Solar Panel Calculator

<h1>Solar Panel Calculator</h1>

<p>Work out how many solar panels you need in about ten seconds. Enter your daily electricity use, the peak sun hours where you live, and the wattage of the panels you are considering — the calculator returns the array size in kilowatts, the number of panels, and the roof area they will take up.</p>

<h2>How the calculator sizes your array</h2>

<p>Three lines of arithmetic sit behind the result.</p>

<p><code>System size (kW) = daily use (kWh) ÷ (peak sun hours × (1 − losses))</code></p>

<p><code>Panels = system size (W) ÷ panel wattage, rounded up</code></p>

<p><code>Roof area = panels × panel area (about 21.5 sq ft, or 2 m², for a 400 W module)</code></p>

<p>Take a household using 30 kWh a day, in a place averaging 4.5 peak sun hours, with 23% system losses. Dividing 30 by 4.5 × 0.77 gives an <strong>8.66 kW</strong> array. Dividing 8,660 W by 400 W a panel gives <strong>22 panels</strong>, which cover roughly <strong>474 sq ft</strong> of unshaded roof.</p>

<p>Each input pulls harder than it looks. Halve the peak sun hours and the array doubles. Move losses from 23% down to 14% and the same house drops three panels. Switch from 400 W to 500 W modules and it drops four more — the array stays the same size in kilowatts, it just arrives in fewer pieces.</p>

<p>The calculator always rounds panel count up. A fractional panel does not exist, and rounding down leaves the array short in exactly the months you notice.</p>

<h2>How many solar panels do you need?</h2>

<p>Most US homes land between 15 and 25 panels. The table below runs the same formula across typical monthly bills, using 400 W panels, 4.5 peak sun hours and 23% losses.</p>

<table>

<thead>

<tr>

<th scope="col">Monthly use</th>

<th scope="col">Daily use</th>

<th scope="col">System size</th>

<th scope="col">400 W panels</th>

<th scope="col">Typical household</th>

</tr>

</thead>

<tbody>

<tr><th scope="row">400 kWh</th><td>13.2 kWh</td><td>3.8 kW</td><td>10</td><td>215 sq ft</td><td>Apartment, or a small gas-heated home</td></tr>

<tr><th scope="row">600 kWh</th><td>19.7 kWh</td><td>5.7 kW</td><td>15</td><td>323 sq ft</td><td>Two people, gas heat and hot water</td></tr>

<tr><th scope="row">800 kWh</th><td>26.3 kWh</td><td>7.6 kW</td><td>19</td><td>409 sq ft</td><td>Close to the US average household</td></tr>

<tr><th scope="row">1,000 kWh</th><td>32.9 kWh</td><td>9.5 kW</td><td>24</td><td>516 sq ft</td><td>Family home with central air conditioning</td></tr>

<tr><th scope="row">1,200 kWh</th><td>39.5 kWh</td><td>11.4 kW</td><td>29</td><td>624 sq ft</td><td>All-electric, or one EV</td></tr>

<tr><th scope="row">2,000 kWh</th><td>65.8 kWh</td><td>19.0 kW</td><td>48</td><td>1,032 sq ft</td><td>Large all-electric home, pool, two EVs</td></tr>

</tbody>

</table>

<p>Assumes 4.5 peak sun hours, 23% system losses and 400 W panels at 21.5 sq ft (2 m²) each. Panel counts rounded up.</p>

<p>Read the table as a starting point, not a quote. Two houses of identical square footage can sit four rows apart: an all-electric home with a heat pump and an EV uses two to three times the electricity of a gas-heated home the same size. Size the array against your actual kWh, which is printed on every utility bill, not against floor area.</p>

<p>If you plan to add an EV or replace a gas furnace within a few years, size for that load now. Adding panels later means a second permit, a second inspection, and often an inverter that no longer has headroom.</p>

<h2>Peak sun hours are not daylight hours</h2>

<p>One peak sun hour is one hour of sunlight at 1,000 W/m² — the irradiance panels are rated at in the lab. It is the input people get wrong most often, because it has nothing to do with how long the sun is up. A June day with fourteen hours of daylight may deliver only five peak sun hours: early and late sun arrives at a shallow angle, through more atmosphere, and does far less work.</p>

<table>

<thead>

<tr>

<th scope="col">Annual average</th>

<th scope="col">December</th>

<th scope="col">What it means for sizing</th>

</tr>

</thead>

<tbody>

<tr><th scope="row">Phoenix, AZ</th><td>6.5</td><td>7.5</td><td>4.8</td><td>The smallest array in the country for a given load</td></tr>

<tr><th scope="row">Los Angeles, CA</th><td>5.6</td><td>6.9</td><td>4.1</td><td>Mild seasonal swing, so off-grid math stays simple</td></tr>

<tr><th scope="row">Denver, CO</th><td>5.5</td><td>6.8</td><td>4.0</td><td>Altitude offsets the short winter days</td></tr>

<tr><th scope="row">Miami, FL</th><td>5.3</td><td>5.9</td><td>4.4</td><td>Cloud cover, not latitude, is the limit</td></tr>

<tr><th scope="row">Austin, TX</th><td>5.2</td><td>6.4</td><td>3.9</td><td>Heat losses matter more here than sun hours</td></tr>

<tr><th scope="row">Atlanta, GA</th><td>5.0</td><td>5.9</td><td>3.7</td><td>Tree shade is the usual constraint, not irradiance</td></tr>

<tr><th scope="row">Chicago, IL</th><td>4.4</td><td>5.9</td><td>2.4</td><td>Going off-grid here needs roughly double the array</td></tr>

<tr><th scope="row">Boston, MA</th><td>4.3</td><td>5.6</td><td>2.6</td><td>A steep tilt sheds snow and catches low winter sun</td></tr>

<tr><th scope="row">New York, NY</th><td>4.2</td><td>5.5</td><td>2.5</td><td>Net metering does most of the heavy lifting</td></tr>

<tr><th scope="row">Seattle, WA</th><td>3.6</td><td>5.4</td><td>1.3</td><td>Grid-tied only, realistically</td></tr>

</tbody>

</table>

<p>Daily averages of full-irradiance equivalent hours, rounded. For your exact coordinates, run the free <a href="https://pvwatts.nrel.gov/" rel="noopener">PVWatts tool</a> from the US National Renewable Energy Laboratory.</p>

<p>Which figure you feed the calculator depends on the system:</p>

<ul>

<li><strong>Grid-tied with net metering</strong> — use the annual average. Summer surplus is banked against winter shortfall, so the array only has to balance over twelve months.</li>

<li><strong>Off-grid</strong> — use the December figure, which across most of the US runs 55–70% of the annual average. The array has to carry the darkest month on its own, and no amount of July sunshine helps in January.</li>

</ul>

<p>Local figures beat regional ones. A coastal marine layer, a valley that holds fog until eleven, or six weeks of wildfire smoke each fall all show up in the measured data, and none of them show up in a map of latitude.</p>

<h2>What “system losses” actually covers</h2>

<p>System losses are everything that happens between the silicon and the meter. A 23% default sounds pessimistic until the components are listed separately, at which point it usually turns out to be about right.</p>

<table>

<thead><tr><th scope="col">Loss</th><th scope="col">Typical</th><th scope="col">What it is</th></tr></thead>

<tbody>

<tr><th scope="row">Temperature</th><td>8%</td><td>Cells run hotter than their 25°C rating; output drops about 0.35% per degree above it</td></tr>

<tr><th scope="row">Inverter conversion</th><td>4%</td><td>DC to AC. Modern inverters are 96–98% efficient at load, less at low light</td></tr>

<tr><th scope="row">Soiling</th><td>3%</td><td>Dust, pollen, salt and bird droppings. Higher in dry and agricultural areas</td></tr>

<tr><th scope="row">Shading</th><td>3%</td><td>Partial and intermittent. Far higher without microinverters or optimizers</td></tr>

<tr><th scope="row">Wiring and connections</th><td>2%</td><td>Resistance in the DC and AC runs; worse with undersized cable</td></tr>

<tr><th scope="row">Module mismatch</th><td>2%</td><td>No two panels perform identically, and a string follows its weakest member</td></tr>

<tr><th scope="row">Availability</th><td>1%</td><td>Downtime for maintenance, faults and grid outages</td></tr>

</tbody>

<tfoot><tr><th scope="row">Total</th><th>23%</th><th>The calculator’s default</th></tr></tfoot>

</table>

<p>Degradation is separate and cumulative: panels lose a further 0.4–0.5% of output every year.</p>

<p>Temperature is the largest single item and the one most people leave out. Panels are rated at a 25°C cell temperature; a module on a dark roof in July runs 20–30°C hotter than the air around it. That is why a 400 W panel almost never produces 400 W outside a spec sheet.</p>

<p>Use 14–18% only for a new, clean, unshaded, well-ventilated array with a modern inverter. Use 25–30% for a roof with morning shade, a string inverter, or a location with dust, pollen or snow.</p>

<h2>Grid-tied and off-grid are sized differently</h2>

<p>The same house needs a noticeably bigger array off-grid than on-grid, and the calculator’s output is only the starting figure for the off-grid case.</p>

<p><strong>Grid-tied.</strong> The grid absorbs the surplus and covers the gaps, so the array is sized to annual consumption. Overbuilding rarely pays: most utilities credit exported energy well below the retail rate, and many cap system size at a percentage of your historical usage.</p>

<p><strong>Off-grid.</strong> Three adjustments stack on top of the base figure:</p>

<ul>

<li>Size on the <strong>worst month</strong>, not the annual average.</li>

<li>Add <strong>battery round-trip losses</strong> — 10–20% of everything that passes through storage, on top of the array losses already counted.</li>

<li>Oversize the array by <strong>20–30%</strong> so the bank can actually recharge after a cloudy stretch, instead of only breaking even on the day’s load.</li>

</ul>

<p>In practice an off-grid array often ends up 1.5 to 2 times the grid-tied size for the same household. Run the base number here, then take it to the <a href="/calculators/off-grid-solar-system">off-grid system calculator</a> and the <a href="/calculators/battery-bank-size">battery bank calculator</a>.</p>

<h2>What this calculator does not account for</h2>

<p>The arithmetic is honest about energy. It knows nothing about your particular roof, and five things move the real answer:</p>

<ul>

<li><strong>Orientation and tilt.</strong> South-facing at roughly latitude tilt is the reference case. East or west faces give up 10–20%; in the northern hemisphere a north-facing slope is rarely worth covering.</li>

<li><strong>Shade.</strong> Partial shade on one module can drag down an entire string unless the system uses microinverters or optimizers. A chimney shadow that crosses the array for two hours a day costs far more than two hours of production.</li>

<li><strong>Usable roof area.</strong> The roof-area figure assumes a clean rectangle. Vents, chimneys, skylights and the setbacks required by fire codes usually take back about a third of what the tape measure says.</li>

<li><strong>Your rate structure.</strong> Net metering, time-of-use rates and export caps decide whether offsetting 100% of consumption is the right target or an expensive one.</li>

<li><strong>Cost, incentives and payback.</strong> Panel count is not price. Labor, racking, inverters, permits and whatever incentives currently apply in your state move the total far more than the modules do.</li>

</ul>

<p>For a bankable design, take these numbers to an installer or to <a href="https://pvwatts.nrel.gov/" rel="noopener">NREL’s PVWatts</a>, which models your exact coordinates against decades of measured weather data. This calculator is for the stage before that: deciding whether the project is a 6 kW conversation or a 16 kW one.</p>

<h2>Keep going with the numbers you just got</h2>

<p>Array size is the first answer, not the last one. Each question that follows has its own tool:</p>

<ul>

<li>Not sure of your daily kWh? Build it up appliance by appliance with the <a href="/calculators/daily-energy-consumption">daily energy consumption calculator</a> and the <a href="/calculators/appliance-wattage">appliance wattage calculator</a>.</li>

<li>Need the peak sun hours for your location? Start with the <a href="/calculators/peak-sun-hours">peak sun hours calculator</a>.</li>

<li>Want production month by month instead of a yearly average? Use the <a href="/calculators/solar-panel-output">solar panel output calculator</a>.</li>

<li>Mounting the array: <a href="/calculators/solar-panel-angle">panel tilt</a> and <a href="/calculators/solar-panel-spacing">row spacing</a>.</li>

<li>Storage: <a href="/calculators/battery-bank-size">battery bank size</a> and <a href="/calculators/depth-of-discharge">depth of discharge</a>.</li>

<li>Electrical: <a href="/calculators/solar-inverter-sizing">inverter sizing</a>, <a href="/calculators/solar-charge-controller">charge controller</a>, <a href="/calculators/solar-wire-size">wire size</a> and <a href="/calculators/fuse-breaker-sizing">fuse and breaker sizing</a>.</li>

<li>Money: <a href="/calculators/solar-payback-period">payback period</a> and <a href="/calculators/energy-savings">energy savings</a>.</li>

</ul>

<h2>Frequently asked questions</h2>

<h3>How do I calculate how many solar panels I need?</h3>

<p>Divide your daily energy use in kWh by your peak sun hours, divide again by one minus your system losses, then divide the result by the wattage of one panel. A home using 30 kWh a day at 4.5 peak sun hours with 23% losses needs an 8.66 kW array, which is 22 panels of 400 W.</p>

<h3>How many solar panels does a 2,000 sq ft house need?</h3>

<p>Floor area does not determine panel count — electricity use does. A 2,000 sq ft home using around 1,000 kWh a month needs roughly 24 panels of 400 W. The same house with gas heat and no EV might need 15; with a heat pump and two EVs, closer to 40.</p>

<h3>How much power does a 400 W solar panel actually produce?</h3>

<p>About 1.4 kWh a day at 4.5 peak sun hours after 23% system losses, or roughly 500 kWh a year. The 400 W rating is a laboratory peak measured at 1,000 W/m² and a 25°C cell temperature, conditions a roof rarely holds for long.</p>

<h3>How long does a 400 W panel take to charge a 200 Ah battery?</h3>

<p>A 200 Ah 12 V battery holds about 2.4 kWh. A 400 W panel delivering roughly 300 W after losses needs around eight hours of peak sun, which is about two average days or one long summer day. A lead-acid bank discharged to 50% needs half that, since only half its capacity is usable.</p>

<h3>How many solar panels do I need to run 50 amps?</h3>

<p>Fifty amps at 240 V is 12 kW of instantaneous draw, which would take 30 panels of 400 W to match in full sun. In practice nobody sizes an array to a breaker rating: a 50 A circuit spends most of the day drawing nothing. Size to the kWh that circuit actually consumes over a day, or add a battery to cover the peaks.</p>

<h3>Is a bigger array always better?</h3>

<p>No. Past the point where exports stop being credited at a useful rate, extra panels earn very little, and many utilities cap system size against your past consumption. Size to your real annual use, plus any load you know is coming — an EV, a heat pump — and stop there.</p>