Solar panel calculator

How solar panels work: converting sunlight into electricity

Solar panels - also called photovoltaic (PV) panels - convert sunlight directly into direct current (DC) electricity using the photovoltaic effect. Each panel is made up of dozens of individual solar cells, typically constructed from silicon wafers. When photons from sunlight strike the silicon, they knock electrons loose, creating an electric current.

A complete residential solar system includes more than just panels. An inverter converts the DC electricity produced by the panels into alternating current (AC) electricity that powers your home. A production meter tracks how much electricity your system generates. The utility net meter measures the difference between what you produce and what you consume - running forward when you draw from the grid, backward when you send excess solar power back.

When your panels produce more electricity than your home needs - which commonly happens on sunny afternoons when you are at work - the surplus flows to the grid. Under net metering policies (available in most US states), your utility credits you for that exported power, usually at the retail rate. Those credits reduce your monthly bill, and in high-production months, your bill can drop to zero or even go negative.

The path of solar energy in your home

Sunlight hits the panels on your roof. The panels generate DC power. The inverter converts DC to AC. That AC power flows through your home's electrical panel, powering anything running at that moment - refrigerator, lights, TV, air conditioning. Any surplus exits via your meter to the grid. After sunset, your home draws from the grid as normal. Your monthly bill reflects only the net difference between production and consumption.

Battery storage (such as the Tesla Powerwall or Enphase IQ Battery) adds a step: excess solar energy charges the battery during the day, and the battery powers your home after sunset or during a grid outage. Batteries add $10,000–$15,000 to system cost but provide backup power and can maximize self-consumption in states where net metering rates have been reduced.

Peak sun hours: the most important variable in solar sizing

Peak sun hours are the most commonly misunderstood concept in solar sizing. They do not mean hours of daylight. A peak sun hour is defined as one hour during which solar irradiance averages 1,000 watts per square meter (W/m²) - full, direct sunlight. A typical day might have 10–14 hours of daylight but only 4–5 peak sun hours because clouds, atmosphere, and low sun angles reduce the intensity below the 1,000 W/m² threshold for much of the day.

Peak sun hours vary dramatically by location and season:

• Phoenix, AZ: ~6.5 hours/day annual average - one of the highest in the US
• Los Angeles, CA: ~5.5–6.0 hours/day
• Dallas, TX: ~5.0–5.5 hours/day
• Atlanta, GA: ~4.5–5.0 hours/day
• Chicago, IL: ~4.0–4.5 hours/day
• New York, NY: ~4.0 hours/day
• Seattle, WA: ~3.5–4.0 hours/day - lowest in the contiguous US

The NREL PVWatts Calculator (pvwatts.nrel.gov) provides precise peak sun hour estimates for any US ZIP code based on 30 years of NASA satellite weather data. It is the industry standard tool for solar production estimation and is freely available to anyone.

For this calculator, the default of 4.5 hours/day represents a reasonable US continental average. If you are in the Southwest, use 5.5–6.5. If you are in the Pacific Northwest or New England, use 3.5–4.0 for more conservative estimates.

How to size a solar system for your home

Proper system sizing ensures your solar installation meets your actual needs without over-investing in capacity you cannot use. The sizing process starts with your electricity consumption data and works backward to the required system capacity.

Step 1: find your baseline consumption

Pull your last 12 electric bills and calculate the average monthly kWh. This annual average smooths out seasonal variation. Many utility websites let you download 12–24 months of hourly smart meter data - use this to understand your peak usage patterns, which helps with battery sizing if you are considering storage.

Step 2: account for future usage changes

If you plan to buy an electric vehicle in the next few years, add roughly 3,000–5,000 kWh per year to your consumption estimate (for average EV driving). Planning a home addition? Add 300–500 kWh per 500 sq ft. Installing a heat pump? Add 2,000–4,000 kWh if replacing a gas furnace. Solar systems are designed for 25+ year lifespans - size for where you expect to be in 5–7 years, not just today.

Step 3: apply the sizing formula

The system size formula is: System kW = Monthly kWh ÷ (Peak Sun Hours × 30 × Efficiency). This accounts for the number of productive sun hours in your area and the system's real-world efficiency losses. At 80% efficiency, a system loses 20% of theoretical production to inverter conversion, wiring resistance, soiling, and temperature derating.

Step 4: check available roof space

A 400W panel typically measures about 18 sq ft (1.7 m²). A 10 kW system with 25 panels needs roughly 450 sq ft of unshaded south-facing roof space. East- and west-facing roofs produce 10–20% less than south-facing and may require a larger system to offset the same consumption. North-facing roofs are generally not viable for solar in the northern hemisphere.

Types of solar panels: monocrystalline, polycrystalline, and thin-film

Not all solar panels are created equal. The three main technologies differ in efficiency, cost, aesthetics, and performance in specific conditions.

Monocrystalline silicon panels

Monocrystalline panels are made from a single continuous crystal of silicon, giving them a uniform dark appearance and the highest efficiency ratings of any mainstream panel technology - typically 20–23% for current residential panels. Their higher efficiency means you need fewer panels and less roof space for the same output. Leading brands include SunPower (22–23%), LG, and REC Alpha. They perform better in low-light conditions and lose less efficiency at high temperatures than polycrystalline panels. The premium over polycrystalline panels has narrowed significantly, making monocrystalline the dominant choice for residential installations.

Polycrystalline silicon panels

Polycrystalline panels (also called multicrystalline) are manufactured by melting multiple silicon fragments together, giving them a distinctive blue, speckled appearance. They are slightly less efficient (15–18%) but have historically been less expensive per watt. The efficiency gap between mono and polycrystalline has narrowed, and many manufacturers have shifted production toward monocrystalline technology. Polycrystalline panels are still available at lower price points and work well in climates with consistently high direct sun.

Thin-film panels

Thin-film panels deposit photovoltaic material in thin layers on glass, plastic, or metal substrates. They are lightweight, flexible, and can be integrated into building materials (BIPV). However, their efficiency is lower (10–13%) and they degrade faster than crystalline silicon panels. Thin-film is mainly used in commercial, industrial, and utility-scale applications where large roof areas are available and cost-per-watt is optimized over efficiency. Common thin-film technologies include CdTe (Cadmium Telluride, used by First Solar) and CIGS (Copper Indium Gallium Selenide).

Emerging technologies: TOPCon and perovskite

Tunnel Oxide Passivated Contact (TOPCon) cells represent the leading edge of mass-production crystalline silicon technology, with efficiencies reaching 24–25%. They are becoming increasingly available from brands like Jinko Solar and Longi. Perovskite solar cells are being developed in laboratories with theoretical efficiencies above 30%, but long-term durability and stability remain challenges to commercialization.

Solar inverters: string, microinverter, and power optimizers

The inverter is the brain of your solar system, converting DC power from the panels into usable AC power. The choice of inverter architecture significantly affects system performance, monitoring capability, and cost.

String inverters

String inverters connect all panels in series (a "string") to a single central inverter unit typically installed near your electrical panel. They are the lowest-cost option and are highly reliable with a simple design. The main drawback: the entire string performs at the level of the lowest-producing panel. If one panel is shaded by a chimney or tree limb in the afternoon, all panels in that string reduce output proportionally. String inverters work best on unshaded, single-orientation roofs.

Microinverters

Microinverters (popularized by Enphase Energy) attach to each panel individually, converting DC to AC at the panel level. Each panel operates independently, so shading on one panel has no effect on the others. This "panel-level optimization" can increase total system output by 20–30% on roofs with any partial shading. Microinverters also provide panel-level monitoring - you can see the output of every individual panel through an app. They cost $100–$150 more per panel than string inverters. For shaded or multi-orientation roofs, they typically deliver ROI within 2–3 years through increased production.

Power optimizers

DC power optimizers (sold by SolarEdge) are a hybrid approach - they attach to each panel and perform maximum power point tracking at the panel level, then send optimized DC to a central string inverter. Like microinverters, they mitigate shading losses and provide panel-level monitoring. They cost less than full microinverter systems while providing most of the same benefits.

Net metering and state incentives

Net metering

Net metering is the policy mechanism that makes rooftop solar economically viable for most homeowners. When your panels produce more than your home uses, the surplus flows to the grid and your utility credits your account. When your consumption exceeds production (nights, cloudy days, winter), you draw from the grid and those credits offset the cost.

Most US states require utilities to offer net metering at full retail rates (1:1), meaning the credit rate equals the rate you pay. However, several states have reduced net metering rates in recent years. California moved to NEM 3.0 in 2023, which credits exported solar at much lower avoided-cost rates rather than retail rates. This significantly increased payback periods for new California installations and made battery storage more attractive. Check your state utility's current net metering policy before sizing your system.

The federal solar credit has ended

The Residential Clean Energy Credit is not available for any system placed in service after December 31, 2025, according to the IRS. Nothing in this calculator subtracts it, so the system cost you see is what you pay before any state or utility incentive. An installer's proposal that still shows a 30 percent credit is out of date.

State incentives and utility rebates

Many states and utilities offer incentives that lower the net cost of a system. The Database of State Incentives for Renewables and Efficiency (DSIRE at dsireusa.org) lists them by state, and that is the place to check, because programs open and close often.

Solar payback period: what to expect and how to improve it

The payback period is the time it takes for cumulative electricity savings to equal the upfront system cost. The simple payback formula is: Payback Years = Net System Cost ÷ Annual Electricity Savings.

Payback depends on your location, your electricity rate and what the system costs, and those vary enough that a national figure is not much use. Work it through instead. A $29,000 system saving $1,400 a year takes about 20.7 years to pay back, which is inside a 25-year production warranty but not by much, and it is why the rate you pay and the price per watt you can get matter so much.

Factors that shorten payback

• High electricity rates: California (about $0.30/kWh) and Hawaii (about $0.47/kWh) pay back much faster than the EIA national average of $0.18/kWh
• High peak sun hours: Arizona and Nevada systems produce 30–40% more than Seattle for the same capacity
• State incentives: A state or utility rebate directly reduces net cost
• Rising utility rates: Historical rate increases of 2–3%/year make future savings larger than today's projections
• Competitive installation quotes: Getting 3+ bids can reduce cost by $3,000–$8,000

Factors that lengthen payback

• Low electricity rates: Louisiana (about $0.13/kWh) and other low-rate states have longer paybacks
• Low sun hours: Pacific Northwest and New England have 20–30% less production than the Sun Belt
• Reduced net metering: States that credit exported solar at wholesale rather than retail rates reduce the effective savings rate
• High installation costs: Premium installers or difficult roof conditions increase per-watt cost

Battery storage: should you add a battery to your solar system?

Battery storage has become increasingly popular with residential solar, driven by falling battery prices, grid outage concerns, and reduced net metering rates in some states. The two most popular residential options are the Tesla Powerwall (13.5 kWh capacity, ~$12,000 installed) and the Enphase IQ Battery (10–19.2 kWh configurations, ~$10,000–$18,000 installed).

Adding battery storage increases system cost by $10,000–$20,000 and extends the payback period significantly. The case for batteries is strongest when:

• Your utility has reduced net metering rates, making self-consumption more valuable than grid export
• You are in an area with frequent power outages (wildfire zones, hurricane-prone regions)
• Your utility offers time-of-use rates with high peak-hour costs, and you can discharge the battery during peak hours
• You have a critical backup power need (medical equipment, well pump, home office)

The federal tax credit for batteries ended at the same time as the one for panels, so count the full price.

Permits, HOA rules, and maintenance

Permits and interconnection

All residential solar installations in the US require building permits from your local jurisdiction. Licensed solar installers handle permit applications as part of the installation contract. Permits ensure the structural attachment meets load requirements and the electrical work meets code. After installation, your utility must approve the interconnection before you can turn the system on and start net metering. Interconnection approval typically takes 2–8 weeks and is entirely managed by your installer.

HOA rules

Homeowners Association restrictions on solar panels are limited by law in most states. The majority of US states have enacted solar rights or solar access laws that prohibit HOAs from banning solar installations outright. However, HOAs may impose reasonable aesthetic requirements, such as requiring panels to be flush-mounted, limiting rooftop equipment visibility from the street, or requiring specific equipment colors. Before installing, submit a formal request to your HOA - most approvals are granted within 30–60 days. Your solar installer may have experience navigating HOA requirements in your area.

Maintenance

Solar panels have no moving parts and require minimal maintenance. Annual cleaning - rinsing panels with a garden hose to remove dust, pollen, and bird droppings - is typically sufficient in most climates. Rainfall naturally cleans most panels in wetter regions. Monitor your system's production monthly through the manufacturer's app and compare actual production to projected production. A sudden unexplained drop in output can indicate a faulty panel, inverter issue, or wiring problem. Professional inspection every 5 years is recommended. Most installers offer monitoring and maintenance packages at $100–$200/year.

Common mistakes to avoid

• Using a national average for peak sun hours: Your local hours matter more than any national figure - find yours at pvwatts.nrel.gov
• Ignoring shading: Even 10–15% shading without microinverters can disproportionately reduce total output
• Leasing instead of buying: Solar leases lock you into a contract for 20+ years and may complicate home sales; purchasing with a loan typically builds more financial value
• Not comparing multiple installers: Prices for the same system can vary $5,000–$10,000 between installers in the same market
• Forgetting about roof condition: If your roof needs replacement within 5–10 years, replace it before or during solar installation to avoid the cost of removing and reinstalling panels later