Solar Panel Calculator
Estimate solar array size, panel count, and daily or annual energy production from consumption, peak sun hours, panel wattage, and system losses.
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Practical context, assumptions, examples, and next steps for using the result well.What this solar panel calculator estimates
This solar panel calculator estimates the direct-current array capacity needed to match a chosen daily electricity consumption. It divides the daily energy target by equivalent sunshine and a useful-output efficiency, then converts the resulting capacity into a whole number of identical panels. The result is an energy balance with stated assumptions. It is not a roof design, a quotation, or a promise that the array can supply every appliance at every moment.
Two units matter throughout the calculation. Kilowatts describe power, which is a rate of transferring energy. Kilowatt-hours describe energy accumulated over time. A device drawing one kilowatt for two hours consumes two kilowatt-hours. Panel manufacturers usually state module power in watts under specified test conditions. This calculator converts those watts to kilowatts by dividing by one thousand before calculating an installed array rating.
The required DC capacity is continuous: it can contain fractions of a panel. The installed DC capacity uses the rounded-up panel count. Production is calculated from that installed capacity, so it can slightly exceed the consumption target. Neither figure is the inverter's AC power rating. The tool makes no recommendation about the appropriate DC-to-AC ratio, export limit, or electrical equipment. Those questions require more information than a daily consumption number.
Choose a representative daily electricity load
Start with the energy demand you actually want the array to offset. For a whole-home estimate, use electricity readings or utility bills over a representative period. Divide measured kilowatt-hours by the number of days in that same period. A 600 kWh total over thirty days corresponds to 20 kWh per day. Do not divide an annual bill total by twelve and enter the monthly result as a daily load.
For a selected group of appliances, estimate each device's energy from its average power and operating time. A 100 W load running five hours uses 0.5 kWh. Add the daily contributions, including loads that remain on continuously. Refrigerators, pumps, heat pumps, and other cycling devices do not necessarily draw their rated power throughout the day. Measured consumption is generally more informative than multiplying a nameplate rating by twenty-four hours.
Consider whether the measured period reflects planned changes. Adding an electric vehicle or replacing gas heating can change electricity consumption substantially. Reducing a large discretionary load can have the opposite effect. Enter alternate demand values and compare the results rather than quietly embedding an unexplained margin in the calculation. This tool requires a positive load; a zero-energy scenario is outside its sizing purpose. It does not assume any particular household size, climate, occupancy pattern, or national average.
Peak sun hours are not daylight hours
Peak sun hours express a day's solar irradiation as an equivalent duration at a reference intensity of 1,000 watts per square meter. Four peak sun hours do not mean the panels receive full rated power for exactly four clock hours. Instead, weaker morning light, stronger midday light, and fading afternoon light are summarized into an equivalent daily energy resource. Sunrise-to-sunset duration is therefore not an appropriate substitute.
The relevant resource depends on location, season, panel orientation, tilt, and local obstructions. An annual-average value can support a rough annual energy comparison. A winter value may be more relevant when exploring a winter demand target, but it produces a different interpretation of the annual result. This calculator does not retrieve irradiation, determine your location, or infer a roof angle. The default of four hours is an editable example rather than a claim about your property.
Use a documented source when choosing a design assumption. The linked national laboratory tools can support more detailed location-based modeling. Record whether your figure describes sunlight on a horizontal surface or on a tilted panel plane, because these quantities need not match. Also check whether a production estimate already includes shading or other reductions. Combining an already adjusted sunshine value with the same reduction in the loss input would count that effect twice.
Use a consistent aggregate system-loss convention
The system-loss input represents the combined reduction between the idealized rated-panel energy calculation and the useful energy output being compared with consumption. The model defines efficiency as one minus the loss percentage divided by one hundred. A 20% loss therefore gives an efficiency of 0.8. Zero loss is allowed as an idealized boundary case; 100% loss is rejected because no finite array could deliver a positive energy target.
Potential contributors include inverter conversion, operating temperature, soiling, mismatch, shading, and other system effects. The calculator does not estimate each contributor separately. Choose an aggregate allowance that is consistent with the resource and equipment assumptions you already use. A simple constant cannot reproduce hourly temperature changes, clipping, partial shading, or a site's unique operating conditions. The editable default is a scenario input, not a universal engineering recommendation.
Loss percentages also require careful arithmetic. If two independent stages retain 95% and 90% of energy respectively, the combined retained fraction is 0.95 times 0.90, or 0.855. The combined loss is 14.5%, not automatically 15%. Conversely, compensating for a 20% loss requires division by 0.8, not multiplication by 1.2. Keep battery charging and discharging outside this basic array estimate unless you deliberately include them in a clearly documented aggregate energy path.
Solar sizing formulas and a worked example
Let daily consumption be E in kWh, peak sun hours be H, the loss fraction be L, and each panel's rated power be W in watts. The useful efficiency is η = 1 − L. Required DC array power is E ÷ (H × η), expressed in kW. Divide that power by W ÷ 1000 to obtain the theoretical panel count, then round upward to the next whole panel.
For example, enter 20 kWh per day, four peak sun hours, 20% aggregate losses, and 400 W modules. Efficiency is 0.8. Required DC power is 20 ÷ (4 × 0.8) = 6.25 kW. Each module contributes 0.4 kW of rated DC power, so the theoretical count is 15.625. Rounding up gives sixteen panels. Their combined installed rating is 16 × 0.4 = 6.4 kW DC.
Modeled useful daily production is then 6.4 × 4 × 0.8 = 20.48 kWh. The small excess over 20 kWh comes solely from whole-panel rounding. Annual production under the calculator's constant-day assumption is 20.48 × 365 = 7,475.2 kWh. These equations use the panel's electrical power rating, not a separate cell-efficiency percentage. Panel efficiency becomes important for estimating required physical area, but roof area is not modeled here.
Interpret daily and annual production carefully
The daily result is the useful production predicted for a day represented by your inputs. It is not an hourly schedule. Actual production can arrive when your home consumes little electricity, while evening demand can occur after solar output has fallen. Matching total daily energy does not remove that timing mismatch. Grid imports, exports, storage, or load shifting may still be necessary, depending on the system and your goals.
The annual result repeats the modeled day 365 times. It does not account for leap years, monthly weather, degradation, equipment downtime, snow cover, or year-to-year resource variation. If your sunshine input represents a reasonable annual average, multiplying by 365 can serve as an initial annual energy estimate. If you instead enter a low winter design value, the annual figure repeats that low value and should not be described as an independent annual weather forecast.
For seasonal planning, compare scenarios explicitly. A home with large summer cooling demand may have a different consumption pattern from one with winter electric heating. The average annual energy balance can conceal a difficult winter deficit. A month-by-month or hourly model can investigate those issues more honestly than adding extra decimal places to this result. Detailed production tools also allow assumptions about tilt, azimuth, inverter sizing, and local weather that this quick calculator intentionally leaves editable or unmodeled.
Compare assumptions without confusing precision and confidence
Scenario comparisons are often more useful than a single exact-looking answer. Keep consumption fixed and reduce peak sun hours to examine a less favorable resource assumption. Increase the aggregate loss to explore a conservative performance case. Change module wattage to see how the whole-panel count changes. Before rounding, required DC power depends on consumption, sunshine, and losses; module wattage affects how many physical modules represent that power.
With the worked example's 20 kWh demand and 20% losses, reducing sunshine from four to three peak sun hours increases the continuous requirement from 6.25 to about 8.333 kW. At 400 W per panel, that becomes twenty-one panels and 8.4 kW installed. The larger count is a consequence of the changed assumption, not evidence that either scenario accurately describes a particular roof or climate.
Displayed decimal places describe arithmetic, not forecast certainty. A result expressed to three decimals does not imply that real production is known to one watt or one watt-hour. Inputs with uncertain weather and loss estimates dominate the uncertainty. Keep a record of each scenario's assumptions when sharing a result. The form stores inputs and numerical outputs in the URL, but recalculates from the current inputs instead of treating a previously shared output as authoritative.
What this estimate cannot establish
A panel count does not establish that the modules fit on a roof or meet structural and electrical requirements. Roof orientation, setbacks, access paths, wind and snow loads, mounting details, and local rules can alter a feasible design. Module dimensions are not included. Neither are string voltage, temperature-adjusted electrical limits, controller ratings, conductors, protection, grounding, or utility interconnection requirements. Do not use the result as wiring or installation guidance.
Energy production is also different from bill savings. Import rates, export compensation, fixed charges, time-of-use pricing, and self-consumption affect the value of generated electricity. This calculator retrieves no tariffs, incentives, prices, or equipment quotes. An array that produces the same annual energy as a home consumes may still leave substantial grid purchases and fixed bills. A financial comparison needs a separate tariff-aware analysis and current local information.
Finally, solar modules alone do not establish backup capability. Grid-connected equipment may stop operating during an outage unless the system has suitable backup functionality. If storage is needed, estimate usable battery energy, allowed depth of discharge, conversion losses, and autonomy separately. Inverter continuous power and starting surge still require independent checks. Use this calculator to frame questions and compare transparent assumptions, then consult qualified professionals and detailed modeling before choosing or installing equipment.
Frequently Asked Questions
How many solar panels do I need for 20 kWh per day?
With four peak sun hours, 20% aggregate losses, and 400 W panels, the energy balance requires 6.25 kW DC. Dividing by 0.4 kW and rounding upward gives 16 panels, or 6.4 kW installed. The modeled output is 20.48 kWh per day, not a guarantee for every actual day.
Are peak sun hours the same as daylight hours?
No. Peak sun hours express daily solar irradiation as an equivalent number of hours at 1,000 W per square meter. A long day with weak light can have fewer peak sun hours than a shorter, clearer day. Use a location- and season-appropriate estimate for the actual panel orientation rather than entering sunrise-to-sunset duration.
Does 20% system loss mean multiplying the load by 1.2?
No. A 20% loss means the modeled useful fraction is 0.8, so required generation is divided by 0.8. That is a 25% increase above an otherwise identical loss-free estimate. Use one consistent aggregate allowance and avoid adding the same inverter or shading loss twice.
Why does the calculator round the panel count upward?
The continuous power requirement can correspond to part of a panel, but the estimate assumes identical whole modules. Rounding upward is the smallest whole-panel count that meets the modeled daily energy target. It does not verify roof layout, string voltage, structural capacity, or electrical compatibility.
Can this estimate tell me my electricity savings?
No. Savings depend on when you consume electricity, import and export tariffs, fixed charges, curtailment, and any storage behavior. This calculator models energy only and retrieves no live rates. An annual energy match also does not mean the home is independent of the grid.
Why is annual production just daily output multiplied by 365?
This is an explicit simplifying assumption, not a monthly weather simulation. If you enter an annual-average sun-hours value, the result is a rough annual energy estimate. If you enter a winter design value, the annual figure repeats that winter assumption and should not be presented as a weather-based forecast.
Does a larger solar array automatically provide power during an outage?
No. Standard grid-connected inverters commonly stop exporting when the grid fails, and backup operation requires suitable equipment and an approved system design. Battery energy, inverter power, surge demand, and transfer arrangements need separate evaluation. Ask a qualified local professional about an appropriate installation.