2026-09-24 8 min read
Solar Panel Size Calculator
Rooftop array size from monthly usage and local sun hours — panel count plus roof footprint.
Calculator
Quick presets
Monthly use(kWh) Help
kWh
Utility bill kWh
Sun hours(hrs) Help
hrs
Peak sun hrs/day
Derate Help
Losses ~0.8
Panel watts(W) Help
W
Per-panel watts
Installed ($/W)($/unit) Help
$
Leave blank for engineering only
Diagram
Sketch for solar panel size calculator with your values. Not to scale — verify on site before ordering.Plan
| Item | Value |
|---|---|
| Array sizeHeadline array size | 7.50 kW DC |
| PanelsSupporting figure | 19.00 panels |
| Roof areaSupporting figure | 342.00 sq ft |
| Priced value at $2.80/unitMaterials before labor + contingency | $21,000.00 |
Cost plan
| Materials | $21,000.00 |
| Labor and install | $6,000.00 |
| Contingency | $2,700.00 |
| Project total | $29,700.00 |
Estimates only — get 2–3 local quotes. Labor varies by region.
Comparisons
| Panels | 19.00 |
| Roof (sq ft) | 342.00 |
| Annual kWh | 10,950.00 |
Results
Normal
Array size
7.50kW DC
From kW = monthly kWh / (30 x sun x derate)
Panels
19.00panels
Supporting conversion
Roof area
342.00sq ft
Supporting conversion
Estimated cost
$21,000.00
At $2.80/unit
Live results update as you type — currently showing 7.50 kW DC for solar panel size calculator. Replace the defaults with your own values for an exact takeoff.
Visualizations
Your array vs homes (kW)
7.5Your result
4Small (4kW)
10Large (10kW)
Project cost split
Materials71%
Labor20%
Contingency9%
Formula
Rooftop array size from monthly usage and local sun hours — panel count plus roof footprint.
kW = monthly kWh / (30 x sun x derate)
Alternative Form
panels = ceil(kW x 1000 / panel W)
Variables
| Symbol | Name | Unit | Description |
|---|---|---|---|
| V1 | Monthly use | kWh | Utility bill kWh |
| V2 | Sun hours | hrs | Peak sun hrs/day |
| V3 | Derate | Losses ~0.8 | |
| V4 | Panel watts | W | Per-panel watts |
| 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 = 900 / (30 x 5 x 0.80)
Result: 7.50 kW DC
Step 2
Panels plus roof footprint at ~18 sq ft each.
N = ceil(kW x 1000/400)
Result: 19 panels (~342 sq ft)
Final Answer
7.50
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:900 kWh
Sun hours:5 hrs
Derate:0.8
Panel watts:400 W
Calculation
1. Array kW = monthly kWh / (30 x sun hours x derate). → 7.50 kW DC 2. Panels plus roof footprint at ~18 sq ft each. → 19 panels (~342 sq ft)
Result
7.50 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: 7.50 kW DC
Typical range: US homes land 5-10 kW; add ~2 kW per EV.
What This Means
Your result is 7.50 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 19.00 panels (panels), about $21,000.00 at your unit price. Keep same-lot spares for repairs.
How this is calculated
kW = monthly kWh / (30 x sun x derate). Rooftop array size from monthly usage and local sun hours — panel count plus roof footprint.
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
Rooftop array size from monthly usage and local sun hours — panel count plus roof footprint.
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 Panel Size Calculator do?
The Solar Panel Size Calculator rooftop array size from monthly usage and local sun hours — panel count plus roof footprint. Enter your values above and every result, step, chart and plan updates live.
What is the formula behind the Solar Panel Size Calculator?
The core formula is kW = monthly kWh / (30 x sun x derate). The alternate form shown on the page is panels = ceil(kW x 1000 / panel W). All math runs at full precision; only the display is rounded.
What inputs does the Solar Panel Size Calculator need?
Enter Monthly use (kWh, default 900) and Sun hours (hrs, default 5), plus Derate () and Panel watts (W). Live steps show each substitution.
What units does the Solar Panel Size Calculator report?
The headline result is reported in kW DC (array size), with panels in panels and roof area in sq ft beside it. Keep one consistent input unit per field.
How accurate is the Solar Panel 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 Panel Size Calculator?
With the defaults (monthly use 900 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 Panel 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 Panel 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 Panel 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 Panel 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 Panel 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 Panel Size Calculator plans conduit, wire and disconnect placement.
How many sun hours does the Solar Panel 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 Panel 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 Panel Size Calculator production for the shaded array.
How do tilt and azimuth move the Solar Panel 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 Panel Size Calculator tilt/azimuth specs quantify both.
How many panels fit my roof per the Solar Panel Size Calculator?
A: each panel needs ~18 sq ft plus IRC fire-access setbacks (typically 3-ft ridge and eave paths). The Solar Panel 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 Panel Size Calculator?
A: size AC watts near 80-100% of DC array watts (DC/AC ratio 1.1-1.3 is standard). The Solar Panel Size Calculator rounds to the next standard inverter and flags clipping on oversized arrays.
Do I need batteries with the Solar Panel 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 Panel Size Calculator battery specs run that sizing separately.
How do I size batteries from the Solar Panel 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 Panel Size Calculator applies your chemistry automatically.
How does net metering affect the Solar Panel Size Calculator payback?
A: full 1:1 credit is the fastest payback; low export rates or time-of-use tariffs stretch it. The Solar Panel Size Calculator prices pre-incentive energy value — enter your actual buy/sell rates for the true line.
How does the Solar Panel Size Calculator handle panel degradation?
A: panels lose ~0.5% per year; 25-year warranties guarantee ~80-87% of nameplate. The Solar Panel Size Calculator compounds degradation so lifetime savings never assume day-one output forever.
What maintenance does the Solar Panel 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 Panel 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 Panel 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 Panel Size Calculator reports.
When should I hire a pro per the Solar Panel Size Calculator?
A: hire out structural/roof review, permitting and utility interconnect, main-panel upgrades, and battery commissioning. Use the Solar Panel Size Calculator for sizing, takeoffs and quote checks.
How does the Solar Panel 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 Panel Size Calculator page.
What wire size does the Solar Panel Size Calculator recommend for solar?
A: hold DC homerun drop to 2-3% and upsize long roof-to-inverter runs a gauge. The Solar Panel Size Calculator voltage-drop specs flag anything past ~3% before you buy copper.
How does temperature derate the Solar Panel 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 Panel Size Calculator derate factor absorbs this plus soiling and mismatch.
What incentives stack on the Solar Panel Size Calculator cost?
A: the federal investment tax credit plus state/utility rebates, SRECs and (commercial) MACRS. The Solar Panel Size Calculator reports pre-incentive cost — subtract your stack for the net line.
How does roof age interact with the Solar Panel 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 Panel Size Calculator material estimate assumes sound mounting surface.
How do EVs change the Solar Panel 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 Panel Size Calculator output?
A: revenue-grade metering or per-panel app data against the Solar Panel 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 Panel Size Calculator for permit and utility?
A: one-line electrical diagram, equipment spec sheets, site/plot plan and NEC 690 placards. The Solar Panel Size Calculator steps attach as the sizing backup utilities ask for.
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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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Page Information
Last Updated:
2026-09-24
Version:
2.0.0
References:
License:
Free to use for educational and professional purposes.