What you'll need
- Your average monthly electricity usage in kWh (from a recent utility bill)
- An idea of your region's peak sun hours (low, average, or high)
- The panel wattage you're comparing (350W, 400W, or 450W)
What you'll get
- Panel count — How many panels of your chosen wattage you'll need — the number itself
- Recommended system size — Sized in kilowatts (kW) from your usage and sun hours
- Roof area needed — Approximate square footage required
- Ballpark install cost — A rough range at this size — see the cost calculator for the full estimate
What this page answers
This calculator answers how many solar panels — and what size system — you need, from your monthly usage and local sun hours. It is not the tool for a final price: once you have a target size, price it with the solar panel installation cost calculator, which prices your system by size, panel type, and roof mounting. For the full list of solar tools, see Solar Calculators.
How many solar panels do I need? Examples by usage
| Monthly usage | System size | Panel count (400W panels) |
|---|---|---|
| 600 kWh/month | ~5.48 kW | 14 panels |
| 900 kWh/month | ~8.22 kW | 21 panels |
| 1,200 kWh/month | ~10.96 kW | 28 panels |
| 1,800 kWh/month | ~16.44 kW | 42 panels |
Panel counts assume 400W panels (round up to the next whole panel) at average peak sun hours (4.5 hrs/day) with an 80% system derate factor covering inverter, wiring, soiling, and temperature losses — the same disclosed formula used throughout this page. Fewer, higher-wattage panels (e.g. 450W) lower the count; more, lower-wattage panels (e.g. 350W) raise it. Your actual panel count depends on your usage, regional sun exposure, and chosen panel wattage — use the calculator above for a figure specific to your home and roof.
Recommended System Size by Monthly Usage
| Monthly Usage | Annual Usage | Recommended System Size |
|---|---|---|
| 600 kWh | 7,200 kWh | ~5.48 kW |
| 900 kWh | 10,800 kWh | ~8.22 kW |
| 1,200 kWh | 14,400 kWh | ~10.96 kW |
| 1,800 kWh | 21,600 kWh | ~16.44 kW |
Estimates assume average (4.5 hr) peak sun hours and an 80% system derate factor. A site assessment from a local installer will give the most accurate final system size.
How to size a solar system
The standard formula is system size (kW) = annual usage (kWh) ÷ (peak sun hours × 365 × ~0.8). The 0.8 is a "derate" that accounts for real-world losses (inverter, wiring, heat, soiling, and shading). For example, a home using 10,800 kWh/year in a 5-sun-hour region needs roughly a 7.4 kW system.
- Annual usage. Add up 12 months of kWh from your utility bills — the most important input.
- Peak sun hours. ~4 in the Northeast/Pacific NW, ~5 in the Midwest/mid-Atlantic, ~5.5–6+ in the Southwest.
- Panel wattage. At 400W per panel, a 7.4 kW system is about 18–19 panels.
Panel count & roof area
Each residential panel is roughly 18 sq ft, so plan on about that much unshaded, well-oriented roof per panel plus setbacks required by fire code. A 7–8 kW system (18–20 panels) typically needs ~350–400 sq ft of usable roof. South-facing is ideal; east/west works with a modest production penalty; heavy shade from trees or chimneys can rule out sections entirely.
Should you oversize?
- Plan for new loads. Adding an EV, heat pump, or pool later can raise usage 20–40% — size for it now if the roof and budget allow.
- Mind utility caps. Many utilities cap system size at ~100–120% of your historical usage for net metering, so overbuilding can be uneconomic.
- Battery vs. bigger array. Under net-billing rules, a battery to store your own power can beat oversizing for export.
Once you have a target size, price it with the Solar Panel Installation Cost Calculator.
System size is your annual usage divided by peak sun hours × 365 × a 0.80 derate factor for real-world losses. A home using 1,000 kWh/month at 4.5 peak sun hours needs a 9.13 kW system — 23 panels at 400 W each, about 414 sq ft of roof, and $22,825–$34,238 installed before any incentive. Fewer peak sun hours or lower-wattage panels both push every downstream number up.
How this calculator sizes a system
The core formula is: system size (kW) = annual usage (kWh) ÷ (peak sun hours × 365 × 0.80). Annual usage comes from your monthly usage × 12. Peak sun hours is a regional average — the number of hours per day sunlight arrives at an intensity equivalent to full solar noon, not the number of daylight hours, which is why a place with long summer days can still have a modest peak-sun-hours figure. The 0.80 derate factor accounts for real-world losses: inverter conversion, wiring resistance, soiling, and temperature effects between the panels' rated output and what the system actually delivers.
Why peak sun hours is the input that moves everything else
Because peak sun hours sits in the denominator, a lower value inflates the recommended system size, and every downstream figure — panel count, roof area, cost — scales with it directly. Holding the same 12,000 kWh annual usage constant: at a lower 3.5 peak-sun-hours region the system grows to 11.74 kW, while at a higher 5.75 peak-sun-hours region it shrinks to 7.15 kW — a 64% swing in recommended size driven by location alone, before a single input about the house itself changes.
This is the calculation's most common failure point when someone reuses a system-size figure from a friend or a different region: the same electricity usage in a lower-sun-hours climate genuinely needs more panels, not the same count installed less efficiently. Getting an accurate peak-sun-hours number for your specific location, rather than a rough regional guess, is worth more to the accuracy of this result than precision on any other input.
Panel wattage: fewer panels, same roof, same output
Panel wattage changes panel count and roof area without changing the system's kW size or its cost — cost here is calculated per watt of system size, not per panel, so swapping panel wattage is purely a packing decision. Using the same 9,130 W system from above: at 400 W panels that's 23 panels needing 414 sq ft; at 450 W panels it drops to 21 panels needing 378 sq ft — two fewer panels and 36 fewer square feet of roof for the identical system size and identical price range.
| Panel wattage | Panel count (⌈watts ÷ wattage⌉) | Roof area (panels × 18 sq ft) |
|---|---|---|
| 350 W | 27 | 486 sq ft |
| 400 W | 23 | 414 sq ft |
| 450 W | 21 | 378 sq ft |
System size and installed cost range ($22,825–$34,238) are identical across all three rows — this calculator prices by system watts, not panel count.
Higher-wattage panels are worth prioritizing specifically when roof space, not budget, is the binding constraint — a smaller usable roof area may only fit a given system size with the higher-wattage option.
What a usage-based size doesn't account for
This calculator sizes a system to match your historical usage — it does not know about shading from trees or neighboring structures, roof orientation and tilt relative to true south, or future usage increases like an EV or a home addition, all of which a local installer's site assessment would catch and this form cannot. It also doesn't include any federal tax credit in its cost range, since Section 25D expired for property placed in service after December 31, 2025 — the $22,825–$34,238 figure above is a pre-incentive number for a 2026 installation.
Sizing to exactly 100% of current usage is also a choice, not a requirement. Many homeowners size somewhat below 100% to reduce upfront cost, or somewhat above it to leave headroom for an EV charger or an addition — your utility's net metering policy determines how much value, if any, production beyond your own usage actually returns.
Checking an installer's proposed size against this one
If an installer's proposal lands meaningfully above or below the size this calculator recommends, the gap usually traces back to one of the same three inputs. A larger proposed system often means the installer used your actual annual usage from utility bills rather than an estimated monthly figure, or priced in future usage you mentioned like an EV. A smaller one often means a site visit found better sun exposure — a higher effective peak-sun-hours figure — than the regional average this calculator defaults to, or a different derate assumption for a newer, more efficient inverter.
It is worth asking directly which of the three inputs — usage, sun hours, or derate factor — an installer's proposal is built on, since a size disagreement that traces to real site data (an actual bill, a shading study) is more trustworthy than one that traces to a different default assumption.
Methodology
System size, panel count, roof area, and the per-watt cost range are the literal formula and constants in this calculator's own module: system kW = annual kWh ÷ (sun hours × 365 × 0.80 derate); panel count = ⌈watts ÷ panel wattage⌉; roof area = panel count × 18 sq ft; cost = watts × $2.50 to watts × $3.75. The federal-credit exclusion follows the IRS residential credit expiry below. No number here is estimated beyond running the module's own arithmetic.
Sources
- IRS — Residential Clean Energy Credit (Section 25D) — accessed 2026-09-05
Frequently asked questions
How do I calculate the solar system size I need?
Take your annual electricity usage in kWh and divide it by your area's peak sun hours per day, multiplied by 365 days and a system derate factor (typically 0.80 to account for inverter, wiring, and soiling losses). The result is your recommended system size in kilowatts (kW). This calculator does that math for you automatically.
How many solar panels do I need for my house?
Once you know your system size in watts, divide it by the wattage of the panel you're considering. For example, a 7,200-watt system using 400W panels needs 18 panels, while the same system using 450W panels needs only 16. Higher-wattage panels reduce the total panel count for the same output.
How much roof space do solar panels need?
A standard residential solar panel takes up roughly 18 square feet, including spacing for mounting hardware. A typical 20-panel system needs about 360 square feet of usable, unshaded roof area. Panel count and roof area both scale with your system size and chosen panel wattage.
Does shading affect how many panels I need?
Yes. Shading from trees, chimneys, or neighboring structures can significantly reduce a panel's output, sometimes requiring a larger system or microinverters/power optimizers to compensate. This calculator provides a baseline estimate and does not account for site-specific shading — a local installer can assess your roof for actual sun exposure.
Should I size my system for 100% of my usage?
Many homeowners size their system to offset 80-100% of their annual usage, since exceeding 100% often provides diminishing financial returns depending on your utility's net metering policies. Some choose a smaller system to reduce upfront cost, while others size up to cover future usage like an EV or home addition. Your ideal size depends on your budget, roof space, and local utility rules.