SettingsSettings for this calculationUS
The amount of energy (in kWh) you want the array to produce per day, on average.
Check a recent electric bill and divide monthly kWh by roughly 30 to estimate your daily usage.
The rated output of a single panel under standard test conditions.
Common residential panels are rated 350-450 watts each.
Your location's average 'peak sun hours' per day, not total daylight hours.
This is a standardized solar resource metric — sunnier regions average 5-6+ peak sun hours, cloudier regions average 3-4.
Accounts for inverter losses, wiring losses, panel soiling, and temperature derating.
80% is a commonly used general planning assumption for a well-designed grid-tied system.
Solar panels needed
7 panels
Actual output varies with roof orientation, tilt, shading, and seasonal sun-hour changes — a solar installer's site-specific production estimate (often via satellite/aerial modeling) is significantly more accurate than this general planning figure.
- Estimated daily output per panel
- 1,600 Wh
They open the calculator with your figures already in it
Solar Panel Array Sizing Calculator: 7 panels — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Panels needed = daily energy target (Wh) / (panel wattage x average daily sun hours x system efficiency)
Inputs used
- Daily Energy Usage Target
- 10
- Panel Wattage
- 400
- Average Daily Sun Hours
- 5
- System Efficiency (%)
- 80
Intermediate steps
- Estimated daily output per panel
- 1,600 Wh
Confidence note: Actual output varies with roof orientation, tilt, shading, and seasonal sun-hour changes — a solar installer's site-specific production estimate (often via satellite/aerial modeling) is significantly more accurate than this general planning figure.
What this calculation does not cover
- This is an energy-balance estimate, not an electrical design. It does not check module voltage and current against an inverter's operating window, conductor and overcurrent sizing, disconnect or rapid-shutdown requirements, or whether your service panel can accept the array's back-fed breaker. Grid-tied work normally needs a permit and a utility interconnection approval before anything is ordered.
- Nothing here checks that the panels fit. The count takes no account of module dimensions, the usable area of each roof plane, row spacing, walkways and fire-access setbacks, or obstructions such as vents, chimneys and dormers - on most roofs the space left after those subtractions is the real limit, not the energy target.
- The sun-hours figure is taken at face value with no correction for orientation, tilt or shade. Published peak-sun-hour averages assume a well-oriented, unobstructed plane, so an east/west, shallow or shaded roof produces less than this count implies. A single efficiency percentage also cannot represent shade that moves across the array through the day and the year, or the difference between a string inverter, where one shaded module drags its whole string, and microinverters or optimisers.
- One average sun-hours number hides the seasonal swing. An array sized to a yearly average falls well short in midwinter and overshoots in summer; off-grid systems are normally sized on the worst month instead. Batteries, days of autonomy and depth of discharge are outside this calculation entirely.
- No allowance is made for panel output declining over the system's life, for load growth such as an EV charger or a heat pump, or for utility rules that cap system size or restrict export. Any of those can change the number of panels you should actually buy.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Panels needed = daily energy target (Wh) / (panel wattage x average daily sun hours x system efficiency)
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