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Number of Solar Panels Needed

This tells you how many solar panels you need to meet your daily energy needs. You divide your total daily energy consumption (in Wh) by the energy one panel produces per day (panel power times effective sun hours). For example, if you need 5 kWh/day and one panel gives 1 kWh/day, you need 5 panels. It is a simple way to size a solar array for a home or business.

Written by the Vorzep Team

Renewable Energy Solar Power

Solar Panel Sizing Calculator

How many panels do you need? Enter any three of the number of panels N, daily energy E, panel rating P and peak sun hours H. The fourth is solved, with system losses included, plus array size, roof area, annual output and cost.

N = E / (P × H × η)E = N · P · H · η

Solve for

System efficiency η (derate)

Panel area

m²

Installed cost

$/W

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Energy coverage with whole panels

0%50%100%120%

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Step-by-step solution

System summary

System diagram

Panel count shown is the whole-panel result. Each rectangle is one panel (up to 24 are drawn).

Panels needed versus peak sun hours

Reference data

Typical annual-average peak sun hours (approximate)

Region	Peak sun hours / day	Panels for 10 kWh/day (400 W, η 80%)

Typical system losses (derate factors)

Loss	Typical factor	Cause
Inverter	0.96	DC to AC conversion
Temperature	0.90 to 0.95	Output falls as cells heat up
Soiling	0.95 to 0.98	Dust, dirt and pollen
Wiring	0.98	Resistive cable loss
Mismatch	0.98	Panel-to-panel variation
Shading	0.90 to 1.00	Trees, roof structures
Overall (product)	≈ 0.75 to 0.85	Use 80% if unsure

Typical household electricity use (approximate)

Household	kWh / day	kWh / month
Small apartment	5 to 8	150 to 240
Typical UK or EU home	8 to 11	240 to 330
Average US home	about 29	about 880
Home with electric heating or EV	30 to 50	900 to 1,500

Check your own electricity bill for the real figure. Regional sun hours and household use vary a lot by site and year.

FAQ

What are peak sun hours?

The number of hours per day the sun would need to shine at 1,000 W/m² to deliver the same energy as the real day. Equivalently, the daily solar energy in kWh/m². It is not the number of daylight hours.

Why include a system efficiency?

Panels never deliver their rated power in practice. Inverter, heat, dust, wiring and mismatch losses typically remove 15 to 25%, so the formula divides by η.

Does this size for batteries or off-grid use?

No. It sizes the array for daily energy only. Off-grid designs also need battery capacity, days of autonomy, depth of discharge and a seasonal worst-case month for sun hours.

Why round up?

You can only install whole panels, so N is rounded up to avoid a shortfall. The coverage gauge shows how much extra energy that adds.

Educational tool only. Results are estimates using annual-average sun hours and a single derate factor. They do not model tilt, orientation, shading, seasonal variation, degradation or net-metering rules. Get a site assessment from a qualified installer before purchasing.

Variables

Symbol	Quantity	Unit
N	Number of solar panels needed (rounded up to next integer)	dimensionless
E_load	Total daily energy consumption (load demand)	kWh/day
P_panel	Rated output power of one solar panel (STC)	kW
H_sun	Average peak sun hours per day (insolation)	h/day
η_sys	Overall system efficiency factor (inverter, wiring, derating, etc.)	dimensionless (optional)

What it means

The number of solar panels required for a given daily energy consumption is N = Total Load Energy / (Panel Output × Sun Hours). This calculation assumes that the panels produce their rated power for the average peak sun hours per day. Example: A daily load of 30 kWh, a panel output of 300W, and 5 sun hours require N = 30000 / (300 × 5) = 30000 / 1500 = 20 panels. This is a basic sizing method; actual design must consider inverter efficiency, battery storage, and seasonal variations. This formula is a starting point for off‑grid and grid‑tied system design.

Worked example

Solar Panels Needed – Practical Example

Real‑World

Scenario: A home uses 30 kWh/day. Each 300 W panel produces 1.5 kWh/day (5 h peak sun). How many panels are needed?

Parameter	Value
Daily energy load	30 kWh
Panel output per day	1.5 kWh
Formula	`N = Energy load / Panel output`

1N = 30 / 1.5 = 20 panels

Final Design 20 panels ✓ System sizing

Why: The number of panels is determined by energy demand and panel yield – also account for system losses.

Common mistakes

Watch for unit mismatches (W vs kW, single- vs three-phase) and remember to include power factor or efficiency where the formula requires it.

Applications

Number of solar panels needed N = Total Load Energy / (Panel Output × Sun Hours) sizes a PV system to meet daily energy requirements. Engineers use it to determine the number of panels, to design battery storage, and to perform cost analysis. This is a key step in photovoltaic system design.

PV system sizing for residential, commercial, and off‑grid
Battery storage and inverter sizing
Cost‑benefit and energy payback analysis
Design of hybrid renewable energy systems
Educational understanding of solar sizing

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Published July 10, 2026
Last updated October 4, 2026