2026-09-24 8 min read

Solar Array Size Calculator

Array Watts and string layout from consumption — panel, string and footprint lines.

Calculator

Quick presets

Monthly use(kWh) Help

kWh

Bill kWh

Sun hours(hrs) Help

hrs

PSH

Derate Help

Losses

Panel watts(W) Help

Panel rating

Diagram

Sketch for solar array size calculator with your values. Not to scale — verify on site before ordering.

Plan

Item	Value
Array sizeHeadline array size	8.33 kW DC
PanelsSupporting figure	21.00 panels
FootprintSupporting figure	378.00 sq ft
Priced value (enter a unit price)Materials before labor + contingency	—

Cost plan

Materials	$41.67
Labor and install	$20.83
Contingency	$6.25
Project total	$68.75

Estimates only — get 2–3 local quotes. Labor varies by region.

Comparisons

Panels	21.00
Strings @10	2.10
Roof (sq ft)	378.00

Results

Normal

Array size

8.33kW DC

From kW = monthly kWh / (30 x sun x derate)

Panels

21.00panels

Supporting conversion

Footprint

378.00sq ft

Supporting conversion

Live results update as you type — currently showing 8.33 kW DC for solar array size calculator. Replace the defaults with your own values for an exact takeoff.

Visualizations

Your array vs sizes (kW)

8.3Your result

5Small (~5kW)

13Large (~13kW)

Result vs 10% allowance

Result91%

Allowance (10%)9%

Formula

Array Watts and string layout from consumption — panel, string and footprint lines.

kW = monthly kWh / (30 x sun x derate)

Alternative Form

Strings = panels / per-string

Variables

Symbol	Name	Unit	Description
V1	Monthly use	kWh	Bill kWh
V2	Sun hours	hrs	PSH
V3	Derate		Losses
V4	Panel watts	W	Panel rating
Q	Result	kW DC	Headline answer at full precision.

Explanation

Inputs convert to consistent units first, the formula runs at full precision, then waste and rounding produce the order figure. Cost multiplies by your unit price.

Step-by-Step Calculation

Follow each step to understand how the result is calculated from your inputs.

Step 1

Array kW = monthly kWh / (30 x sun hours x derate).

kW = 1000 / (30 x 5 x 0.80)

Result: 8.33 kW DC

Step 2

Panels plus roof footprint at ~18 sq ft each.

N = ceil(kW x 1000/400)

Result: 21 panels (~378 sq ft)

Final Answer

8.33

Worked Examples

See how the calculator works with different scenarios.

Scenario

The live inputs above run through the full path below — not just the headline answer.

Inputs

Monthly use:1000 kWh

Sun hours:5 hrs

Derate:0.8

Panel watts:400 W

Calculation

1. Array kW = monthly kWh / (30 x sun hours x derate). → 8.33 kW DC 2. Panels plus roof footprint at ~18 sq ft each. → 21 panels (~378 sq ft)

Result

8.33 kW DC

Interpretation

Order the uplifted figure, never the net: cuts, defects, rounding and code minimums consume the allowance.

Understanding Your Result

Your Result: Array size: 8.33 kW DC

Typical range: 10-14 panels per string at 400V MPPT sweet spot.

What This Means

Your result is 8.33 kW DC (array size). That is the engineered figure — verify it against the code table before buying.

Why It Matters

Sizing drives budget, schedule, deliveries and inspections. Small input errors compound across runs, so modelling before buying is the cheapest due diligence.

Professional Interpretation

Plan around 21.00 panels (panels). Keep same-lot spares for repairs.

How this is calculated

kW = monthly kWh / (30 x sun x derate). Array Watts and string layout from consumption — panel, string and footprint lines.

What drives the result

Input size, the selected option, waste % and whole-unit rounding. Unit price never changes quantity — only cost.

Recommendations

•Split irregular scopes into measurable runs and add the sections.
•Add waste before ordering: x1.10 straight, x1.15 cut-up, x1.20 complex.
•Measure twice — lasers beat tapes past 20 ft; photograph nameplates.
•Keep spares: lots, dye and grain shift between batches.

Educational Content

Introduction

Array Watts and string layout from consumption — panel, string and footprint lines.

Concept Explanation

Every calculation reduces to kW = monthly kWh / (30 x sun x derate), evaluated at full precision after unit conversion. The Steps section shows each substitution so the path is auditable.

Applications

▸Planning and takeoffs

▸Bid backup and quote checks

▸Permit documentation

▸Delivery and crew scheduling

▸Verification against supplier quotes

Benefits

•One live calculation — no separate tools per size or material
•Steps show every substitution for bid backup
•Waste and rounding separated from net math
•Instant cost, charts and lists from the same numbers

Limitations

•Planning math only — it does not replace stamped design where required
•Allowances use national planning rates; confirm local stock and prices
•Irregular geometry should be split into measurable sections

Best Practices

•Confirm nameplate and table values before ordering
•Keep one unit per field; watch the steps for conversions
•Photograph measurements and labels for dispute-proof records
•Add the trade waste factor before ordering, not after

Common Misconceptions

Myth: Order exactly the computed figure.

Truth: Cuts, defects, fittings and rounding consume 10-20%. Ordering net guarantees a second trip.

Myth: Sticker price picks the cheapest option.

Truth: True cost is installed cost per net unit after waste, labor and contingency — compare that line.

Myth: One rule fits every job.

Truth: Straight runs, retrofits and complex layouts need different allowances — use the sensitivity example.

Practical Tips

•Run one calculation per run, room or phase, then add the order lines.
•Pick stock sizes that divide your runs evenly to cut waste several points.
•Photograph each measurement next to the tape for bid backup.
•Enter your actual unit price to turn the takeoff into a budget in one step.

Common Mistakes

Ordering the net figure with no waste.

Use the order line: net x (1 + waste%) rounded up to whole units.

Mixing units in one run.

Keep one unit per field; the steps show every conversion explicitly.

Forgetting fittings, slack and extras.

Add equivalents and slack first, then apply waste — in that order.

Ignoring lot/batch differences on the second trip.

Order spares from the same lot up front.

Skipping the verification step.

Recompute one line by hand using the FAQ method before paying.

Assumptions

This calculator makes the following assumptions to provide estimates:

Consistent units: Each field uses its stated unit; conversions run before the formula.

Standard conditions: Water at ~60 F; copper electrical at 75 C terminations unless noted.

Code minimums: Results at or above model-code minimums; local amendments may require more.

Limitations

Be aware of these limitations when using this calculator:

Not a stamped design: Planning and takeoff math; get licensed design where the AHJ requires it.

Estimates, not quotes: Cost plans use allowances and your unit price — get 2-3 local quotes.

Frequently Asked Questions

Answers to common questions about this calculator.

What does the Solar Array Size Calculator do?

The Solar Array Size Calculator array Watts and string layout from consumption — panel, string and footprint lines. Enter your values above and every result, step, chart and plan updates live.

What is the formula behind the Solar Array Size Calculator?

The core formula is kW = monthly kWh / (30 x sun x derate). The alternate form shown on the page is Strings = panels / per-string. All math runs at full precision; only the display is rounded.

What inputs does the Solar Array Size Calculator need?

Enter Monthly use (kWh, default 1000) and Sun hours (hrs, default 5), plus Derate () and Panel watts (W). Live steps show each substitution.

What units does the Solar Array Size Calculator report?

The headline result is reported in kW DC (array size), with panels in panels and footprint in sq ft beside it. Keep one consistent input unit per field.

How accurate is the Solar Array Size Calculator?

Arithmetic follows directly from your inputs using kW = monthly kWh / (30 x sun x derate), so accuracy equals measurement quality. Verify one line by hand (see the verification FAQ), confirm nameplate values, and check against the governing code table before buying.

Can you walk through an example with the Solar Array Size Calculator?

With the defaults (monthly use 1000 kWh, sun hours 5 hrs), the calculator substitutes each value into kW = monthly kWh / (30 x sun x derate) in the Steps section and lands on the dashboard headline. Change any input to replay the path with your numbers.

Which code or standard governs the Solar Array Size Calculator?

NEC Article 690, IRC fire-access setbacks and utility interconnect rules govern the work; the calculator implements the standard math, and your local amendment has the final word.

How does the Solar Array Size Calculator price the job?

Enter any unit price and the dashboard, cost plan and pie chart split materials, labor allowances and a 10% contingency instantly. Leave price blank for an engineering-only answer.

What is the most common mistake with the Solar Array Size Calculator?

Mixing units (inches vs feet, kW vs BTU/h, yards vs bags) and skipping waste, coverage and rounding allowances. The Steps section exposes every conversion so a slipped unit is visible immediately.

When should I hire a pro instead of trusting the Solar Array Size Calculator?

Use the calculator for planning, takeoffs and quote checks; hire a licensed licensed solar installer/electrician for permitted work, structural/roof changes, fuel and power connections, and anything the inspector must see. Bring the printed steps — pros quote faster with documented numbers.

How do I stay safe using the Solar Array Size Calculator?

A: rooftop work needs fall protection and a structural check; DC strings run 600-1,000V with real arc-fault energy. Only licensed electricians make the interconnect — the Solar Array Size Calculator plans conduit, wire and disconnect placement.

How many sun hours does the Solar Array Size Calculator assume?

A: peak sun hours equal local kWh per square meter per day — roughly 3 to 6 across the US. Look yours up (NREL PVWatts) and enter it; every 0.5 sun-hour moves system size ~10%.

How does shading change the Solar Array Size Calculator?

A: even 10% shade can cut string output 10-30%. Microinverters or optimizers isolate shaded panels; survey with a shade tool and derate the Solar Array Size Calculator production for the shaded array.

How do tilt and azimuth move the Solar Array Size Calculator?

A: tilt near your latitude facing true south (180°) is the annual optimum in the northern hemisphere; east/west arrays lose ~10-20% but match time-of-use peaks. The Solar Array Size Calculator tilt/azimuth specs quantify both.

How many panels fit my roof per the Solar Array Size Calculator?

A: each panel needs ~18 sq ft plus IRC fire-access setbacks (typically 3-ft ridge and eave paths). The Solar Array Size Calculator converts array kW to panel count to roof area so the layout check is one step.

What size inverter goes with the Solar Array Size Calculator?

A: size AC watts near 80-100% of DC array watts (DC/AC ratio 1.1-1.3 is standard). The Solar Array Size Calculator rounds to the next standard inverter and flags clipping on oversized arrays.

Do I need batteries with the Solar Array Size Calculator system?

A: with 1:1 net metering, usually not; for backup, time-of-use arbitrage or off-grid, yes — sized to critical loads and autonomy days. The Solar Array Size Calculator battery specs run that sizing separately.

How do I size batteries from the Solar Array Size Calculator?

A: usable kWh = amp-hours x volts x depth-of-discharge / 1,000. Keep lithium (LiFePO4) near 80-90% DoD and lead-acid at 50%; the Solar Array Size Calculator applies your chemistry automatically.

How does net metering affect the Solar Array Size Calculator payback?

A: full 1:1 credit is the fastest payback; low export rates or time-of-use tariffs stretch it. The Solar Array Size Calculator prices pre-incentive energy value — enter your actual buy/sell rates for the true line.

How does the Solar Array Size Calculator handle panel degradation?

A: panels lose ~0.5% per year; 25-year warranties guarantee ~80-87% of nameplate. The Solar Array Size Calculator compounds degradation so lifetime savings never assume day-one output forever.

What maintenance does the Solar Array Size Calculator assume?

A: rinse panels once or twice a year, expect inverter replacement around year 10-15, and monitor per-panel output. Soiling alone costs 2-5% where dust and pollen rule.

How do I verify the Solar Array Size Calculator by hand?

A: system kW = monthly kWh / (30 x sun hours x 0.8); annual kWh = kW x sun hours x 365 x 0.8. If hand math differs, check DC vs AC watts and the derate factor.

What is the biggest mistake the Solar Array Size Calculator prevents?

A: sizing to summer bills only and treating nameplate watts as delivered AC power. Real AC yield is DC x sun x ~0.8 derate — the number the Solar Array Size Calculator reports.

When should I hire a pro per the Solar Array Size Calculator?

A: hire out structural/roof review, permitting and utility interconnect, main-panel upgrades, and battery commissioning. Use the Solar Array Size Calculator for sizing, takeoffs and quote checks.

How does the Solar Array Size Calculator treat off-grid vs grid-tied?

A: off-grid sizes to 2-3 sunless autonomy days plus a generator; grid-tied targets annual net-zero against the meter. Different batteries, different math — pick the right Solar Array Size Calculator page.

What wire size does the Solar Array Size Calculator recommend for solar?

A: hold DC homerun drop to 2-3% and upsize long roof-to-inverter runs a gauge. The Solar Array Size Calculator voltage-drop specs flag anything past ~3% before you buy copper.

How does temperature derate the Solar Array Size Calculator?

A: cells lose ~0.3-0.4% per degree C above 25 C cell temperature, so hot roofs underperform nameplate at noon. The Solar Array Size Calculator derate factor absorbs this plus soiling and mismatch.

What incentives stack on the Solar Array Size Calculator cost?

A: the federal investment tax credit plus state/utility rebates, SRECs and (commercial) MACRS. The Solar Array Size Calculator reports pre-incentive cost — subtract your stack for the net line.

How does roof age interact with the Solar Array Size Calculator?

A: with under ~10 years of roof life left, reroof first — mounts, flashing and labor warranties all reset with the roof. The Solar Array Size Calculator material estimate assumes sound mounting surface.

How do EVs change the Solar Array Size Calculator size?

A: each EV adds roughly 2,500-4,000 kWh per year (~8-12 extra panels). Add the charger load before sizing so the array covers driving, not just the house.

What monitoring proves the Solar Array Size Calculator output?

A: revenue-grade metering or per-panel app data against the Solar Array Size Calculator prediction, trued up yearly. Persistent 10%+ shortfalls mean shade, soiling or a failed optimizer — not a math error.

How do I document the Solar Array Size Calculator for permit and utility?

A: one-line electrical diagram, equipment spec sheets, site/plot plan and NEC 690 placards. The Solar Array Size Calculator steps attach as the sizing backup utilities ask for.

Related Calculators

Explore more calculators related to this topic.

Related Guides

Deepen your understanding with these educational resources.

Measure once, order once: takeoff workflow

Net-to-order pipeline with waste, rounding and spare stock.

From takeoff to bid: labor and contingency

Turning quantities into a bid with rates and 10% contingency.

Permit-ready documentation

Steps, photos and nameplate records inspectors accept.

Glossary

Technical terms used in mortgage calculations explained in plain language.

Array size: Headline result of this calculator, reported in kW DC.
Net vs order quantity: Net is pure math before waste; order adds waste %, coverage and whole-unit rounding.
Waste factor: Uplift for cuts, offcuts, defects and rework — typically 10% straight runs, 15% cut-up, 20% complex.
Peak sun hours: Daily kWh/m² of solar resource — numerically the hours at 1,000 W/m².
Derate factor: Combined losses (temperature, soiling, wiring, inverter) — ~0.75-0.85 typical.
DC/AC ratio: Array DC watts over inverter AC watts; 1.1-1.3 is standard.
Depth of discharge (DoD): Usable share of battery capacity; chemistry-dependent.
Net metering: Utility credit for exported kWh against imported kWh.

Authoritative References

Trusted sources and standards referenced by this calculator.

NFPA 70 — National Electrical Code, Article 690 (Solar PV) NREL — PVWatts Solar Resource Calculator U.S. DOE — Energy Saver: Planning a Home Solar System Energy Star — Home Energy Guidance DSIRE — Incentives Database Solar Energy Industries Association (SEIA)

Applicable Standards

NEC 690 governs PV wiring, disconnects, rapid shutdown and labelingAnnual kWh = kW x sun hours x 365 x derate (~0.8 planning)Hold DC homerun voltage drop to 2-3%; utility interconnect rules applyPanels degrade ~0.5%/yr; 25-yr warranty output ~80-87% of nameplateMoney rounded for display only; all calculations run at full precision

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Last Updated:

2026-09-24

Version:

2.0.0

References:

License:

Free to use for educational and professional purposes.