Enter your energy use and peak sun hours below, then choose a panel type. Formula: Array Size = Energy Use ÷ Peak Sun Hours ÷ System Efficiency, plus a safety margin.
Your Energy Profile
Not sure what a field means? Tap the ⓘ next to its label.
Panel Type & Wattage
Choose a preset to auto-fill a typical wattage and module efficiency, or select Custom to enter your own panel's exact spec.
Multiple silicon fragments melted together. Lower cost per panel, lower efficiency (~15–17%), larger footprint per watt. Good when space isn't the limiting factor.
Single-crystal silicon. Higher efficiency (~19–21%) and smaller footprint than poly, at a moderate premium. The most common residential choice today.
Advanced high-efficiency cell technology (~21–23%). Best when roof or ground space is tight, or you want maximum output per square foot, at the highest per-panel price.
Your Results
This chart shows how many panels your recommended array size would need at common residential and commercial panel wattages. If your entered panel wattage matches one of the bars, that bar is highlighted red — useful for weighing panel count against physical footprint before you shop.
Results are for reference and estimation only. Verify all values before use in any solar system design or purchase. See full disclaimer below.
How to Use the Solar Panel Size Calculator
Solar Panel Sizing Guide
A deeper look at what goes into a correctly sized solar array, and how to choose between panel types — written for first-time buyers and job-site reference alike.
Peak Sun Hours — Not the Same as Daylight
A peak sun hour is the equivalent number of hours per day that sunlight arrives at a standard intensity of 1,000 W/m². A location can have 12 hours of daylight but only 4.5 peak sun hours, because morning and evening sun is weaker than midday sun. Always use peak sun hours, not total daylight hours, when sizing an array — using daylight hours will significantly undersize your system.
System Efficiency (Derate Factor)
No solar system delivers its full nameplate wattage in the field. Wiring resistance, inverter conversion losses, temperature effects (panels lose output as they heat up), dust and soiling, and connector losses all reduce real-world output below the panel's rated Standard Test Conditions (STC) value. An 85% system efficiency is a common, conservative planning assumption; hotter climates or longer wire runs may warrant a lower figure.
Why Add a Safety Margin
A safety margin (commonly 20–25%) sits on top of the derate factor and accounts for panel degradation over the system's lifespan (typically 0.3–0.5% output loss per year), unusually cloudy stretches, and future increases in your energy use. Sizing exactly to today's minimum need often leaves no headroom once the panels have aged a few years.
Polycrystalline vs Monocrystalline — The Real Trade-off
Polycrystalline cells are cut from multiple silicon fragments melted together, giving them a distinctive blue, speckled look, lower manufacturing cost, and efficiency typically in the 15–17% range. Monocrystalline cells are cut from a single silicon crystal, giving them a uniform black appearance, higher manufacturing cost, and efficiency typically in the 19–23% range depending on cell technology. Because efficiency determines how much power a panel produces per square foot, monocrystalline panels need less roof or ground space for the same array size — the deciding factor when space is limited. When space is not a constraint, polycrystalline can be the more budget-friendly choice for the same total wattage.
Common Sizing Mistakes
The most frequent errors are: using total daylight hours instead of peak sun hours (oversizing the assumed sunlight and undersizing the array), skipping the system efficiency and safety margin entirely (leaving no headroom for real-world losses or panel aging), and sizing the panel array without checking that a paired battery bank can actually be recharged within a single sunny day. Always size panels and batteries together — use the Solar Battery Storage Calculator alongside this tool to confirm both numbers are compatible.
This guide is provided for educational and reference purposes. Always confirm system specifications with a licensed solar installer before purchasing or installing equipment.
Panel Count Quick Reference Table
Common daily energy-use levels mapped to an approximate panel count and array size, assuming 400W panels, 5 peak sun hours, 85% system efficiency, and a 25% safety margin. Use this to sanity-check a calculated result at a glance — your own inputs above will differ from these defaults.
| Daily Use (kWh) | Panels (400W) | Array Size | Typical Household |
|---|---|---|---|
| 10 | 8 | 3.2 kW | Small apartment, minimal AC |
| 15 | 12 | 4.8 kW | Small home, gas heat |
| 20 | 15 | 6.0 kW | Average home, moderate AC |
| 25 | 19 | 7.6 kW | Larger home, central AC |
| 30 | 23 | 9.2 kW | Home with EV charging |
| 40 | 30 | 12.0 kW | Large home, electric heat |
| 50 | 37 | 14.8 kW | Large home, pool, EV charging |