Calculate your ideal solar system size, estimated annual production, monthly savings, and payback period. Free solar calculator for homeowners and solar professionals.
Most solar calculators ask for a ZIP code, give you a vague "you could save $X per year" estimate, and immediately hand you off to a lead form. This one is different. SurgePV's Solar Calculator is designed for solar professionals and educated homeowners who want accurate, transparent results based on their actual inputs — not industry-average assumptions hidden behind a sales funnel.
Enter your monthly electricity usage, rate, location's peak sun hours, roof area, and panel specifications. The calculator returns seven distinct outputs: recommended system size, number of panels needed, estimated annual production, estimated monthly savings, simple payback period, 25-year cumulative savings, and CO2 offset in tons per year. Every formula is transparent and explained in the methodology section below.
Use this tool to understand whether solar makes sense for your situation before you ever contact an installer, or use it to validate the numbers on a proposal you've already received. The math is the same either way.
From system size and panel count through monthly savings, payback period, 25-year cumulative savings, and annual CO2 offset — everything you need to evaluate a solar installation from a single entry form.
Select your state to auto-populate average electricity rate and peak sun hours for your region, so you get accurate results immediately without needing to look up NREL data separately.
Every calculation is based on industry-standard formulas — no black-box algorithms. The methodology section documents exactly how each output is derived so you can verify or adjust the approach for your specific situation.
Use this tool before contacting a single installer. Know your expected system size, approximate cost (use $2.50–$3.50/W as a 2026 benchmark), and payback period going into any conversation. This prevents you from being quoted a system that is significantly over- or under-sized for your actual energy needs.
Received a solar proposal? Run the installer's numbers — system size, production estimate, and pricing — through this calculator using your own data. If the proposal's production estimate is more than 15% higher than the calculator's output, ask the installer to explain their assumptions. Inflated production estimates are a common issue in the industry.
Solar sales professionals can run this calculator live during a customer consultation to demonstrate system sizing logic in real time. Showing a customer exactly how their monthly usage translates into a system size, production estimate, and savings figure builds credibility and helps customers make informed decisions without pressure.
Output 1
Recommended System Size (kW)
The DC system size required to offset your monthly electricity consumption, calculated from usage, peak sun hours, and system efficiency. This is the starting point for all other outputs. If your roof area is insufficient, the calculator flags this and shows the maximum system size your roof supports instead.
Typical residential: 5–12 kWOutput 2
Number of Panels Needed
System size in watts divided by your selected panel wattage, rounded up to the nearest whole panel. This tells you how many panels will physically need to fit on your roof. A standard 400W panel occupies approximately 21–22 square feet; confirm your usable roof area accommodates this count.
Typical residential: 15–30 panelsOutput 3
Estimated Annual Production (kWh)
The kilowatt-hours of electricity your system is expected to produce in a typical year, calculated using system size, daily peak sun hours, system efficiency, and 365 days. Compare this to your annual electricity consumption to understand what percentage of your usage the system will offset — typically 80–110% for properly sized residential systems.
Typical: 6,000–15,000 kWh/yr for residentialOutput 4
Estimated Monthly Savings ($)
Annual production multiplied by your electricity rate, divided by 12. This is the average monthly bill reduction you can expect. Actual monthly savings vary seasonally — higher in summer when production peaks, lower in winter. This figure assumes net metering at full retail rate; if your utility has a reduced export rate (like California NEM 3.0), adjust accordingly.
Typical: $80–$250/monthOutput 5
Simple Payback Period (Years)
Net system cost divided by Year 1 annual savings. This is the most conservative payback estimate — it ignores rate escalation, which would shorten real payback by 1–3 years. For 2026 residential installs without ITC, typical payback is 9–14 years. Use our dedicated ROI Calculator for adjusted payback with rate escalation modeling.
Typical 2026: 9–14 years (no residential ITC)Output 6
25-Year Cumulative Savings ($)
A conservative estimate of total lifetime savings using a flat electricity rate (no escalation). This represents the worst-case total savings scenario. With 3% annual rate escalation (US historical average), real 25-year savings would be 30–45% higher than this figure. Use this as your conservative floor estimate.
Typical: $25,000–$80,000Output 7
CO2 Offset (Tons/Year)
Annual carbon dioxide emissions avoided by generating clean solar electricity instead of grid power. Calculated using the EPA's national average grid emission factor of 0.386 kg CO2 per kWh (2024). This equals approximately 8,800 miles not driven per year for a typical 8 kW system — a powerful environmental impact talking point for proposals.
Typical: 3–6 tons CO2/yearAll calculations are based on NREL's PVWatts methodology, adapted for instant single-page calculation. The formulas below show exactly how each output is derived from your inputs — fully auditable and consistent with industry-standard solar design practice.
System efficiency (performance ratio) of 0.80 accounts for: inverter losses ~4%, wiring losses ~2%, temperature derating ~3%, soiling ~2%, shading ~2%, module mismatch ~1%, other losses ~2%. Total: ~80% performance ratio, consistent with NREL PVWatts default.
Example: 8.33 kW × 4.5 PSH × 365 × 0.80 = 10,960 kWh/year
Assumes full net metering at retail rate. For reduced net metering (NEM 3.0, etc.), effective savings will be lower — adjust electricity rate input accordingly.
For escalated 25-year savings accounting for rate increases, use our Solar ROI Calculator. CO2 factor varies by state grid mix: coal-heavy states (WY, IN) have higher factors; hydro-heavy states (WA, OR) have lower factors. Use EPA eGRID for state-specific values.
Calculations sourced from SurgePV’s Solar Calculator — surgepv.com/tools/solar-calculator/
Recommended system size using 80% efficiency, state average peak sun hours, state average electricity rate. Assumes south-facing roof at optimal tilt. Cost estimates use $2.90/W installed (2026 national average).
| State | Avg Rate (¢/kWh) | Peak Sun Hours | 900 kWh/mo System | 1,200 kWh/mo System | 1,500 kWh/mo System | Est. Cost (900 kWh) | Annual Production |
|---|---|---|---|---|---|---|---|
| Arizona | 13.0¢ | 6.5 PSH | 5.8 kW | 7.7 kW | 9.6 kW | $16,820 | 13,505 kWh |
| California | 28.0¢ | 5.5 PSH | 6.8 kW | 9.1 kW | 11.4 kW | $19,720 | 10,962 kWh |
| Texas | 13.5¢ | 5.2 PSH | 7.2 kW | 9.6 kW | 12.0 kW | $20,880 | 10,886 kWh |
| Florida | 14.5¢ | 5.3 PSH | 7.1 kW | 9.5 kW | 11.8 kW | $20,590 | 10,950 kWh |
| New York | 22.0¢ | 4.0 PSH | 9.4 kW | 12.5 kW | 15.6 kW | $27,260 | 11,006 kWh |
| Massachusetts | 25.0¢ | 4.2 PSH | 8.9 kW | 11.9 kW | 14.9 kW | $25,810 | 10,967 kWh |
| Colorado | 15.0¢ | 5.4 PSH | 7.0 kW | 9.4 kW | 11.7 kW | $20,300 | 10,979 kWh |
| Washington | 11.5¢ | 3.8 PSH | 9.9 kW | 13.2 kW | 16.4 kW | $28,710 | 10,978 kWh |
| Hawaii | 38.0¢ | 5.7 PSH | 6.6 kW | 8.8 kW | 11.0 kW | $19,140 | 10,960 kWh |
| National Average | 16.0¢ | 4.5 PSH | 8.3 kW | 11.1 kW | 13.9 kW | $24,070 | 10,898 kWh |
Running your December bill through a solar calculator and using that single month as your usage input will significantly oversize your system. Always use a 12-month average. If you don't have 12 months of data, use your utility's online portal — most utilities display 12 months of consumption history for free. For new homes, use local average consumption data by home size from the EIA.
Payback period must use net system cost after incentives, not gross cost. If you're entering gross cost to calculate payback, you'll get an inflated payback period. Enter total project cost and then subtract any applicable incentives (state rebates, utility rebates, SREC credits) before using the result as your payback denominator. ITC for residential in 2026 is 0% — do not include it if your install date is in 2026.
A 22%-efficient solar panel does not produce 22% of all sunlight that hits it — that's the panel's conversion efficiency. System efficiency is the overall performance ratio accounting for all real-world losses: typically 75–85% for complete systems. Never input panel efficiency (22%) as system efficiency — a common beginner mistake that produces grossly incorrect production estimates.
This calculator uses your peak sun hours input to represent irradiance, which implicitly assumes optimal orientation (south-facing, 30° tilt in the US). If your roof faces east or west, reduce your effective PSH by 15–20%. A north-facing roof in the northern hemisphere may reduce effective PSH by 30–40%. For accurate site-specific estimates, use NREL PVWatts or Aurora Solar with your actual roof orientation and tilt data.
Solar system size (kW) = Monthly electricity usage (kWh) ÷ (Peak Sun Hours × 30 days × System Efficiency). For a home using 900 kWh/month in a 4.5 PSH location with 80% system efficiency: 900 ÷ (4.5 × 30 × 0.80) = 8.33 kW. Round up to account for real-world variability. This calculator performs this calculation automatically along with six other outputs the moment you enter your inputs.
Number of panels = System Size (W) ÷ Panel Wattage (round up). For an 8.33 kW system using 415W panels: 8,330 ÷ 415 = 20.1, rounded up to 21 panels. Most residential homes need 15–30 panels depending on energy usage, panel wattage, and location. Higher-wattage panels (430–440W) reduce panel count and may be preferred for space-constrained roofs. Verify your roof has enough usable area — each standard panel needs approximately 21–22 square feet including spacing.
Peak sun hours (PSH) represent the equivalent number of full-sun hours per day when solar irradiance averages 1,000 W/m² (the standard test condition for solar panels). A location with 4.5 PSH receives the same total daily solar energy as 4.5 hours of direct noon sun. PSH is the most location-sensitive variable in solar sizing — Arizona averages 6.5 PSH while Seattle averages only 3.5 PSH. The same 8 kW system produces ~18,980 kWh/year in Arizona versus only ~10,220 kWh/year in Seattle. PSH data is available from NREL's National Solar Radiation Database (NSRDB).
This calculator is accurate to within ±10–15% for typical residential installations with accurate inputs. The main sources of error are: (1) PSH variation year-to-year and micro-climate differences not captured in regional averages; (2) shading not accounted for in a simple PSH input; (3) roof orientation and tilt deviating from optimal south-facing 30°; (4) panel degradation over time not modeled in Year 1 estimates. For permit-level accuracy, use NREL PVWatts or Aurora Solar with your actual roof geometry, orientation, and measured shading data.
The default of 80% (performance ratio of 0.80) is appropriate for most residential string inverter systems in average conditions. Use 82–85% for microinverter systems or power optimizer systems in low-shading environments. Use 75–78% for older equipment, systems with significant shading, or string inverter systems in very hot climates where temperature derating is pronounced. NREL PVWatts defaults to 86% for DC-AC ratio adjustments — if comparing, account for this difference in methodology.
Electricity rate directly multiplies your annual production to determine savings — so it has a linear impact on ROI. A customer paying $0.28/kWh (California) saves 75% more per kWh than a customer paying $0.16/kWh (national average) for the exact same system producing the same kWh. This is why solar ROI varies so dramatically by state: Hawaii and California have some of the best solar economics in the country not primarily because of sun (though that helps) but because of high electricity rates.
Yes — this calculator is accurate enough to give you a strong ballpark for system size, production, and savings before contacting any installer. It will tell you if a 6 kW system is approximately right for your usage or if you actually need a 10 kW system. This prevents you from accepting a dramatically over- or under-sized proposal. For a binding financial commitment, always review the installer's full proposal with site-specific production modeling from PVWatts, Aurora Solar, or a comparable professional design tool.
Watch how to use your electricity bill, local peak sun hours, and roof data to get accurate system size and savings estimates in under 3 minutes.
Solar Calculator — Full Walkthrough
Learn how to calculate recommended system size, number of panels, annual production, monthly savings, payback period, and CO2 offset from your real utility bill data.
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