can be expanded later by adding more panels, provided the inverter has spare capacity and the roof has additional space. String inverters are typically sized close to the original array capacity, so expansion may require a second inverter or upgrading to a larger unit. Microinverter and power optimizer systems are easiest to expand because each panel operates independently. Planning for future expansion by oversizing the inverter slightly at installation is a cost-effective approach for growing energy needs.

A solar array continues producing electricity on cloudy days, typically at 10%–25% of its rated capacity. Output drops proportionally with reduced sunlight intensity. During grid power outages, a standard grid-tied array automatically shuts down for safety to protect utility workers. To maintain power during outages, the array must be paired with a battery storage system and a hybrid or off-grid inverter. Adding battery storage allows the array to charge batteries during the day and power the home through outages or at night.