Pv Watt Calculator
A “PV watt calculator” has to answer two different questions. The first: given a solar panel’s specs and your local sunlight, how much energy will it actually produce each day? The second: given your household’s electricity use, how big a system do you need? This tool does both. The output mode estimates daily, monthly and yearly energy from the panel’s area, module efficiency and solar irradiance, and it cross-checks your datasheet specs for consistency. The sizing mode works backwards from your electricity bill to the system size and panel count you need.
Annual output:
Math:
Spec check:
Math:
Output mode uses the panel’s physical specs (irradiance × area × module efficiency); the rated wattage feeds only the consistency check. Sizing mode uses whole-system efficiency including inverter, wiring and heat losses. Both results stay hidden until you calculate.
Quick Answer
A 400-watt panel covering 2.0 m² at 20% efficiency, in a location with 5.0 kWh/m²/day of sunlight, produces 2.000 kWh per day, about 60.9 kWh a month and 730 kWh a year. A home using 30 kWh a day with 5 peak sun hours and 85% system efficiency needs a 7.059 kW system: eighteen 400-watt panels. Both figures are pre-filled in the calculator above.
Key takeaways
- Daily output = irradiance × panel area × module efficiency. Every input multiplies, so a wrong irradiance figure skews the whole answer.
- System size = daily kWh ÷ (peak sun hours × system efficiency). The panel count always rounds up: you buy whole panels.
- The rated wattage on the sticker does not feed the energy math. It feeds the spec check, which flags datasheet typos at a 5% disagreement.
- Module efficiency (15 to 23%) and system efficiency (80 to 85%) are different numbers for different modes. Mixing them up is the most common input error.
How to use the PV watt calculator
Pick a mode with the tabs, fill in four figures, and click Calculate. Results stay hidden until you calculate, and each mode has its own Reset that restores the worked example.
- Estimate panel output: enter the rated power, panel area, module efficiency and solar irradiance. Read the daily, monthly and yearly output, the math line and the spec check.
- Size my system: enter your average daily electricity use, peak sun hours, system efficiency and the wattage of the panels you plan to buy. Read the required system size and the whole-panel count.
- Read the spec check before trusting the output: it compares your entered wattage against area × efficiency × 1000. A mismatch over 5% means a datasheet value is wrong, and every downstream number inherits the error.
- Copy or share: each mode has its own Copy and Share buttons inside its results box.
The formulas, in plain terms
Output mode multiplies three things: the daily solar energy per square metre, the panel's area, and the fraction of that energy the panel converts. Sizing mode divides your daily consumption by what each installed kilowatt actually delivers per day, then converts kilowatts to a whole number of panels.
Monthly ≈ daily × 30.44 | Yearly = daily × 365
Required size (kW) = daily use (kWh) ÷ (peak sun hours × system efficiency)
Panels = round up(required kW × 1000 ÷ panel watts)
The monthly figure uses 30.44 days, the average month, rather than a flat 30. The sizing formula treats system efficiency as a single derate covering the inverter, wiring, heat, soiling and shade; it is the number you tune when you know your roof runs hot or partly shaded. Once you have a system size, the cost per watt calculator turns kilowatts into a budget figure, and the cost per solar panel calculator breaks that budget down panel by panel.
Worked examples: one panel, one home
The pre-filled output example is a standard 400-watt residential panel: 2.0 m² at 20% efficiency under 5.0 kWh/m²/day of irradiance. Daily energy = 5.00 × 2.00 × 0.200 = 2.000 kWh. Monthly = 2.000 × 30.44 = 60.9 kWh. Yearly = 2.000 × 365 = 730 kWh. The spec check confirms the datasheet: 2.00 m² × 20.0% × 1000 = 400 W, matching the entered 400 W.
The sizing example is a home using 30 kWh a day with 5 peak sun hours and 85% system efficiency: 30.0 ÷ (5.0 × 0.85) = 7.059 kW required, which at 400 W per panel is 17.65 panels, rounded up to 18. Every row below is a real run of this calculator.
| Scenario | Daily output | Monthly | Yearly |
|---|---|---|---|
| 400 W, 2.0 m², 20%, 5.0 irradiance | 2.000 kWh | 60.9 kWh | 730 kWh |
| 700 W, 3.5 m², 20.0%, 7.2 irradiance | 5.040 kWh | 153.4 kWh | 1,840 kWh |
| Home: 30 kWh/day, 5 PSH, 85%, 400 W panels | 7.059 kW → 18 panels | ||
| Home: 45 kWh/day, 4.2 PSH, 82%, 450 W panels | 13.066 kW → 30 panels | ||
Why the spec check exists, and what it catches
Under Standard Test Conditions, a panel's rated wattage must equal its area times its efficiency times 1,000 W/m². That is not a second formula; it is the same physics rearranged. If you enter a 400 W panel with 2.0 m² at 20%, the check computes 400 W and agrees. Enter 500 W with the same area and efficiency and it warns you, because those three numbers cannot all be true at once.
The 5% tolerance exists because datasheets round: a panel listed as 400 W might compute to 398 W from its rounded area and efficiency, and that is fine. A 25% disagreement is not rounding; it is a copied value from the wrong row, a square-foot area entered as square metres, or an efficiency entered as a decimal (0.20) instead of a percent (20). The check catches all three, and the output math refuses to run on unchecked specs only in the sense that you should not trust an output whose inputs disagree. Converting the result into the current your wiring must carry is the job of the amp watt calculator.
What competing calculators miss
Solar output tools split into camps with clear blind spots:
- Derate-factor estimators (rule-of-thumb pages): watts × sun hours × 0.75. Fast, and the 0.75 is doing a lot of unexplained work: it bundles temperature, inverter, wiring and soiling into one number that is never justified and never adjustable.
- Formula-and-table pages (installer guides): the correct watts × peak sun hours ÷ 1,000 arithmetic with worked tables, but no interactive tool and no way to test your own numbers.
- Popular output estimators (web calculator pages): interactive, with efficiency and loss factors as inputs, but at least one publishes a formula its own worked examples do not reproduce: 300 W × 5 hours × 20 panels × 0.18 × 0.85 is 4,590 kWh/day by its own formula, not the 255 kWh/day the page shows. An example that cannot be reproduced from the stated formula is a warning sign, not a feature.
- Engineering suites (PVWatts, Global Solar Atlas, performance-ratio tools): the serious models, with hourly weather and loss stacks. Strong, but they answer at project scale; none gives you a single panel's daily kWh in four inputs, and none checks your datasheet arithmetic.
- The gap this page fills: physics-based output from irradiance, area and module efficiency instead of a bundled derate factor; a sizing mode that shares the page rather than a second site; the math line shown for every result so the arithmetic is auditable; and the spec check, which no competitor runs: area × efficiency × 1000 must equal the sticker wattage, or something you typed is wrong.
Frequently asked questions
One hour of sunlight at 1,000 W per square meter, the intensity panels are rated at. It is not a daylight hour. Your irradiance in kWh per square meter per day equals your peak sun hours numerically.
The sticker wattage is measured under Standard Test Conditions: 1,000 W per square meter, 25 C cell temperature, perfect spectrum. Real roofs run hotter, sunlight is rarely at full intensity, and inverters and wiring lose energy.
Use the Global Solar Atlas for worldwide locations or NREL's PVWatts for US addresses; both are free. Avoid guessing: output scales linearly with this input, so a wrong value skews everything.
In output mode, enter the module efficiency from the panel datasheet, usually 15 to 23 percent. In sizing mode, enter system efficiency including all losses: 80 to 85 percent for a standard well-sited residential system.
kW is power, the rate of production at one moment. kWh is energy, the total over time: a 7 kW array at full power for one hour makes 7 kWh. Your electricity bill is in kWh, so that is the figure to match.
Yes. Panels are rated at 25 C cell temperature, but cells on a hot roof can reach 50 to 70 C. Most silicon panels lose about 0.3 to 0.5 percent of output per degree above 25 C.
To verify your specs. Under Standard Test Conditions, rated power must equal area times efficiency times 1000. The spec check warns you if your entered wattage disagrees by more than 5 percent.
It depends on usage and sunlight, but one industry estimate puts the average home near seventeen 400-watt panels. Run the sizing mode with your own daily kWh and peak sun hours for an accurate figure.
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References
- National Renewable Energy Laboratory. PVWatts Calculator: the reference photovoltaic performance model for estimating system energy production. Accessed October 24, 2026.
- World Bank / ESMAP. Global Solar Atlas: worldwide solar resource maps and photovoltaic power potential. Accessed October 24, 2026.
- National Renewable Energy Laboratory. Best Research-Cell Efficiency Chart: record photovoltaic cell efficiencies by technology. Accessed October 24, 2026.
- Solar Energy Industries Association. Solar Industry Research Data: US solar market data on installed capacity and system costs. Accessed October 24, 2026.
- U.S. Department of Energy. Solar Energy Technologies Office: federal research on photovoltaic performance and cost reduction. Accessed October 24, 2026.
- Sandia National Laboratories. PV Performance Modeling Collaborative: open photovoltaic performance models and validation data. Accessed October 24, 2026.