Solar Panel kW System for Your Home
This calculator determines the solar panel system size (in kilowatts, kW) you need to offset your home's electricity consumption. It uses your monthly energy usage in kilowatt-hours (kWh) and the average daily Peak Sun Hours (PSH) at your location to compute the minimum DC capacity required. The core formula is: Required kW = (Monthly kWh ÷ 30) ÷ Peak Sun Hours × Efficiency Correction (÷ 0.80). Use it when comparing installer quotes, applying for federal tax credits (IRS Form 5695), or sizing battery backup systems. According to the U.S. Energy Information Administration, the average American household consumes about 899 kWh/month, requiring roughly a 6–7 kW system in a location with 5 PSH/day.
When to use this calculator
- Homeowner comparing two 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.
- Applicant calculating the qualifying system size before filing IRS Form 5695 to claim the 30% federal Residential Clean Energy Credit for tax year 2025.
- Off-grid cabin owner determining whether a 4 kW array is sufficient to power a 400 kWh/month load in a high-sun rural Nevada location (6.0 PSH/day) without utility backup.
Calculation Example
- Example
- Result
How it works
3 min readHow It's Calculated
The two-step formula converts monthly consumption into a real-world DC system size, accounting for inevitable energy losses in wiring, inverters, and temperature derating (collectively modeled as an 80% system efficiency factor):
Step 1 – Daily energy needed:
Daily kWh = Monthly kWh ÷ 30
Step 2 – Required DC capacity:
System kW = Daily kWh ÷ Peak Sun Hours (PSH) ÷ 0.80
Full formula:
System kW = (Monthly kWh ÷ 30 ÷ PSH) ÷ 0.80Why 0.80? The National Renewable Energy Laboratory (NREL) PVWatts model uses a default DC-to-AC derate factor of ~0.83, but real installations typically land at 0.75–0.82 due to soiling, shading, wiring losses, and inverter inefficiency. Using 0.80 is a widely accepted conservative standard for residential sizing.
Peak Sun Hours ≠ daylight hours. PSH is the number of equivalent hours per day when solar irradiance averages 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, regardless of whether it's actually sunny for 10 hours.
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Reference Table
Average monthly residential electricity consumption and peak sun hours by U.S. state/city (EIA 2023 data + NREL solar resource data):
| State / City | Avg. Monthly kWh | Avg. PSH/Day | Calculated System kW* |
|---|---|---|---|
| Louisiana (Baton Rouge) | 1,273 | 4.9 | 10.8 kW |
| Texas (Houston) | 1,176 | 5.3 | 9.3 kW |
| Florida (Miami) | 1,142 | 5.6 | 8.5 kW |
| Arizona (Phoenix) | 1,114 | 6.5 | 7.2 kW |
| Georgia (Atlanta) | 1,040 | 4.7 | 9.2 kW |
| National Average | 899 | 5.0 | 7.5 kW |
| California (Los Angeles) | 689 | 5.8 | 5.0 kW |
| New York (New York City) | 614 | 4.1 | 6.3 kW |
| Washington (Seattle) | 981 | 3.5 | 11.7 kW |
| Colorado (Denver) | 696 | 5.5 | 5.3 kW |
*Calculated using: Monthly kWh ÷ 30 ÷ PSH ÷ 0.80, rounded to one decimal.
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Typical Case Examples
Example 1 — Average U.S. Home (Phoenix, AZ)
Example 2 — High-Consumption Home (Seattle, WA)
Example 3 — Off-Grid Cabin (Rural Nevada)
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Common Errors
1. Using daylight hours instead of Peak Sun Hours. Phoenix gets ~14 hours of daylight in summer but only 6.5 PSH. Confusing these two values can undersize your system by 50% or more, leaving you with an unexpected utility bill.
2. Ignoring the efficiency/derate factor (÷ 0.80). Skipping this step produces an "ideal" DC figure that assumes perfect panels operating at lab conditions. Real-world soiling, heat, and inverter losses reduce actual output by 15–25%. Always apply the derate.
3. Sizing to a low-consumption month. Many homeowners calculate based on their spring electric bill (the lowest of the year) and end up with a system that can't handle summer A/C loads. Use your highest monthly consumption — or average all 12 months — for a balanced design.
4. Forgetting to account for future load growth. Adding an electric vehicle (EV) charger adds ~300–500 kWh/month. Adding a heat pump water heater adds ~50–100 kWh/month. NREL recommends over-sizing the array by 10–20% if major electric appliances are anticipated within 5 years.
5. Using panel wattage instead of system kW. A single 400 W panel produces 0.4 kW at STC. A "6 kW system" means 15 × 400 W panels, not 6 panels of any size. Always convert your system kW result: Number of panels = System kW × 1,000 ÷ Panel wattage (W).
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Related Calculators
Solar sizing connects to several other home energy calculations. While no direct related calculators are currently linked for this tool, useful companion calculations include energy cost savings estimators, battery backup sizing tools, and federal tax credit (IRS Form 5695) benefit calculators — all available on Hacé Cuentas.
Frequently asked questions
What are Peak Sun Hours and how do I find mine?
Peak Sun Hours (PSH) represent the equivalent daily hours of solar irradiance at exactly 1,000 W/m². They are NOT the same as daylight hours. The NREL PVWatts Calculator (pvwatts.nrel.gov) lets you enter any U.S. ZIP code and retrieve average annual and monthly PSH values. Typical U.S. ranges: 3.0–4.0 PSH (Pacific Northwest), 4.5–5.5 PSH (Southeast/Midwest), 5.5–7.0 PSH (Southwest/Desert states).
Why does the formula divide by 0.80?
The 0.80 factor represents system efficiency — accounting for real-world losses including inverter conversion loss (~4–6%), wiring resistance (~2–3%), panel soiling and shading (~3–5%), temperature derating (~5–10%), and module mismatch (~2%). NREL's PVWatts uses a similar composite derate of 0.83 for typical grid-tied residential systems. Using 0.80 gives a conservative, safe sizing estimate that avoids under-producing.
What is the 30% federal solar tax credit and how does system size affect it?
The Residential Clean Energy Credit (Section 25D of the Internal Revenue Code) allows homeowners to deduct 30% of total solar installation costs from their federal tax liability through 2032, stepping down to 26% in 2033 and 22% in 2034. A larger system costs more and therefore generates a larger credit. For example, a 7.5 kW system costing $22,500 yields a $6,750 credit (filed on IRS Form 5695). There is no cap on system size for this credit.
How many solar panels do I actually need for a calculated system size?
Divide your required system kW by individual panel wattage: Number of panels = (System kW × 1,000) ÷ Panel wattage. Using today's common 400 W residential panels: a 6 kW system needs 15 panels; an 8 kW system needs 20 panels; a 10 kW system needs 25 panels. Higher-efficiency panels (420–440 W) can reduce panel count by 1–2 units for the same system output.
Should I size my system for summer peak or annual average consumption?
It depends on your utility's billing structure. If you have net metering (most U.S. states), size for your annual average consumption — excess summer production banks credits that offset winter bills. If you lack net metering or are going off-grid, size for your highest monthly consumption (typically July or August for A/C-heavy households, or January for electric-heat homes). Over-sizing by 10–15% is generally recommended when net metering policies are uncertain.
Does roof orientation and tilt angle affect how many kW I need?
Yes. This calculator assumes an ideally south-facing roof at the optimal tilt angle for your latitude (roughly equal to your latitude in degrees — e.g., 33° for Atlanta, GA). East or west-facing roofs produce approximately 15–20% less energy than a true south orientation. If your roof is east- or west-facing, increase your calculated system kW by 15–20% to compensate. North-facing roofs in the U.S. are generally not viable for solar generation.
Can I use this calculator for a battery backup or off-grid system?
Yes, but with an important addition. This calculator sizes the solar array only. For off-grid or battery backup, you also need to size battery storage using: Battery kWh = Daily kWh × Backup Days ÷ Depth of Discharge (DoD). Most lithium iron phosphate (LiFePO4) batteries allow 80–90% DoD; lead-acid batteries should not exceed 50% DoD. For example, 30 kWh/day × 1.5 backup days ÷ 0.85 DoD ≈ 53 kWh of battery capacity needed (roughly 4 Tesla Powerwall 3 units at 13.5 kWh each).
What is the average cost per kW of solar in the U.S. in 2025?
According to Lawrence Berkeley National Laboratory's Tracking the Sun report, the median installed cost for residential solar in the U.S. was approximately $3.00–$3.50 per DC watt in 2024–2025, before the 30% IRS federal tax credit. That translates to $21,000–$24,500 for a 7 kW system gross, or $14,700–$17,150 net of the credit. Costs vary by state: California and New York average higher (~$3.50–$4.00/W), while Texas and Florida tend to run lower (~$2.50–$3.00/W).