Solar Panel kW System for Your Home
Calculate how many solar panels and how many kW you need to cover your home's electricity bill. Real formula: monthly kWh ÷ (panel kWp × Peak Sun Hours × efficiency). Free, instant result.
See step-by-step calculation
When to use this calculator
- Homeowner comparing installer proposals to verify the quoted system size actually covers their 1,100 kWh/month bill in Phoenix, AZ (6.5 PSH/day), before signing a 25-year lease.
- Recent mover sizing a rooftop array for a new home in Seattle, WA (3.5 PSH/day) where winter energy use spikes to 1,400 kWh/month due to electric heating.
- Off-grid cabin owner determining whether a 3 kW array is sufficient to power a 400 kWh/month load in rural Nevada (6.0 PSH/day) without utility backup.
- Applicant calculating the qualifying system size before filing IRS Form 5695 to claim the 30% federal Residential Clean Energy Credit for tax year 2025.
Average Peak Sun Hours & Estimated System Size by U.S. City (NREL NSRDB, 80% efficiency, 400 W panels)
| City / State | PSH (h/day) | Avg. Monthly kWh | Estimated System Size (kW) |
|---|---|---|---|
| Phoenix, AZ | 6.5 | 1,114 | 7.2 |
| Las Vegas, NV | 6.4 | 931 | 6.1 |
| Miami, FL | 5.6 | 1,142 | 8.5 |
| Houston, TX | 5.3 | 1,176 | 9.3 |
| Denver, CO | 5.5 | 696 | 5.3 |
| Atlanta, GA | 4.7 | 1,040 | 9.2 |
| Chicago, IL | 4.4 | 736 | 6.9 |
| New York City, NY | 4.1 | 614 | 6.3 |
| Portland, OR | 3.9 | 862 | 9.2 |
| Seattle, WA | 3.5 | 981 | 11.7 |
Fuente: NREL National Solar Radiation Database (NSRDB) & U.S. EIA Average Monthly Residential Electricity Consumption (2024). System size calculated at 80% efficiency with 400 W panels, rounded up.
How it works
How It's Calculated
The standard residential PV sizing formula (per IEC 61724-1 and NREL PVWatts methodology) is:
Panels required = Monthly kWh ÷ (Panel kWp × PSH × 30 days × System Efficiency)
Where:
- Monthly kWh = from your utility bill
- Panel kWp = panel wattage ÷ 1,000 (e.g., 400 W → 0.40 kWp)
- PSH = Peak Sun Hours at your location (h/day)
- 30 days = monthly average
- Efficiency = system derate factor (typically 0.75–0.85)
Total installed capacity (kWp) = Panels (rounded up) × Panel kWp
Estimated generation (kWh/month) = Total kWp × PSH × 30 × EfficiencyStep-by-step example — Houston, TX:
Panels = 900 ÷ (0.40 × 5.3 × 30 × 0.80)
Panels = 900 ÷ 50.88 = 17.69 → round up to 18 panels
Total kWp = 18 × 0.40 = 7.20 kWp
Generation = 7.20 × 5.3 × 30 × 0.80 = 916.8 kWh/month---
What Are Peak Sun Hours?
Peak Sun Hours (PSH) are not the same as daylight hours. PSH represents the number of equivalent hours per day when solar irradiance averages exactly 1,000 W/m² (1 kW/m²). A city with 5 PSH receives the same daily solar energy as 5 hours of perfect noon-intensity sun, even if it's daylight for 12+ hours. PSH values come from satellite irradiance databases like NREL's National Solar Radiation Database (NSRDB) and NASA POWER.
Average PSH by U.S. State/City (NREL NSRDB)
| City / State | PSH (h/day) | Avg. Monthly kWh | Sized System kW* |
|---|---|---|---|
| Phoenix, AZ | 6.5 | 1,114 | 7.2 kW |
| Las Vegas, NV | 6.4 | 931 | 6.1 kW |
| Miami, FL | 5.6 | 1,142 | 8.5 kW |
| Houston, TX | 5.3 | 1,176 | 9.3 kW |
| Denver, CO | 5.5 | 696 | 5.3 kW |
| Atlanta, GA | 4.7 | 1,040 | 9.2 kW |
| Chicago, IL | 4.4 | 736 | 6.9 kW |
| New York City, NY | 4.1 | 614 | 6.3 kW |
| Portland, OR | 3.9 | 862 | 9.2 kW |
| Seattle, WA | 3.5 | 981 | 11.7 kW |
*Calculated at 80% efficiency, 400 W panels, rounded up.
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Why Divide by System Efficiency?
No solar system converts 100% of irradiance into usable AC electricity. Typical losses:
| Loss Source | Typical Range |
|---|---|
| Inverter (DC→AC conversion) | 3 – 6% |
| Wiring / connector resistance | 1 – 3% |
| Temperature derating (>25°C) | 5 – 12% |
| Soiling / dust accumulation | 2 – 5% |
| Partial shading | 0 – 10% |
| Module mismatch / degradation | 1 – 3% |
| Total system efficiency | 75 – 85% |
NREL PVWatts uses a default derate of ~0.83 for grid-tied systems. Using 0.80 is a practical conservative estimate that prevents undersizing.
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Common Sizing Errors
1. Using daylight hours instead of PSH. Phoenix gets ~14 hours of daylight in summer but only 6.5 PSH. Confusing these can undersize your system by 50% or more.
2. Skipping the efficiency factor. Dividing by 1.0 (100%) produces a figure that assumes lab-condition output — real systems generate 15–25% less.
3. Sizing to your lowest-consumption month. Spring bills are often the lowest of the year. Size for your annual average or your highest month if you want full offset year-round.
4. Not accounting for planned load growth. Adding an EV charger adds ~300–500 kWh/month; a heat-pump water heater adds ~50–100 kWh/month. NREL recommends over-sizing by 10–20% if major electric loads are anticipated.
5. Confusing panel W with system kW. A 400 W panel produces 0.4 kW at STC. A 7 kW system needs 7,000 ÷ 400 = 17.5 → 18 panels of that size.
Example: average U.S. home in a sunny climate
Frequently asked questions
What are Peak Sun Hours and how do I find mine?
Why does the formula use a system efficiency of 80% (dividing by 0.80)?
How many solar panels does my calculated kW translate to?
What is the 30% federal solar tax credit and how does system size affect it?
Should I size my solar system for summer peak or annual average consumption?
Does roof orientation and tilt affect the sizing calculation?
Can I use this calculator for a battery backup or off-grid system?
What is the average cost per kW of solar in the U.S. in 2025–2026?
How do solar panels perform on cloudy days?
How does temperature affect solar panel output?
Sources & references
Methodology & trust
Calculadora de construcción revisada por el equipo editorial de Hacé Cuentas, contrastada con NREL PVWatts Calculator — National Renewable Energy Laboratory, según nuestra política editorial y metodología.
Última revisión: June 20, 2026. Los parámetros se verifican periódicamente con las fuentes citadas.
Calculations run 100% in your browser. We do not store or transmit your data.
Indicative results. For critical decisions, consult a professional.
Rodríguez, M. (2026). Solar Panel kW System for Your Home. Hacé Cuentas. https://hacecuentas.com/solar-panel-kw-home-calculator
Contenido bajo licencia CC-BY 4.0 — reutilizable citando la fuente con enlace a Hacé Cuentas.