How much power a portable solar panel makes on a trip

A portable panel adds energy to a power bank for the hours it is in direct sun, and nothing for the rest of the day. The daily energy is the panel’s rated watts, multiplied by the equivalent sun hours it receives and by the end-to-end efficiency of the charging path. Because the sun hours and the angle vary through the day and from day to day, the result is a scenario, not a forecast.

The daily energy formula

The power budget tool uses one line for the solar side of the budget:

solar Wh = panel W × sun hours per day × efficiency × days

For a 10 W panel, two equivalent sun hours a day and 80% efficiency, the daily figure is 10 × 2 × 0.8 = 16 Wh. Over three days it is 16 × 3 = 48 Wh. The 80% is the same end-to-end assumption used for the bank’s own output, covering the panel’s regulator or charger, the cable and the bank’s input path. It is an assumption, and the methodology page says so.

The term that does the most work is the sun hours. Panel watts are a rating, not an average: manufacturers rate panels at standard test conditions, which NREL gives as 1,000 W/m² of sunlight, a 25 °C cell temperature and an air mass 1.5 spectrum. NREL also notes that in strong sun, modules typically run much hotter than 25 °C, so the sun-hour figure converts the rating into a day’s energy.

The sun-hour number is an input, and it differs from the length of daylight. A day can last 12 hours and still deliver only 2 sun hours of useful energy if the sky is hazy or the panel is poorly aimed.

What a peak sun hour is

A peak sun hour is one hour of sunlight at an irradiance of 1 kW/m², which is the strength of direct sun at noon in clear conditions. A day that delivers 4 kWh/m² of solar energy on a horizontal surface is therefore equal to 4 ÷ 1 = 4 peak sun hours. Choose the sun-hour figure for the place and season of the trip, from a solar-resource source for that region, and lower it for the angle and shade a portable panel actually sees.

The solar tool on this site gives sunrise, sunset, sun elevation and sun bearing. Those values describe when and where the sun is, which is useful for aiming a panel and for planning the day. They do not give the solar energy received, because that depends on cloud and haze. Use them together: the sunrise and sunset times set the window, and the sun-hour figure says how much of that window is productive.

How panel angle reduces the direct beam

Output is highest when the panel faces the sun squarely. For direct sunlight, the power on a flat panel falls with the cosine of the angle between the sun’s direction and the panel’s perpendicular. This is pure geometry, and it applies to the direct beam only. The table shows the fraction of direct-beam output at several angles.

At 60° off the sun the panel receives half the direct-beam power, so the sun-hour figure you use should reflect the average angle over the day, not the best angle at noon. A panel hung on a moving pack changes angle constantly as you walk, so the effective figure is often lower than a panel laid flat on a rock. The diffuse light from a bright sky is not in this table, and it is not modelled here.

The solar tool helps with the aiming. It gives the sun’s bearing and elevation for a chosen time and place, so you can set the panel towards the sun for the longest part of the charging window. The sun direction at a campsite guide applies the same idea to where to place the shelter and the panel.

Scenario table for a 10 W panel

The table shows the daily energy in watt-hours for three panel sizes and three sun-hour values, at 80% efficiency. The values are multiplied out from watts × hours × 0.8.

A 5 W panel gives a quarter of the 20 W figure in every column, because the rating is the only term that changes across a row. Doubling the panel doubles the energy, as doubling the sun hours does. The sun hours are the term the weather controls, so they often move the result more than the panel size does.

A three-day trip with a 10 W panel

Take the 2 W average device load for 3 hours a day over 3 days from the phone guide. That is 18 Wh of demand. The bank alone, at 80%, delivers 29.6 Wh, so the bank covers the demand without any sun. The panel is a margin, and its value depends on the days.

daily solar  = 10 × 2 × 0.8 = 16 Wh
three days   = 16 × 3 = 48 Wh
bank usable  = 37.0 × 0.8 = 29.6 Wh
demand       = 2 × 3 × 3 = 18 Wh
balance      = 29.6 + 48 − 18 = 59.6 Wh

The balance is the stored energy plus the solar energy minus the demand. With a sunny forecast of two sun hours a day, the panel supplies most of the trip’s energy, and the bank is a reserve rather than the main source. That is a strong position to plan from, but it depends on the sun hours being right.

The zero-sun day

Solar energy is zero on a day with no direct sun. A panel is not a steady source, so the plan must work when a day gives nothing. Take the same panel with one day of no sun in the three-day trip:

day 1: 10 × 2 × 0.8 = 16 Wh
day 2: 10 × 2 × 0.8 = 16 Wh
day 3: 0 × 0.8 = 0 Wh (no direct sun)
solar total = 32 Wh
balance     = 29.6 + 32 − 18 = 43.6 Wh

The balance stays positive, because the bank covers the 18 Wh demand on its own. The panel only changes the margin here. Without the bank the picture is different. The panel would need to supply the whole 18 Wh over three days, which is 18 ÷ 3 = 6 Wh a day. At 10 W and 80% that is 6 ÷ (10 × 0.8) = 0.75 sun hours a day, so a panel without a bank depends on fairly steady sun.

Shade, cloud and temperature

Several things reduce the output of a portable panel below the rated figure, and the simple formula does not model most of them. The list below names each one and how to allow for it.

• Shade on any part of the panel. Partial shade on any part of the panel reduces output, often by more than the shaded share of its area suggests, so keep the whole face clear.
• Cloud and haze. These lower the sun-hour figure. They also add diffuse light, which the direct-beam table does not cover.
• Angle. The cosine table above gives the direct-beam effect only, and a moving pack changes the angle all day.
• Cell temperature. Panel output changes with temperature, and the panel’s datasheet is the source for the figure. This page gives no per-degree value.
• Charging losses. The 80% assumption stands in for the regulator, cable and input path, and the real value depends on the hardware.

The effect of these is to move the sun-hour figure down, or the efficiency down, or both. The formula handles both, so a cautious plan uses lower values for both rather than a single optimistic number.

Planning around the sun

Plan the charging window with the daylight times rather than the whole day. The remaining daylight guide explains how to count the hours left, and the sunrise and sunset in the solar tool give the start and end of that window. Charging is most productive in the hours near midday, when the panel can face the sun most directly, so put the panel where it will be in sun during those hours rather than at the start or end of the day.

The same window matters for the cold. Solar charging in winter has less sun and a lower sun, and the batteries in cold weather guide covers how cold changes the bank’s own output. For how the panel and the phone share the same budget, the phone charges guide shows the count, and the mAh to Wh guide sets out the bank’s stored energy.