Solar Panel Size Calculator

This calculator runs the charge-time math in reverse, in three steps:

1. energy needed (Wh) = capacity × (target% − current%)
   How many watt-hours you have to put back into the battery.
2. required effective input (W) = energy needed ÷ available peak sun hours
   If you set the deadline in days, available peak sun hours = days × peak sun hours per day.
3. required panel rating (W) = required effective input ÷ real-world efficiency
   Divide by the derating factor (say 0.7) to turn the effective watts you need into a nameplate rating to shop for.

This is a linear approximation that assumes a steady effective input for every sun hour and ignores the slowdown near a full charge, so treat the answer as a floor and size up from it. To go the other way — from a panel you already own to a charge time — use the Solar Charge Time Calculator.

The wattage printed on a panel is its output under Standard Test Conditions: a cool 25°C cell temperature, bright 1000 W/m² light, and the sun striking it head-on. Outdoors, several things pull the real figure below that number:

• Sun angle. Output peaks when light hits the panel perpendicularly. A flat or poorly aimed panel gives up a large share of its potential, especially early and late in the day.
• Panel temperature. Cells lose efficiency as they warm, so a hot panel in still air produces less than its rating even in strong sun.
• Haze, dust, and glass losses. Thin cloud, atmospheric haze, and a dirty surface each trim output.
• Wiring and controller losses. Long or thin cables, connectors, and the MPPT or PWM controller each take a small cut.

A common planning approach is to expect 60 to 80% of a panel's rating under good conditions, and less when it's cloudy or the panel is fixed flat. That factor is what the "real-world efficiency" field represents, and it's why a bare formula result should be rounded up before you buy.

Peak sun hours are not hours of daylight. One peak sun hour is one hour of sunlight at an intensity of 1000 watts per square metre — roughly bright midday sun. A day that runs 13 hours from a dim sunrise to a dim sunset might only add up to 4 or 5 peak sun hours once the weak early and late light is counted properly.

The figure varies widely with location, season, and weather. Many places average somewhere between 3 and 6 peak sun hours per day over the year, with sunny desert or high-altitude areas higher and cloudy northern winters much lower. Local solar resource maps and databases publish monthly averages for a given latitude.

This calculator uses peak sun hours in two ways: as the window your recharge has to finish in, and — when you set the deadline in days — as the daily figure that converts days into a total number of sun hours. Fewer sun hours in the window means a larger panel is needed to move the same energy.

The deadline you set is the biggest lever on panel size. The same 800 Wh recharge for a 1,000 Wh power station (20% to 100%) needs roughly 229 W of panel to finish in one 5-sun-hour day, but only about 114 W if you can spread it over two days. Doubling the time roughly halves the panel.

Allowing more time is often the cheaper, lighter, and less fiddly choice — a smaller panel is easier to carry and aim. A tight one-day target makes sense when you genuinely cycle the battery hard every day; if you don't, sizing for a two- or three-day catch-up keeps the panel manageable and still recovers from a run of cloudy days.

With margin added, that's about a 250 W panel for the one-day target versus roughly 150 W for two days.

Cloud cover is the single biggest source of variation. A bright overcast day can drop panel output to somewhere around a quarter to a half of clear-sky levels, and heavy storm cloud takes it well below that. If your location is often cloudy, either lower the real-world efficiency figure you enter or size the panel for more sun hours than a clear day would give.

Shade behaves worse than its size suggests. Because cells are wired in series, a shadow across even a small part of one panel — a branch, a pole, a roof vent, the edge of an awning — can cut that panel's output by far more than the shaded fraction. A modest panel kept in clear sun and aimed well often beats a larger one that spends part of the day shaded or lying flat.

The practical takeaway: the calculator's number assumes good conditions, so buy above it if your site has frequent cloud, limited clear-sky windows, or unavoidable partial shade.

A required panel wattage is only useful if your power station can actually accept it. Every unit's built-in charge controller has three separate limits:

• Maximum solar input watts. Connect more panel wattage than this and the unit caps the intake — the surplus does not charge it faster.
• Voltage window. The controller needs panel voltage above a minimum to start, and it has a maximum voltage that must never be exceeded. Wiring panels in series raises voltage quickly and can overshoot that ceiling.
• Maximum current. Wiring panels in parallel raises current instead, which has its own limit.

If the calculator's required wattage is higher than your unit's rated input, a bigger panel won't help — you'll need to allow more time, accept a slower or partial charge, add a separate charging method the manufacturer supports, or look at a higher-input model. Find the maximum input watts, voltage range, current limit, and connector type in your power station's manual and check the panel's open-circuit voltage and short-circuit current against them. Exceeding the voltage limit can damage the unit or trip its protection. Don't improvise wiring that the manufacturers don't document.

Up to a point, more panel wattage means more input and a shorter charge. But several ceilings can flatten that out:

• The power station's input cap. Once you reach the rated maximum solar input, extra wattage is simply ignored.
• The battery's charge acceptance. As the battery fills, the management system tapers the current regardless of how much sun is available, so the last 10 to 20% takes longer than the panel size implies.
• A fixed number of sun hours. A panel can only work while the sun is up. Beyond the size that fills the battery within your available sun hours, adding more does nothing that day.
• Real-world conditions. Cloud, heat, shade, and poor aiming scale down every panel proportionally, so a larger panel in bad conditions can still underperform a smaller one in good conditions.

The useful target is the panel that comfortably meets your recharge deadline within your power station's input limit — not the largest panel you can find.

Camping. Work out your daily consumption with the Power Station Size Calculator, then size the panel to replace that much within the peak sun hours you actually get on site. Trees and terrain often cut the usable window, so lean toward a larger panel or a multi-day recharge target.

RV and van. Fixed roof panels are convenient but rarely at the ideal angle, so their real-world efficiency is on the low side — enter a lower figure for them. A portable panel you can tilt toward the sun often out-produces a bigger flat array, and it can be added without exceeding the power station's input limit.

Power outage. Here the panel's job is to keep pace with daily use so a fixed battery lasts indefinitely. Size it against your outage load, check how long a charge lasts each device with the Power Station Runtime Calculator, and see the Solar Charge Time Calculator to check a specific panel against a charge gap. Device-specific guidance is available for refrigerators and Starlink.