Solar Panel Array Size Calculator

Project preset:

Choose a representative starting point, then replace it with bill and site values.

Average electricity use:

The monthly energy demand the planned array should offset.

kWh/month

Annual offset target:

Peak sun hours:

Equivalent daily full-sun hours used by this screening shortcut.

h/day

Module profile:

Pick a mainstream module class or enter a proposal-specific module.

Module nameplate power:

Rated DC power for one module.

W DC

Module footprint:

Outside-frame area for one module.

Usable roof or ground area:

Clear area available to the selected module footprint.

System losses:

Design reserve:

Layout allowance:

Inverter planning:

Leave off for a DC-only array screen.

DC/AC ratio:

Measure	Value	Planning basis	Copy
{{ row.label }}	{{ row.value }}	{{ row.detail }}

Check	Status	Review action	Copy
{{ row.label }}	{{ row.status }}	{{ row.detail }}

Energy balance unavailable until the inputs are valid.

An electricity bill measures energy used over time, while a solar module is rated by the power it can deliver under test conditions. Array sizing connects those two quantities: annual kilowatt-hours become a required direct-current capacity, that capacity becomes a whole number of modules, and the module count must fit the available roof or ground area.

The annual offset target is the share of electricity use assigned to solar production. A 100% target does not promise a zero bill. Production and consumption occur at different times, and tariffs, export credits, batteries, minimum charges, and seasonal demand affect the financial result even when annual energy totals match.

Quantities used in an early solar array estimate
Quantity	Meaning	Why it changes the estimate
Peak sun hours	Equivalent full-sun hours per day at the array plane.	More usable sunlight lowers the DC capacity needed for the same annual energy.
System losses	One combined reduction for heat, shade, soiling, wiring, conversion, mismatch, and downtime.	A larger loss percentage requires more nameplate capacity.
Module power	The rated DC watts of one module.	Higher-watt modules reduce count only when their larger footprint and electrical limits still fit.
Layout allowance	Extra area beyond the module rectangles.	Walkways, setbacks, gaps, roof geometry, and obstructions can consume usable space.

A fast annual-energy estimate compresses weather, orientation, tilt, shade, temperature, and equipment behavior into only a few assumptions. That makes it useful for comparing rough scenarios, but it cannot place modules around vents, check roof structure, model hourly clipping, select conductors, or determine utility and permit requirements.

The whole-module result should therefore be read as a screening size. The next step for a real project is a site-specific production model and layout using the actual address, roof planes, shade, equipment, structural limits, electrical service, and interconnection rules.

How to Use This Tool:

Use a full year of electricity bills when possible, then replace every preset with values for the actual site and proposed module.

Enter Average electricity use and choose the annual offset target. A recent 12-month total divided by 12 is stronger than one seasonal bill.
Enter annual Peak sun hours for the planned tilt and direction, then set aggregate losses. Avoid using a best-month sunlight figure for an annual estimate.
Select a module profile or enter the proposal's nameplate watts and outside-frame area. Enter only the roof or ground area that is genuinely usable.
Add design reserve and layout allowance when the project needs them. Enable inverter planning only when an approximate AC size is useful, then set the intended DC/AC ratio.
Check the whole-module estimate, annual offset, and area status together. An energy target that passes while area is short still needs a different module, surface, or target.

Interpreting Results:

The installed DC size is based on a whole-module ceiling, so estimated annual production usually lands slightly above the requested target. The coverage percentage compares that rounded production with the original annual usage, while target energy also includes any design reserve.

Area short means the module rectangles plus layout allowance exceed the entered usable area.
Tight fit means the area is nonnegative but utilization exceeds 85%. Detailed placement can still fail because a single total-area number does not describe roof shape.
An approximate inverter AC size is only DC capacity divided by the selected DC/AC ratio. It does not model clipping, voltage windows, strings, efficiency curves, or equipment compatibility.

Technical Details:

The annual-energy model treats monthly use as a steady average, converts the target to daily energy, and applies one performance ratio equal to one minus the entered loss fraction. It then rounds DC capacity upward to a whole module before recalculating production and space.

Formula Core:

Monthly use, offset, and reserve establish the annual energy target.

E_{target} = E_{month} \times 12 \times \frac{O}{100} \times (1 + \frac{R}{100})

Required DC capacity divides daily target energy by peak sun hours and the performance ratio. Module count always rounds upward.

P_{DC} = \frac{E_{target} / 365}{H \times (1 - \frac{L}{100})}, N = ⌈ \frac{P_{DC} \times 1000}{P_{module}} ⌉

Required area multiplies whole-module count by module footprint and the layout allowance.

A_{required} = N \times A_{module} \times (1 + \frac{G}{100})

For 900 kWh per month, an 80% target, 4.5 peak sun hours, 14% losses, no reserve, and 420 W modules, the accepted reference case needs 6.1166 kW before rounding. Fifteen modules install 6.30 kW DC and estimate 8,899.065 kWh per year, or 82.39875% of the original 10,800 kWh annual use.

Rule Core:

Status labels are screening checks with exact boundaries.

Solar array screening status rules
Check	Rule	Meaning
Area short	Area margin < 0	Entered usable area is smaller than the simple required footprint.
Tight fit	Margin is at least 0 and utilization > 85%	Little room remains for geometry or placement losses.
Loss assumption	< 8% optimistic; > 25% high	Values from 8% through 25% remain in the planning range.
Solar resource	< 3.5 h/day low; > 6.2 h/day very high	Values from 3.5 through 6.2 still need source verification.
Project scale	< 8 modules small; > 40 large	Counts from 8 through 40 receive the typical-screen label.

Accepted inputs include 1 to 1,000,000 kWh/month, 10% to 120% offset, 0.5 to 9 peak sun hours, 0% to 35% losses, 0% to 30% reserve, 0% to 50% layout allowance, 50 to 1,000 W modules, and a 1.00 to 2.00 DC/AC ratio.

Accuracy Notes:

This is a deterministic annual screening model, not a site simulation or design package. It does not use hourly weather, azimuth, tilt, horizon, shade geometry, temperature curves, degradation, snow, module mismatch, inverter clipping, or utility rules.

Use location- and plane-specific solar resource data rather than a regional best-case value.
Confirm usable area with a scaled layout that includes setbacks, access paths, obstructions, module orientation, and structural limits.
Have qualified professionals verify electrical design, equipment compatibility, permits, interconnection, and roof loading.

References:

PVWatts V8 API documentation, National Renewable Energy Laboratory.
Photovoltaic System Design and Energy Yield, U.S. Department of Energy.
Solar Photovoltaic System Design Basics, U.S. Department of Energy.