Core Sizing Engine

Solar System Size Calculator

Determine your required photovoltaic capacity in kilowatts (kW DC) based on your utility bill, geographic solar resource, and system efficiency losses.

Updated for 2026 • NEC & NREL PVWatts Benchmark

Personalized Archetypes

Select Your System Design Strategy

Choose a sizing strategy that matches your home and budget goals to configure the calculator instantly:

Photovoltaic Sizing & Optimization Engine

IEC 61724 / NREL PVWatts Standard

Monthly Electricity Consumption

kWh

Small Home (500 kWh) US Average (900 kWh) Large / High AC (2000+ kWh)

Average Daily Peak Sun Hours (PSH) Solar irradiance equivalent to 1,000 W/m²

hrs/day

Cloudy (3.0 PSH) Moderate (4.5 PSH) Sunny Desert (6.0 PSH)

Target Electricity Bill Offset Percentage of total household demand to generate

System Loss (Derate Factor)

Preferred Panel Wattage

Recommended System Capacity

7.8 kW DC

Generates ~29.6 kWh per day average.

Total Solar Panels 20 Modules

Recommended Inverter 6.2 kW AC (1.25 DC/AC)

Roof Area Needed 390 sq ft (~36 m²)

Sizing Rating Optimal 100% Net Zero

Check Panel Layout → Size Inverter

How It Works

How Photovoltaic Capacity (kW DC) is Engineered

Click through the engineering steps below to see how licensed solar electricians translate utility usage into system kilowatt ratings.

Step 01 of 05 Convert Monthly kWh to Daily Energy Demand

Solar generation occurs on a daily solar cycle. Electric bills provide monthly figures, so engineers divide your monthly usage by 30.417 days: e.g., 900 kWh ÷ 30.417 = 29.59 kWh per day.

Formula: Daily kWh = Monthly kWh ÷ 30.417

Geographic Irradiance

How Geographic Location Shapes Your System Size

The same household power consumption requires drastically different array sizes depending on regional solar irradiance. Click a region to see how your kW DC capacity shifts:

Required Array in Selected Region Mid-Atlantic & Midwest (4.2 PSH) Calculated for 900 kWh/month at 100% offset

8.3 kW DC (21 × 400W Modules)

System Efficiency Physics

Environmental Loss & System Derate Factor Simulator

Per IEC 61724 standards, real-world solar systems never produce 100% of laboratory STC capacity. Adjust individual loss parameters to observe their cumulative effect on your required kW DC capacity:

Inverter Efficiency 96.5%

CEC weighted efficiency

Thermal Cell Derate -8.0%

Roof temp above 25°C

Soiling & Dirt Losses -3.0%

Pollen, dust, bird droppings

AC / DC Wiring Resistance -2.0%

Conductor voltage drop

Composite System Derate Factor: 0.844 (84.4% System Efficiency)

Total system losses: ~15.6%

Future-Proofing

Future Household Electrification Planner

Planning to switch to an Electric Vehicle or replace a gas furnace with a heat pump? Check upcoming additions to size your array with future capacity headroom:

1× Electric Vehicle 12,000 miles/yr

+2.4 kW

Whole-Home Heat Pump Replacing gas furnace

+3.2 kW

Swimming Pool Heat Pump Seasonal heating load

+1.6 kW

Future-Proofed Total Capacity: Base array plus selected electrification additions

7.8 kW DC (20 × 400W Modules)

Your Custom System Blueprint

System Sizing Engineering Blueprint

Reference these metrics when requesting quotes from solar contractors:

Array Rating 7.8 kW DC Target 100% Offset

Inverter Continuous 6.2 kW AC 1.25:1 DC/AC Ratio

Panel Modules 20 Modules 400W Monocrystalline

Daily Production 29.6 kWh Average daily yield

Next Step: Financial Sizing Calculate turnkey equipment costs, 30% Federal ITC tax credits, and monthly loan options.

Price This System →

Reference Table

Solar System Sizing Benchmarks by Monthly Consumption

Standard residential sizing guidelines based on typical US solar irradiance (4.5 PSH) and standard 85% system efficiency.

Monthly Usage	Daily kWh	System Size (kW DC)	Panels (400W)	Inverter (kW AC)	Roof Space
500 kWh	16.4 kWh	4.3 kW DC	11 Panels	3.6 kW AC	~215 sq ft
750 kWh	24.7 kWh	6.5 kW DC	16 Panels	5.0 kW AC	~312 sq ft
900 kWh (US Avg)	29.6 kWh	7.8 kW DC	20 Panels	6.2 kW AC	~390 sq ft
1,200 kWh	39.4 kWh	10.3 kW DC	26 Panels	8.0 kW AC	~507 sq ft
1,500 kWh	49.3 kWh	12.9 kW DC	32 Panels	10.0 kW AC	~624 sq ft
2,000 kWh	65.7 kWh	17.2 kW DC	43 Panels	13.5 kW AC	~839 sq ft

Real-World Case Studies

Worked Solar Sizing Case Studies

Real engineering examples illustrating how geographic climate and lifestyle loads alter system requirements.

Case Study 1 Denver, CO

6.5 kW Suburban Array

A 1,800 sq ft home in Denver consumes 750 kWh/mo (4.8 PSH). Sized for 100% net offset:

• Daily Demand: 24.7 kWh/day

• Capacity: 6.5 kW DC

• Panels: 16 × 400W

• Inverter: 5.0 kW AC

Outcome: Eliminates $125/month electricity bill with a 6.8-year payback.

Case Study 2 Dallas, TX

10.8 kW System + Tesla EV

A 2,400 sq ft home with central AC adds a Model Y (1,200 miles/mo), pushing usage to 1,250 kWh/mo:

• Daily Demand: 41.1 kWh/day

• Capacity: 10.8 kW DC

• Panels: 27 × 400W

• Inverter: 7.6 kW AC (Sol-Ark)

Outcome: Powers home and 100% of EV commuting miles from sunlight.

Case Study 3 Phoenix, AZ

13.6 kW Sunbelt Pool Estate

A 3,200 sq ft home with dual AC and pool pump uses 2,000 kWh/mo under intense 5.8 PSH solar irradiance:

• Daily Demand: 65.7 kWh/day

• Capacity: 13.6 kW DC

• Panels: 34 × 400W

• Inverter: 10.0 kW AC (SolarEdge)

Outcome: High 5.8 PSH keeps array compact despite heavy 2,000 kWh consumption.

Frequently Asked Questions

Expert Sizing & Capacity FAQ

Essential technical answers on system kilowatt sizing, peak sun hours, derating factors, and inverter ratios.

What size solar system do I need for 1,000 kWh per month?

For 1,000 kWh per month with an average 4.5 peak sun hours per day and 15% system losses (0.85 derate), you need an 8.6 kW DC solar panel array (approx. 21–22 modules of 400W each).

How is solar system size calculated mathematically?

The standard engineering formula is: System Size (kW DC) = (Daily kWh Consumption × Target Offset %) ÷ (Peak Sun Hours × System Derate Factor). For example, 29.6 kWh/day ÷ (4.5 PSH × 0.85) = 7.74 kW DC.

What is the difference between kW and kWh?

Kilowatt (kW) measures instantaneous power capacity (how much energy your panels can produce at peak moment under standard test conditions), while kilowatt-hour (kWh) measures the total quantity of energy generated or consumed over time.

Should I oversize my solar panel array?

Oversizing by 10% to 25% is recommended if you plan to adopt an Electric Vehicle, install a heat pump, add air conditioning, or if you live in an area with net billing where winter generation is low.

How does geographic Peak Sun Hours (PSH) affect required system size?

Peak Sun Hours measures solar irradiance equivalent to 1,000 W/m². A home in Phoenix with 5.8 PSH needs only a 6.0 kW system to generate 900 kWh/month, while the same home in Seattle or the UK with 3.0 PSH requires an 11.6 kW array.

What is the system derate factor and what losses does it include?

The derate factor accounts for real-world inefficiencies: inverter DC-to-AC conversion (3-4%), high temperature cell losses (5-10%), wiring resistance (2%), soiling and dust (2-5%), and module mismatch (1-2%). Typical combined derate is 80% to 88% (0.80–0.88).

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