Solar Panel Calculator — Free Online Calculator
Calculate how many solar panels you need based on your electricity usage, location, and panel wattage. Free solar sizing tool with cost estimates.
How to Use This Calculator
Enter your average monthly electricity usage in kWh (from your utility bill), select your area's peak sun hours, choose a panel wattage, and how much of your usage you want to offset. The calculator determines how many panels you need and estimates system cost.
The Formula Explained
Solar system sizing starts with your daily energy need: Daily kWh = Monthly kWh / 30. The system size accounts for production losses (~20%): System kW = Daily kWh / (Sun Hours × 0.8). Number of panels = System kW × 1000 / Panel Watts. Cost estimates use $2.50–$3.50 per watt installed (national average before the 30% federal tax credit).
Solar System Size by Monthly Usage (5 Peak Sun Hours, 400W Panels)
| Monthly kWh | Panels | System Size | Est. Cost |
|---|---|---|---|
| 500 kWh | 6 | 2.4 kW | $6,000–$8,400 |
| 750 kWh | 8 | 3.2 kW | $8,000–$11,200 |
| 1000 kWh | 11 | 4.4 kW | $11,000–$15,400 |
| 1500 kWh | 16 | 6.4 kW | $16,000–$22,400 |
| 2000 kWh | 21 | 8.4 kW | $21,000–$29,400 |
Sizing a Solar System: The Complete Picture
Solar system sizing is both simpler and more complex than most first-time buyers realize. The simple part: you need enough panels to produce roughly the annual kWh you use. The complex part: figuring out what "enough" means involves climate, orientation, shading, future consumption changes, net metering rules, budget constraints, and roof space limitations. The calculator above gives you the starting point; refining the design requires balancing all the variables.
The core formula: system size in kW = annual kWh consumption / (sun-hours × system efficiency × 365). Sun-hours range from 3.5 in cloudy northern areas to 6.5+ in desert southwest. System efficiency (panel to grid) is typically 80-85% after losses from wiring, inverter conversion, temperature, dust, and degradation. An Arizona home using 14,000 kWh per year at 6.0 sun-hours and 82% efficiency needs about 7.8 kW of panels. A Seattle home with the same consumption needs about 11.5 kW — nearly 50% larger — to produce the same annual energy.
Worked Example: Arizona Full Offset
An Arizona home uses 14,500 kWh annually. Phoenix area: 6.2 sun-hours, 82% system efficiency. Target: 100% offset.
System size: 14,500 / (6.2 × 0.82 × 365) = 14,500 / 1,855 = 7.8 kW. Round up to 8 kW standard size. At 400W panels: 20 panels. At 450W panels: 18 panels.
Roof space required: about 500 square feet for 20 panels (25 sq ft each including racking spacing). Most homes have ample south-facing roof for this. Cost in 2026: about 20,000 USD after 30% tax credit. Annual production: 8 × 1,850 = 14,800 kWh. Full offset achieved with slight excess for future EV charging.
Payback at 15 cents per kWh Arizona rate: 20,000 / (14,800 × 0.15) = 9 years. Not as fast as California due to lower rates, but still positive with 16+ years of free electricity after payback and 45,000+ USD in lifetime savings.
Worked Example: Midwest Partial Offset
An Ohio home uses 16,000 kWh annually. Cleveland area: 4.3 sun-hours, 80% efficiency. Available roof space: 400 sq ft on south face (enough for about 16 panels).
100% offset size: 16,000 / (4.3 × 0.80 × 365) = 16,000 / 1,256 = 12.7 kW. That is 32 × 400W panels, requiring 800 sq ft — twice the available roof.
Practical option: install 16 × 400W panels = 6.4 kW on the south roof. Annual production: 6,400 × 1,256 / 1000 = 8,040 kWh. Covers 50% of consumption. Payback at 13 cents per kWh: 16,000 / (8,040 × 0.13) = 15 years (before rate escalation). Not amazing but still worthwhile when you consider home value increase and 10+ years of savings after payback.
Better option for this home: add east or west roof space if available, or consider a ground-mount array for the remainder. Or accept partial offset and combine with energy efficiency improvements (LED lighting, heat pump, improved insulation) to reduce consumption to match the 6.4 kW production.
Five Panel Sizing Mistakes
1. Using one year of old bills when consumption is about to change. Planning an EV purchase next year? Adding a heat pump? Having a baby? Use projected future consumption, not historical. A 10 kW system that perfectly matches current use becomes 70% of need when you add an EV.
2. Not factoring in net metering rules. Some utilities cap system size at 100% of historical usage. Others allow oversizing up to 120% or 150%. Over-sized systems on NEM 3.0 rules may not pay back the extra panels. Check your local rules before final sizing.
3. Ignoring roof orientation and shading. A 10 kW system on a north-facing shaded roof produces maybe 60% of its rated output. A 7 kW system on a perfect south-facing roof produces nearly its full rating. Nameplate watts mean nothing without real-world production context.
4. Mixing panel brands or types. String inverters require all panels in a string to be the same model. Mixing creates imbalances that hurt production. Microinverters are more forgiving but still cleaner with uniform panels.
5. Forgetting roof age. Installing solar on a 15-year-old asphalt roof means you will remove the panels when the roof needs replacement in 5-10 years, paying thousands for removal and reinstallation. If roof has less than 15 years remaining, replace the roof before installing solar.
Production Factors by Region
Annual kWh per installed kW (production ratio):
Phoenix, AZ: 1,850. Los Angeles, CA: 1,650. San Diego, CA: 1,700. Austin, TX: 1,550. Denver, CO: 1,600. Miami, FL: 1,500. Atlanta, GA: 1,400. Chicago, IL: 1,300. Boston, MA: 1,300. New York, NY: 1,300. Seattle, WA: 1,100. Portland, OR: 1,200. Anchorage, AK: 950.
These are "TMY" (typical meteorological year) values at optimal tilt and azimuth. Real installations with suboptimal orientation lose 5-20% from these numbers. Shaded panels can lose 50%+ for the affected strings. Always use NREL PVWatts for site-specific analysis rather than these rough averages.
Panel count for common residential needs:
Offset 6,000 kWh/year (modest home): 10-15 panels in sunny climate, 18-25 in cloudy. Offset 12,000 kWh/year (average home): 18-25 panels in sunny, 30-40 in cloudy. Offset 20,000 kWh/year (large home or EV): 30-40 panels in sunny, 50-65 in cloudy.
Design Tools and Standards
NREL PVWatts — the authoritative source for solar production estimates. Free, web-based, used by every serious installer. NREL SAM (System Advisor Model) — detailed financial modeling including all cost categories, incentives, and 25-year projections. HelioScope and Aurora Solar — commercial design software with 3D modeling, shade analysis, and detailed string sizing.
For permit-quality designs, always work with an installer or engineer familiar with local requirements. NEC Article 690, Article 691 (large-scale PV), and Article 705 (interconnected power production sources) all apply to grid-tied solar. Local jurisdictions have additional requirements — fire code setbacks, structural load calculations, and utility interconnection specifications. The calculator gives you a starting estimate; permitted installation requires professional engineering.
Solar panel sizing: from kWh usage to a real system size
Sizing a solar PV system is two questions stacked together. First, how much energy do you actually use? Take 12 months of utility bills, average the daily kWh, and that is your target offset. Second, how much sun does your location and roof receive? NREL calls this the solar resource, measured in peak sun hours per day, ranging from 3.5 in Seattle to 6.5 in Phoenix.
The calculator multiplies your daily kWh by an inverse efficiency factor and divides by peak sun hours to give the DC system size. From there it estimates panel count using a typical 400 W residential module, and rough install cost at current EnergySage averages.
The formula and what it does
System efficiency captures real-world losses: inverter (3-5 percent), DC wiring (1-2 percent), soiling (2-5 percent), shading and azimuth deviation from south (variable). NREL PVWatts uses 14 percent total loss as default, so the calculator uses 0.86 efficiency unless you override. Peak sun hours is your location-specific value from NREL or the National Solar Resource Database.
Worked example
Scenario: Home in Austin, Texas, average monthly usage 1,100 kWh.
Daily usage: 1,100 / 30.4 = 36.2 kWh/day. Austin peak sun hours: 5.4 (NREL). System efficiency factor: 0.86 typical. kW_DC = 36.2 / (5.4 x 0.86) = 7.79 kW DC. With 400 W modules: 20 panels. At Texas $2.55/W installed (EnergySage Q1 2026 average): 9,860 gross. After 30 percent federal Residential Clean Energy Credit: 3,902 net. Year 1 production at PVWatts: 11,200 kWh, saving ,608 at Austin Energy 14.4 cents/kWh average. Payback: 8.6 years before any state or utility incentives.
Code references and standards
NEC 690.7 sets maximum system voltage, must be below the inverter input limit (typically 600 V residential or 1000 V commercial). Module Voc x string count must be calculated at the minimum design temperature for the location, not at STC.
NEC 690.8 conductor sizing at 156 percent of Isc (1.25 continuous x 1.25 irradiance correction).
NEC 705.12 120 percent rule for interconnection at the main panel busbar: PV breaker plus main breaker must total no more than 120 percent of busbar rating. 200 A panel: max sum 240 A, so 200 A main + 40 A solar breaker max for backfeed.
Approximate solar payback by state (6 kW system, average insolation)
| State | Avg install $/W (2026) | Net cost after 30% ITC | Yr 1 savings | Payback |
|---|---|---|---|---|
| California | $3.20 | 3,440 | $2,100 | ~6.4 yr |
| Massachusetts | $3.45 | 4,490 | $2,250 | ~6.4 yr |
| Arizona | $2.65 | 1,130 | ,450 | ~7.7 yr |
| Texas | $2.55 | 0,710 | ,180 | ~9.1 yr |
| Florida | $2.50 | 0,500 | ,150 | ~9.1 yr |
| New York | $3.30 | 3,860 | ,950 | ~7.1 yr |
Source: EnergySage Solar Marketplace Q1 2026 averages, NREL PVWatts annual production estimates, and current state net-metering policies. Federal Residential Clean Energy Credit (Section 25D) is 30 percent through 2032.
Common mistakes to avoid
Sizing to annual kWh average without seasonality. Summer production is much higher than winter at most latitudes. A system sized for annual offset will overproduce in summer and underproduce in winter; net metering smooths this but only if your state allows it.
Forgetting shading. Even one shaded panel in a string drags down the whole string output dramatically. Use microinverters or DC optimizers (Enphase, SolarEdge) where shading is unavoidable.
Assuming flat 1.0 production-to-rated. A 7 kW DC system does not produce 7 kWh per peak sun hour. Realistic ratio is 0.78-0.86 of DC nameplate after all losses.
Frequently asked questions
How do I find my peak sun hours?
NREL PVWatts (free, gov.nrel.gov/pvwatts) gives location-specific solar resource data. Enter your zip and it returns monthly and annual peak sun hours plus production estimates.
Should I oversize the system?
Two reasons to: anticipated future load (EV, heat pump, electric appliances), and net-metering policies that allow annual roll-over. Reason not to: most states cap net-metering at 100-110 percent of consumption, so excess is lost or paid at wholesale.
Do panels degrade?
Yes, about 0.5 percent per year for quality modules. A 25-year-old system produces about 87 percent of its original output. Tier-1 manufacturers (Q-Cells, REC, Panasonic) warranty 80-85 percent at 25 years.
What is the difference between DC kW and AC kW?
DC kW is the sum of panel STC ratings. AC kW is what the inverter outputs. DC-to-AC ratio is typically 1.15-1.25 (oversized PV vs inverter), since inverters cost more and panels rarely produce full STC.
How much roof space does this need?
A 400 W module is roughly 21 sq-ft. So a 7 kW DC system (17-18 panels) needs about 360 sq-ft of clear south or west-facing roof. East works too at slightly lower production.
Do I need batteries?
Not for grid-tie. Net metering effectively uses the grid as a battery. Storage makes sense for: blackout backup, time-of-use rate arbitrage (peak vs off-peak), or off-grid. Adds 0-15K to install for typical Powerwall-class storage.
How long is the federal tax credit?
The Residential Clean Energy Credit (Section 25D) is 30 percent through 2032, dropping to 26 percent in 2033 and 22 percent in 2034. After 2034 it expires unless extended.