Solar Charge Time Calculator

The estimate is built from three short steps:

1. energy needed (Wh) = capacity × (target% − current%)
   How many watt-hours you have to put back into the battery.
2. effective input (W) = panel watts × real-world efficiency
   The rated panel power scaled down to what you actually collect.
3. charge time (hours) = energy needed ÷ effective input
   And, if you know your site's days ≈ charge time ÷ peak sun hours per day.

This is a linear approximation. It assumes the panel delivers a steady effective wattage for every hour of charging, which real sunlight never quite does, and it doesn't model the slowdown that usually happens in the last stretch before full. It's a solid planning number, not a stopwatch.

The number 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 hitting it straight on. Outdoors, several things pull the real figure below that:

• Sun angle. Output is highest when light hits the panel perpendicularly. A flat or poorly aimed panel loses a large share of its potential, especially in the morning and late afternoon.
• Panel temperature. Panels lose efficiency as they heat up, so a hot panel in still air can produce noticeably less than its rating even in full sun.
• Haze, dust, and glass losses. Thin cloud, atmospheric haze, and a dusty or dirty surface all trim output.
• Wiring and controller losses. Long or thin cables, connectors, and the charge controller itself each take a small cut, and a controller can only extract so much when panel voltage is low.

A common planning approach is to assume you'll sustain roughly 60 to 80% of a panel's rating under good conditions, and less when it's cloudy or the panel is fixed flat. That derating factor is exactly what the "real-world efficiency" field in the calculator represents.

Peak sun hours are not the same as 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 from a dim sunrise to a dim sunset might last 13 hours but only add up to 4 or 5 peak sun hours once the weak early and late light is accounted for.

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

In this calculator, peak sun hours per day are only used to turn an ideal charge time in hours into a rough number of days. If you leave the field at zero, you just get the hours figure.

Cloud cover is the biggest single variable. A bright overcast day can drop panel output to somewhere around a quarter to a half of clear-sky levels, and heavy storm cloud can take it well below that. Over a run of grey days, a panel that comfortably tops up your battery in summer sun might only replace part of what you use.

Shade behaves worse than people expect. Because of how cells are wired in series, a shadow across even a small part of one panel — a branch, a pole, a roof vent, the corner of an awning — can cut that panel's output by far more than the shaded fraction. Moving a portable panel a short distance to clear a shadow, or angling it toward open sky, often recovers more power than adding another panel would.

For trips longer than a day, it's safer to plan around a cloudy-day partial charge and treat a full sunny-day recharge as a bonus rather than the baseline.

Every power station has a ceiling on how much solar power it will accept, and it's set by three separate limits in the built-in charge controller:

• Maximum solar input watts. Connect more panel wattage than this and the unit simply caps the intake — the surplus does not make it charge faster.
• Voltage window. The controller needs the panel voltage above a minimum to start charging, 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.

Before buying or combining panels, find these three numbers and the 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 in particular can damage the unit or trip its protection. If a combination isn't covered by the manufacturers' own guidance, don't improvise the wiring — use a setup they explicitly support.

A partial charge moves less energy, so it finishes sooner. For a 1,000 Wh power station on a 200 W panel at 70% efficiency, a full 0 to 100% charge needs 1,000 Wh and about 7.1 hours of ideal sun, while a 20 to 80% top-up needs only 600 Wh and about 4.3 hours.

There's a second effect the linear formula doesn't capture. Many batteries slow their charge rate in the last 10 to 20% as the management system switches from constant current to constant voltage, so the stretch from 80 to 100% can take longer than a same-sized band lower down. Stopping around 80 to 90% often gives you more usable energy per hour of sun, and routinely leaving a little headroom rather than sitting at 100% is generally easier on the battery.

Camping. The question that matters is whether a day of sun replaces a day of use. Estimate your daily consumption with the Power Station Size Calculator, then check whether your panel can realistically collect that much in your available peak sun hours. Carry a bit more battery than one perfect day requires so a cloudy afternoon doesn't leave you short.

RV and van. Fixed roof panels are convenient but rarely at the ideal angle, so their real-world efficiency is on the low side; a portable panel you can aim at the sun often out-produces a larger flat array. Watch the power station's maximum solar input if you plan to combine roof and portable panels.

Power outage. Solar turns a fixed battery into a rechargeable one, which is what lets you ride out a multi-day outage. Pair it with restrained loads — see the Power Station Runtime Calculator to see how long a charge lasts a given device. Device-specific guidance is available for refrigerators and Starlink.