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Solar Array Sizing Calculator — Panel Count from Daily kWh Load

Size Grid-Tied and Off-Grid PV Arrays Using Peak Sun Hours, System Losses, and Tilt Factors

Free solar array sizing calculator for solar installers, electricians, and homeowners planning a PV system. Enter your daily kilowatt-hour consumption, select your location's peak sun hours (PSH), specify panel wattage, and the calculator determines the number of modules needed after accounting for inverter efficiency, wiring losses, soiling, temperature derating, and tilt/azimuth correction factors. Supports both grid-tied systems sized to offset a percentage of annual usage and off-grid systems sized for worst-month production. Outputs total array wattage (STC and PTC), estimated annual kWh production, and roof area required.

Pro Tip: The single biggest sizing mistake is using annual-average peak sun hours for an off-grid system. Off-grid arrays must be sized for the worst month, which in most of the continental US is December or January with PSH values 40-60% lower than the annual average. A system sized on 5.0 average PSH that actually sees 2.5 PSH in January will only produce half the expected energy when heating loads are highest. For grid-tied systems, annual average is fine because net metering banks summer overproduction against winter shortfalls. But off-grid has no bank. Size for December, not July.

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Read the complete solar system sizing guide

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Solar Array Sizing Calculator
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How It Works

  1. Enter Your Daily Energy Consumption

    Pull your average daily kWh from your utility bill. Divide the monthly kWh total by the number of days in the billing period. For grid-tied systems, use the annual average. For off-grid, use the highest-consumption month (usually winter if you heat electrically or summer if you cool). A typical US household uses 28-30 kWh/day, but this varies enormously by climate and lifestyle.

  2. Set Your Peak Sun Hours

    Select your location or enter peak sun hours manually. PSH is the number of hours per day that solar irradiance averages 1,000 W/m2. Phoenix averages 6.5 PSH annually; Seattle averages 3.5 PSH. Use the NREL PVWatts database or your installer's irradiance data for precise values. For off-grid, enter the worst-month PSH, not the annual average.

  3. Select Panel Specifications

    Enter the wattage of your chosen panel (e.g., 400W, 450W). The calculator uses STC ratings by default but applies a temperature derating factor to estimate real-world PTC output. Higher-wattage panels reduce count but verify your roof can accommodate the larger physical dimensions.

  4. Configure System Loss Factors

    Adjust loss factors for inverter efficiency (typically 96-97%), DC wiring losses (1-3%), soiling (2-5%), shading (0-25%), module mismatch (1-2%), and temperature derating (5-15% depending on climate). The calculator defaults to industry-standard values but you can override each one based on your site assessment.

  5. Review Array Sizing Results

    The calculator outputs the total number of panels, array wattage at STC and PTC, estimated first-year annual production in kWh, percentage of load offset, and approximate roof area required. For off-grid systems, it also shows the daily Ah production at your battery bank voltage for battery sizing integration.

Built For

  • Solar installers creating proposals with accurate panel counts and production estimates for residential customers
  • Electricians sizing PV arrays for grid-tied systems to offset a specific percentage of annual utility consumption
  • Off-grid homesteaders sizing arrays for worst-month production to avoid generator dependence in winter
  • Engineers designing commercial rooftop systems with precise loss modeling for investor-grade production forecasts
  • Homeowners verifying installer proposals by cross-checking panel count against their actual energy usage and local irradiance
  • Building inspectors reviewing solar permit applications for reasonable array sizing relative to the electrical load

Features & Capabilities

Grid-Tied and Off-Grid Modes

Grid-tied mode sizes the array to offset a target percentage of annual consumption using annual-average PSH. Off-grid mode sizes for worst-month production to ensure year-round self-sufficiency. The two modes produce significantly different panel counts for the same load because off-grid must cover the lean months without net metering.

Comprehensive Loss Modeling

Accounts for inverter efficiency, DC wiring losses, soiling, shading, module mismatch, snow cover, age degradation, and temperature derating. Default values follow PVWatts methodology but every factor is adjustable. Total system derate typically ranges from 0.75 to 0.85, meaning 15-25% of nameplate capacity is lost to real-world conditions.

Temperature Derating

Silicon PV modules lose approximately 0.35-0.45% efficiency per degree Celsius above 25C (STC reference temperature). In Phoenix where rooftop cell temperatures reach 65-70C, temperature derating alone reduces output by 14-20%. The calculator applies location-appropriate temperature corrections using NOCT or user-entered cell temperature data.

Tilt and Azimuth Correction

Adjusts production estimates based on array tilt angle and compass orientation. True south at latitude tilt is the baseline (1.0 correction factor). East or west-facing arrays receive a 0.80-0.85 correction. Flat-mounted arrays at low tilt receive corrections based on latitude. These factors significantly affect panel count for roof-mounted systems constrained by roof geometry.

Roof Area Estimation

Calculates the physical roof area required based on panel dimensions and count. Accounts for standard row spacing, setback requirements from roof edges per fire code (IFC 605.11.3.2), and equipment clearance zones. Helps identify whether the available roof area can accommodate the required array before committing to a design.

Daily Ah Output for Battery Sizing

For off-grid systems, converts the daily kWh production to amp-hours at the battery bank voltage (12V, 24V, or 48V). This output feeds directly into battery bank sizing calculations, ensuring the array and battery bank are properly matched for the same daily energy budget.

Frequently Asked Questions

House size alone does not determine panel count. Energy consumption does. A 2,000 sqft house in Phoenix with a high-efficiency heat pump might use 25 kWh/day, requiring 14-16 panels at 400W each with 6.0 PSH. The same size house in Minnesota with electric resistance heat and a hot tub might use 50 kWh/day, requiring 35-40 panels at 400W with 3.5 winter PSH. Start with your actual kWh consumption from utility bills, not your square footage. The calculator converts your real energy usage into an accurate panel count.
STC (Standard Test Conditions) rates panels at 1,000 W/m2 irradiance, 25C cell temperature, and AM 1.5 spectrum. These are laboratory conditions that rarely exist in the real world. PTC (PVUSA Test Conditions) rates panels at 1,000 W/m2 but with 20C ambient air temperature and 1 m/s wind speed, which results in a higher cell temperature. PTC ratings are typically 10-15% lower than STC for crystalline silicon panels. PTC is more realistic for production estimates, but STC is the industry-standard nameplate rating used for pricing, permitting, and interconnection applications.
Peak sun hours (PSH) is not the same as hours of daylight. PSH is a measure of total daily solar energy expressed as equivalent hours at full 1,000 W/m2 intensity. A location might have 14 hours of daylight in summer but only 5.5 PSH because the sun is weak in early morning and late afternoon. The morning and evening hours contribute partial energy that sums to fewer equivalent peak hours. Denver has longer summer days than Miami, but both have similar annual PSH because Miami has higher solar intensity per hour. Always use PSH data from NREL or your local solar resource database, not sunrise-to-sunset hours.
For grid-tied residential systems with net metering, sizing to 100-110% of annual consumption is typical because net metering credits summer overproduction against winter underproduction at a 1:1 ratio. However, check your utility's net metering policy first. Some utilities cap system size at 100% of historical usage or apply reduced credit rates for excess generation. Some have moved to net billing where export credits are worth less than retail rate, which changes the economics of oversizing. If your utility pays wholesale rate for exports, sizing to 80-90% offset often yields a better ROI than 100%.
Shading is the most destructive loss factor because of how panels are wired internally. A standard 60/72-cell panel has three cell strings wired in series with bypass diodes. Shading even one cell in a string forces the bypass diode to activate, eliminating that entire string's contribution, which is one-third of the panel's output. Ten percent physical shading can cause 33% power loss on the affected panel, and on string inverter systems, the weakest panel drags down every panel on that string. This is why module-level power electronics (MLPEs) like microinverters or DC optimizers are required by NEC 690.12 rapid shutdown and strongly recommended for any site with partial shading.
Disclaimer: This calculator provides solar array sizing estimates for planning purposes. Actual production depends on local weather, shading, installation quality, and equipment specifications. Solar installations require permits, structural engineering review, and must comply with NEC Article 690 and local building codes. Consult a licensed solar installer or professional engineer for final system design. ToolGrit is not responsible for system design decisions or production outcomes.

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