Solar Panel Array Size Calculator

Project preset:

Start with a common solar sizing case, then tune the assumptions for the site or proposal.

Electricity use:

Average monthly consumption to offset with solar production.

kWh/month

Target offset: {{ targetOffsetDisplay }}

Solar production target as a share of annual usage.

Peak sun hours:

Average daily equivalent full-sun hours at the array plane.

h/day

Module profile:

Choose a typical module class or keep the exact wattage from a quote.

Panel wattage:

DC nameplate rating for one solar panel.

Usable roof or ground area:

Area sanity check only; it is not a permit layout or structural review.

Panel footprint:

Approximate area for one module in the same area unit.

System losses: {{ systemLossesDisplay }}

Performance derate applied to the nameplate array output.

Check the solar array inputs

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Design reserve: {{ designReserveDisplay }}

Optional cushion for degradation, shade uncertainty, or load growth.

Footprint allowance: {{ footprintFactorDisplay }}

Area multiplier used only for the fit sanity check.

DC/AC ratio: {{ dcAcRatioDisplay }}

Approximate inverter AC capacity equals rounded DC kW divided by this ratio.

Metric	Value	Basis	Copy
{{ row.metric }}	{{ row.value }}	{{ row.basis }}

Check	Status	Action	Copy
{{ row.check }}	{{ row.status }}	{{ row.action }}

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Solar array sizing starts with energy, not roof area. A household that uses 900 kWh per month and wants an 80% annual offset needs a different array from a smaller cabin aiming for 120% of its measured load, even if both sites have the same roof shape.

The important terms are annual use, target offset, peak sun hours, module wattage, and system losses. Annual use sets the load to cover. Peak sun hours convert local solar resource into daily production. Module wattage turns the required DC size into a whole-panel count. Losses account for inverter conversion, wiring, temperature, soiling, shading, mismatch, and downtime.

Solar array sizing flow from sun hours and annual use to rounded panel count and area fit.

DC nameplate size is the sum of the panel ratings under standard test conditions. Real annual production is lower after losses, then higher or lower depending on the local solar resource. That is why a 6 kW array in a sunny site can produce more annual energy than a larger array in a cloudy or shaded site.

Array sizing is a planning estimate. It can show whether the panel count and roof area are in the right range, but it cannot replace a shade model, structural review, electrical interconnection check, permit layout, or installer proposal.

How to Use This Tool:

Enter the energy target first, then review the whole-panel rounding and area fit checks.

Choose a Project preset if it matches the situation, such as a suburban home, cloudy roof, off-grid cabin, or small business. Change any value that differs from the bill or site.
Enter Electricity use as average monthly kWh, then set Target offset. The offset is annual production as a share of annual use before billing rules, batteries, and export credit policies.
Set Peak sun hours from a location-specific solar resource, PVWatts run, or proposal when available. A generic regional value can undercount or overcount panels.
Choose a Module profile or enter the exact Panel wattage and Panel footprint from a datasheet or quote.
Enter Usable roof or ground area after obvious shade, setbacks, vents, walkways, and keepouts. The area check is a sanity check, not a layout drawing.
Use Advanced for System losses, Design reserve, Footprint allowance, and DC/AC ratio when the proposal has those assumptions.

Read Array Sizing for the panel count and DC size, Sizing Checks for warning rows, Production Balance for annual use versus target and rounded production, and JSON for a structured record.

Interpreting Results:

The rounded panel count is the practical output. It is based on the required DC size, then rounded up to a whole number of modules. That rounding usually produces a small surplus over the target annual kWh.

Estimated usage offset should be read with the Sizing Checks. A target can look met while the area row says Area short, the loss row says assumptions need review, or the peak-sun row suggests the solar resource may be too optimistic.

Do not treat the area badge as a permit answer. A roof can have enough square footage and still fail because of shade, fire setbacks, structural limits, service equipment, inverter clipping, or utility interconnection rules.

Technical Details:

The model uses a simple annual-energy approach. Monthly use becomes annual use, the target offset and optional reserve set the production target, and peak sun hours plus a performance ratio translate that target into required DC kilowatts.

Formula Core

The performance ratio is one minus the selected system-loss percentage. Whole-panel rounding happens after the required DC size is known.

\begin{array}{lcl} AnnualUse & = & MonthlyKWh \times 12 \\ TargetAnnualKWh & = & AnnualUse \times \frac{OffsetPercent}{100} \times (1 + \frac{ReservePercent}{100}) \\ RequiredDCKW & = & \frac{TargetAnnualKWh / 365}{PeakSunHours \times PerformanceRatio} \\ PanelCount & = & ⌈ \frac{RequiredDCKW \times 1000}{PanelWatts} ⌉ \end{array}

With 900 kWh per month, an 80% target, 4.5 peak sun hours, 14% losses, and 420 W modules, the required DC size is about 6.11 kW. Rounding to whole modules gives 15 panels, or 6.30 kW DC.

Solar array sizing variables
Variable	Meaning	Common source
`Peak sun hours`	Daily equivalent full-sun hours at the array plane.	PVWatts, solar-resource map, or proposal.
`System losses`	Derate for ordinary production losses before annual kWh is estimated.	Proposal assumptions, PVWatts loss categories, or conservative planning value.
`Footprint allowance`	Area multiplier for racking gaps, access, and simple layout inefficiency.	Roof layout estimate or installer proposal.
`DC/AC ratio`	Planning ratio used to estimate approximate inverter AC size from rounded DC size.	Inverter design or quote assumption.

Solar array check rows and interpretation
Check	What triggers attention	What to verify
`Target production`	Rounded panels do not reach the target annual kWh.	Increase panel count, lower losses, or reduce the offset target.
`Area fit`	Required module area exceeds the entered usable area.	Roof faces, keepouts, module dimensions, and target offset.
`Loss assumption`	Losses are very low or high compared with ordinary planning values.	Shade, temperature, soiling, mismatch, wiring, inverter, and downtime assumptions.
`System scale`	Very small or large panel counts.	Fixed costs, service-panel limits, roof structure, and interconnection limits.

Accuracy Notes:

This is a deterministic screening estimate. It does not model hourly shade, roof pitch and azimuth, inverter clipping, snow cover, battery dispatch, utility tariffs, structural loading, permitting, or financial return.

Use annual average peak sun hours only when they represent the array plane, not just a broad regional climate value.
Keep system losses conservative when shade, high temperatures, soiling, or downtime are uncertain.
Confirm the final module count with a site-specific design before buying equipment.

Worked Examples:

Suburban home partial offset

A 900 kWh/month home with an 80% target, 4.5 peak sun hours, 14% losses, and 420 W panels rounds to about 15 panels. Array Sizing shows the installed DC size and Production Balance compares annual usage, target kWh, and rounded production.

Cloudy roof with tight area

A cloudy roof with 3.5 peak sun hours and 18% losses needs more DC capacity for the same annual target. If Area short appears, lower the target offset, add another roof face, or confirm the module footprint and keepout assumptions.

Small off-grid-style load

A cabin using 220 kWh/month and targeting 120% offset may still have a modest panel count, but the Sizing Checks can warn that small systems are affected by fixed inverter, permitting, and labor costs.

Validation recovery

If the result says to enter peak sun hours above zero or panel wattage above zero, check whether a blank field, copied comma, or wrong unit made the input nonnumeric. The tables rebuild after the value is valid.

FAQ:

Is DC array size the same as inverter size?

No. DC array size is the sum of panel nameplate watts. The approximate inverter AC size is estimated separately from the selected DC/AC ratio.

Why does the estimated offset exceed the target?

Panel count is rounded up to whole modules. That rounding often produces slightly more annual kWh than the exact target.

Should I enter roof area before or after setbacks?

Use clear usable area after obvious shade, vents, walkways, setbacks, and equipment keepouts. Leave detailed layout validation to a site-specific design.

Does this replace PVWatts?

No. Use PVWatts or a comparable site-specific model when tilt, azimuth, weather data, shading, and monthly production detail matter.

Glossary:

DC nameplate: The sum of the rated DC wattage of all solar modules.
Peak sun hours: Equivalent full-sun hours per day used to estimate annual production.
Performance ratio: The fraction of nameplate production left after the selected loss percentage.
Target offset: The share of annual electricity use the array is planned to produce.
Specific yield: Estimated annual kWh per installed kW DC.

References:

PVWatts API documentation, National Renewable Energy Laboratory.
Planning a Home Solar Electric System, U.S. Department of Energy.
SAM Photovoltaic Models, National Renewable Energy Laboratory.