How Many Solar Panels Do I Need?

Most US homes need somewhere between 15 and 25 solar panels to cover their full electricity use. The exact number comes down to three things: how much electricity you use, how much sun your roof gets, and how powerful each panel is. The good news is the math is straightforward, and our calculator does it automatically — but understanding the logic helps you evaluate quotes from installers and spot oversized or undersized recommendations.

The Quick Formula

You can estimate your panel count in four steps:

Step 1: Find your annual electricity usage. Check your utility bills or your utility’s online portal for your monthly kilowatt-hour (kWh) usage. Most utilities show a 12-month rolling average or a breakdown by month. The US national average is about 10,800 kWh/year (~900 kWh/month), but usage varies enormously — a small condo might use 4,000 kWh/year; a large house with an EV and pool might use 25,000 kWh/year.

Step 2: Find your state’s peak sun hours. Peak sun hours measure daily solar irradiance — essentially, how productive your location is for solar. Not the same as hours of daylight. The range in the US:

• Southwest (Arizona, Nevada, New Mexico): 5.5–6.5 peak sun hours/day
• California: 5.0–6.5 depending on region
• Southeast (Texas, Florida, Georgia): 4.5–5.5
• Midwest and Mid-Atlantic: 3.5–4.5
• Pacific Northwest, New England: 3.0–4.0

Our state pages show peak sun hours by state from NREL data.

Step 3: Calculate the system size you need.

System kW = Annual kWh ÷ (Peak sun hours × 365 days × 0.80)

The 0.80 (80%) is a performance ratio — it accounts for real-world losses from heat, wiring resistance, inverter inefficiency, dust, and other factors. Modern systems typically achieve 75–85%; 80% is a solid middle-ground assumption.

Step 4: Convert kilowatts to panel count. Divide the system’s total watts by the wattage of the individual panels. Modern residential panels are typically 370–450 watts each, with 400 W being a common reference point.

Panel count = (System kW × 1,000) ÷ Panel wattage

Skip the math: our solar calculator does all of this from your monthly bill and state — it sizes the system, estimates production, and calculates payback. Use it for a quick answer, and use this guide to understand what the calculator is doing.

A Worked Example

A home using 10,800 kWh/year in central North Carolina (about 4.8 peak sun hours):

• System size = 10,800 ÷ (4.8 × 365 × 0.80) ≈ 10,800 ÷ 1,401.6 ≈ 7.7 kW
• Panels at 400 W = 7,700 W ÷ 400 W ≈ 19–20 panels

The same home in Phoenix, Arizona (6.5 peak sun hours):

• System size = 10,800 ÷ (6.5 × 365 × 0.80) ≈ 10,800 ÷ 1,898 ≈ 5.7 kW
• Panels at 400 W = 5,700 W ÷ 400 W ≈ 14–15 panels

Same electricity usage, same panel size — but the Arizona home needs only about 75% as many panels because of better sun. More sun means a smaller, less expensive system covers the same load.

Panel Count by Usage and Sun Hours

Monthly usage	Low sun (~4 hrs)	Average sun (~5 hrs)	High sun (~6 hrs)
500 kWh/month	~16 panels	~13 panels	~11 panels
750 kWh/month	~24 panels	~19 panels	~16 panels
900 kWh/month	~28 panels	~23 panels	~19 panels
1,200 kWh/month	~37 panels	~30 panels	~25 panels
1,500 kWh/month	~47 panels	~37 panels	~31 panels

(Assumes 400 W panels and an 80% performance ratio. Round up — installers will fine-tune.)

Should You Size for 100% Offset?

Not necessarily. Here’s the nuance:

100% offset means the system is sized to produce as much energy as you use annually. This gives the most electricity savings and the largest potential net-metering credits, but also the highest upfront cost.

75–90% offset is worth considering if:

• Your roof space is limited
• Your net metering policy pays you less than full retail rate for exported power (like California’s NEM 3.0)
• You want to optimize payback speed rather than total savings
• You plan to add loads later (EV, pool, HVAC upgrade) and don’t want to oversize now

Over-sizing (110%+) rarely makes sense unless you know you’re adding an EV or other large load soon. Without net metering, excess production has low value.

Our calculator defaults to 100% offset but you can select 75%, 90%, or 50% to see how it changes the system size, cost, and payback.

What Changes Your Panel Count

Panel wattage. Higher-wattage panels (420–450 W vs. standard 400 W) produce more per panel. This matters when roof space is limited — you can get the same system output with fewer panels. Premium high-watt panels (SunPower, Panasonic) cost more per panel but may mean fewer panels total on a constrained roof.

Roof orientation and tilt. A south-facing roof at a 30° pitch captures the most sunlight in the continental US. East/west-facing roofs produce roughly 10–20% less per panel, meaning you’d need more panels for the same output. North-facing panels are typically avoided for production reasons.

Shading. Even partial shading from a chimney, dormer, tree, or neighboring building can significantly reduce a panel’s output — and in some configurations, affect nearby panels too. Installers assess shading with tools like Solar Pathfinder, Solargraf, or Google’s Project Sunroof data. Heavily shaded roofs may not be suitable for solar at all, or may require microinverters/optimizers to isolate the shading effect.

Temperature. Counterintuitively, solar panels are less efficient in extreme heat. Most panels lose 0.3–0.5% output per degree Celsius above 25°C. In a Phoenix summer with panels at 60°C+, you might see 15–20% efficiency reduction at peak heat. This is already reflected in the NREL sun-hour data our calculator uses, but it means the summer in very hot climates doesn’t produce quite as much as the raw sun hours suggest.

Performance ratio. The 80% default is conservative and correct for most systems. Extremely clean installations in mild climates can achieve 85%+. Old, dirty, or poorly maintained systems might be closer to 70%. Use 80% for planning.

Will the Panels Fit on My Roof?

A typical 400 W panel is about 17.5 square feet (roughly 3.3 × 5.3 ft). So:

• 15 panels = ~265 sq ft of panel area (plus spacing between panels)
• 20 panels = ~350 sq ft
• 25 panels = ~440 sq ft

You need that space in usable, unshaded, south-facing or southwest-facing sections. A 2,000 sq ft house typically has 800–1,200 sq ft of total roof area — but usable solar area is often 40–60% of that after excluding north-facing slopes, obstructions (vents, skylights, chimneys), and setback requirements.

Most single-family homes have sufficient roof space for a system that covers their entire usage. Problems arise with:

• Very small roofs (condos, townhouses, small bungalows)
• Complex hip roofs with many angles and ridges
• Heavily shaded roofs
• Homes with very high electricity use (1,500+ kWh/month) and limited south-facing exposure

If your roof is constrained, higher-wattage panels produce more per square foot and can help fit the system.

Ground-Mounted Systems

If your roof isn’t suitable, ground mounting is an option. A ground-mounted array sits on a steel structure in your yard, typically at the optimal angle facing south. Benefits:

• Can be placed anywhere on your property with good sun exposure
• Easier to clean and maintain
• Can be installed at the optimal tilt angle regardless of roof pitch

Downsides: costs more to install ($0.30–$0.60/W additional for the ground mount structure), requires sufficient open yard space, and may require setback permits.

Some ground-mounted systems include tracking mechanisms that follow the sun through the day, increasing production by 20–30% — but these cost significantly more and have moving parts that require maintenance.

How This Affects Your Cost in 2026

In 2026, system sizing directly affects your out-of-pocket cost more than it did in prior years, because the 30% federal tax credit that used to offset a third of the cost is now gone for residential buyers.

A 7 kW system at $3/W = $21,000 out of pocket (full price, no federal offset). A 5 kW system at $3/W = $15,000 out of pocket.

Every kW you can reduce through right-sizing — not over-building for theoretical future use — reduces a real dollar amount you’re paying today. This is another reason to model your usage accurately before getting quotes and to critically evaluate whether 100% offset is the right target for your situation.

Reading Your Quote’s Panel Count

When an installer gives you a quote, check the panel count against this formula independently. If they’re quoting you 30 panels when our formula suggests 18–20 for your usage and sun hours, ask why. Common explanations:

• They’re using lower-wattage panels (300 W instead of 400 W)
• They’re sizing for 100% offset on your peak month rather than annual average
• They’re accounting for shading losses with additional panels
• They’re over-sizing to give you headroom for an EV or future loads

All of those can be valid reasons — or they can be upsells. Understanding the baseline calculation lets you ask the right questions.

Bottom Line

Take your annual kWh, divide by your state’s peak sun hours times 292 (that’s 365 × 0.8), to get system kilowatts. Divide by 0.4 to get panel count at 400 W. For most US homes that lands between 15 and 25 panels, in the 5–9 kW range.

For a no-math answer tailored to your state’s real sun and electricity rate, run the solar savings calculator. Once you have a system size, read how much that system costs in 2026 and whether solar makes financial sense for your situation before talking to installers.