Enter your monthly electricity bill or annual kWh usage to instantly calculate the right solar system size, panel count, and 25-year savings projection for any U.S. location.
Sizing a solar system correctly is the foundation of every successful solar project. Too small and your client won't offset enough of their bill; too large and you're wasting roof space and budget. This free calculator uses actual energy consumption data combined with location-specific peak sun hour (PSH) data to deliver an accurate system size recommendation in seconds.
Whether you're a solar installer doing a quick field estimate, a designer building a formal proposal, or a homeowner exploring feasibility, this tool gives you the numbers you need. It factors in a standard 80% system derate accounting for inverter losses, wiring losses, soiling, and temperature derating, and projects 25-year savings using utility rate escalation and panel degradation curves.
All calculations follow industry-standard methodologies aligned with PVWatts and NEC sizing guidelines so you can present results to clients with confidence.
Enter a monthly dollar amount or annual kWh. The calculator handles either format and normalizes to annual energy demand automatically.
Select any U.S. state to apply accurate peak sun hour averages derived from NREL datasets so system size reflects real-world solar resource.
Projects lifetime system value using 3% annual utility rate escalation and 0.5% per-year panel degradation for a realistic long-term ROI picture.
Input your average monthly electricity bill in dollars or your annual kWh consumption. If using the bill amount, also enter your utility's average rate per kWh so the calculator can convert to energy usage accurately.
Choose the installation state from the dropdown. This applies location-specific peak sun hour data so system size accounts for actual solar irradiance in that region. Arizona needs a smaller system than Maine for the same energy output.
Adjust the solar offset slider, typically 80-100% for grid-tied systems. A 100% offset means the system generates as much as the home consumes annually. Consider setting lower if the client plans future load additions.
Input the wattage of the panels you plan to install. Standard residential panels range from 380W to 440W. Higher wattage panels mean fewer panels needed for the same system size, important when roof space is limited.
The calculator instantly displays system size in kW, number of panels, estimated annual generation, and projected 25-year savings. Use these numbers as the starting point for proposals and client conversations.
This calculator uses the standard solar industry sizing methodology, consistent with PVWatts and NEC guidelines. All formulas are transparent so designers and sales professionals can verify every number.
System_kW = Annual_kWh x offset% / (PSH_per_day x 365 x 0.80)Panel_Count = CEIL(System_kW x 1000 / Panel_Wattage)Annual_Gen = System_kW x PSH_per_day x 365 x 0.80Inverter Efficiency: 0.96
DC Wiring Losses: 0.98
AC Wiring Losses: 0.99
Soiling/Dirt Losses: 0.95
Temperature Derating: 0.92
Mismatch Losses: 0.97
Combined Derate Factor: ~0.80For each year Y from 1 to 25:
Generation_Y = Annual_Gen x (1 - 0.005)^Y // 0.5%/yr degradation
Rate_Y = Rate_Year0 x (1 + 0.03)^Y // 3% rate escalation
Savings_Y = Generation_Y x Rate_Y
Total_25yr_Savings = Sum of all Savings_YWorked example: A California home uses 12,000 kWh/year (San Diego: 5.5 peak sun hours/day). System size: 12,000 / (5.5 × 365 × 0.80) = 7.47 kW. At 400W panels: ceil(7,470 / 400) = 19 panels. Annual generation: 7.47 × 5.5 × 365 × 0.80 = 12,007 kWh — offsets 100% of consumption. With $0.28/kWh retail rate: saves $3,362/year.
Calculations sourced from SurgePV’s System Size Calculator — surgepv.com/tools/system-size-calculator/
| State | Avg Peak Sun Hours/Day | Notes |
|---|---|---|
| Arizona (AZ) | 6.5 | Highest PSH in the continental U.S. |
| New Mexico (NM) | 6.2 | Excellent solar resource |
| Nevada (NV) | 6.0 | Desert Southwest high irradiance |
| California (CA) | 5.8 | Varies: coastal 4.5, inland 6.5+ |
| Hawaii (HI) | 5.7 | High rates make solar highly valuable |
| Colorado (CO) | 5.5 | High altitude boosts irradiance |
| Texas (TX) | 5.5 | West TX higher than Gulf Coast |
| Florida (FL) | 5.3 | High humidity reduces slightly |
| Georgia (GA) | 5.0 | Southeast average |
| North Carolina (NC) | 4.9 | Strong solar policy state |
| Virginia (VA) | 4.7 | Mid-Atlantic average |
| New Jersey (NJ) | 4.6 | High SREC value compensates |
| New York (NY) | 4.5 | Upstate lower than NYC metro |
| Massachusetts (MA) | 4.3 | High rates improve economics |
| Michigan (MI) | 4.2 | Great Lakes cloudiness factor |
| Oregon (OR) | 4.0 | Willamette Valley vs Eastern OR |
| Washington (WA) | 3.8 | West of Cascades cloud cover |
| Alaska (AK) | 3.0 | Lowest PSH; extreme seasonal variation |
One month's bill is unreliable. Seasonal variation from summer AC and winter heat can skew sizing by 30% or more. Always pull a full 12-month usage history from the utility or the client's online account before finalizing system size.
If the client plans to add an EV (2,000-4,000 kWh/yr) or pool (2,500-5,000 kWh/yr) within 2-3 years, size for future consumption. Oversizing slightly now is far cheaper than a future system expansion requiring a new permit and interconnection application.
In states with unfavorable net metering such as post-NEM 3.0 California, exporting excess generation yields very little credit. Sizing to 85-90% offset can improve payback period by avoiding excess generation that earns minimal export rates.
The 0.80 system efficiency factor is an industry baseline. Adjust downward for heavily shaded roofs, long wire runs, or extreme climates. Use your design software's actual performance ratio when available, which may range from 0.75 to 0.85.
The average U.S. residential solar system is 7-10 kW DC, generating roughly 9,000-13,000 kWh per year depending on location. A 2,000 sq ft home in the Southeast typically needs 8-9 kW; the same home in Arizona might need only 6-7 kW due to higher sun hours.
Plan for approximately 20-22 square feet per panel including racking clearance and setbacks. A 400W panel is typically 21.5 sq ft (about 3.5 x 6 ft). A 10 kW system with 25 panels at 400W needs roughly 540 sq ft of usable, unshaded south-facing roof area.
Panel ratings are measured under Standard Test Conditions (25°C, 1,000 W/m²). Real-world losses include inverter efficiency (~96%), wiring losses (~2%), soiling (~5%), and temperature derating (~8%). Combined: 0.96 × 0.98 × 0.95 × 0.92 × 0.97 = 0.80. Without this derate a 7.5 kW rated system would actually deliver only ~6 kW of usable power — a 20% shortfall on promised energy offset.
Always ask clients about planned additions. EVs typically add 2,000-4,000 kWh/year and pools add 2,500-5,000 kWh/year. If major load additions are likely within 2-3 years, size for future usage now. The incremental cost of 1-2 extra panels at installation is far less than a future expansion requiring a new permit and interconnection application.
For state estimates use our quick reference table (e.g., CA = 5.8, TX = 5.5, AZ = 6.5). For project-specific work use NREL PVWatts with exact address, tilt, and azimuth. A 10 kW system in Phoenix (6.5 PSH) generates 10 × 6.5 × 365 × 0.80 = 18,980 kWh/year; the same system in Seattle (3.8 PSH) generates only 11,096 kWh/year — 42% less output for identical installed cost.
The EIA reports residential electricity prices have risen approximately 2.5-3.5% annually over the past 20 years. A 3% escalation rate is the standard industry assumption for residential solar financial modeling. Markets with historically higher rate increases such as California, Hawaii, and New England may justify using 4-5%. Always disclose this assumption to clients as it is a projection, not a guarantee.
Most premium solar panels are warranted to produce at least 80-87% of rated output after 25 years, implying 0.5-0.8% per-year degradation. This calculator uses 0.5%/year as the conservative baseline. A 400W panel today would produce approximately 351W in year 25 (400 x 0.995 to the power of 25).
Yes, significantly. Under full retail net metering (NEM 1.0/2.0) sizing to 100% offset maximizes savings. Under NEM 3.0 or avoided-cost policies, export credits are much lower at about $0.05-0.08/kWh versus $0.25-0.35 retail, so oversizing beyond self-consumption yields minimal additional savings. Always confirm the client's net metering policy before finalizing system size.
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