Solar panels for camping: when they earn their weight

A panel is rated in watts under laboratory conditions you will almost never meet outdoors. What decides whether it earns its place is not the rated wattage but the number of equivalent full sun hours you can actually collect, and how much battery those hours replace. On trips shorter than about four days, the panel usually weighs more than the extra battery it saves.

What a rated watt actually means#

Panel ratings come from standard test conditions defined in IEC 60904-3: irradiance of 1000 watts per square metre, the air mass 1.5 reference spectrum, and a cell temperature of 25 degrees C. Every one of those is optimistic in the field.

Irradiance at ground level reaches roughly 1000 W per square metre only near solar noon on a clear day with the panel square to the sun. Cell temperature is the sneakier one: a panel sitting in full sun on a dark pack lid runs well above 25 degrees C, and silicon output falls as cells warm. The rating is not dishonest, it is a comparison basis, in the same way a stated fuel consumption figure is a comparison basis. Treat rated watts as a way to compare two panels, never as a prediction of what you will collect.

Equivalent full sun hours, the only planning unit#

The way to plan solar is to ask how many hours of full rated output the day is worth in total, then multiply by the rating. That number, the equivalent full sun hours, absorbs the angle, the haze, the cloud and the length of the day into one figure.

A 10 watt panel at 2 equivalent full sun hours produces 20 Wh, of which roughly 85 percent survives conversion into a bank, so about 17 Wh lands in storage. That is the number to plan with, and it is a long way from "10 watts, eight hours of daylight, 80 watt-hours".

Why one branch shadow costs so much#

Cells in a panel are wired in series, and a series string passes the current of its weakest member. Shade one cell and it does not simply contribute less: it limits the current for the whole string, so output can fall out of all proportion to the shaded area. Bypass diodes limit the damage by routing current around a shaded group, but they work at group level, not cell level.

The practical consequences are worth memorising:

• A thin branch shadow crossing the whole panel is worse than a solid shadow over one corner, because it touches every string.
• A guy line, a strap end or a tent pole shadow can cost a disproportionate share of your day.
• Dappled forest light does not average out. A panel under moving canopy shade delivers far less than the average illumination suggests.
• Check the panel every time you move: the shadow that was not there at 10 am owns the panel at 2 pm.

Angle beats area#

Output scales with the cosine of the angle between the panel surface and the incoming sunlight. A panel lying flat on the ground in the morning is receiving light at a steep angle and losing most of it. Propping it towards the sun costs nothing and no weight.

This makes repositioning the highest-value habit in camp solar. Three adjustments a day, at breakfast, at lunch and mid-afternoon, is the difference between the 2 hour row and the 3 to 3.5 hour row in the table above, which is a 50 to 75 percent increase in harvest for a few minutes of attention. Buying a panel 50 percent larger to achieve the same thing costs mass on every step of the walk.

Which explains the awkward truth about walking with a panel on your pack: it is at a fixed and usually poor angle, it is shaded by your own head and shoulders, and it swings. It collects something, but treat it as a bonus rather than the plan.

The break-even, in grams#

Here is the arithmetic we use, and it is the only honest way to answer whether a panel is worth carrying.

A panel replaces battery mass. Lithium banks store roughly 5.4 grams per watt-hour, from the typical specification of a 10,000 mAh (37 Wh) bank weighing 180 to 220 grams: 200 divided by 37 is 5.4. A folding panel in the 10 to 15 watt class typically weighs 400 to 600 grams with its cable, so we model 500 grams for a 10 watt panel.

Each day the panel is out, it saves you the battery mass that would have held the energy it collected, after the same 85 percent conversion loss.

That is the case for the common advice that solar starts making sense around four days. But the table hides a second condition that matters more, and almost nobody states it.

The panel only replaces battery you would otherwise have carried. If your daily consumption is 6 Wh, a phone in airplane mode and a headlamp, the panel cannot save you 27.6 Wh of battery per day. It saves you 6 Wh of battery per day, because that is all you were going to carry. Redo the last two columns on that basis:

• 6 Wh per day times 5.4 g per Wh is 32 g of battery saved per day.
• 500 g divided by 32 g is about 15 days before the panel breaks even.

A disciplined hiker with one phone essentially never carries a panel profitably. A photographer charging camera batteries, a group sharing one panel across four phones, or a party running a satellite messenger and a tracker daily crosses the line in a few days. Consumption sets the break-even far more strongly than trip length does, which is why the honest first step is to total your loads in the power budget calculator or by the method in power banks and batteries, then come back to this table.

When solar is the wrong answer#

• Trips under four days. A larger bank is lighter, more reliable and needs no attention. This is most weekend and long weekend camping.
• Forest walking. Canopy plus movement is the worst case for a panel, and it is also where you are least likely to stop and reposition.
• Winter and high latitudes. Short days, low sun angles and persistent cloud stack against you exactly when cold is also cutting your battery capacity.
• Any trip that passes a socket. A cafe stop, a hut, a resupply town or a trailhead vehicle beats any panel. Plan around charging opportunities before you plan around generation.
• Low daily consumption. Under roughly 10 Wh a day, the break-even runs past two weeks. An efficient headlamp used at the output the job needs, as set out in headlamps and the FL1 standard, keeps you in that bracket.
• Rushed schedules. Solar rewards a camp that sits still in the open for hours. If you are walking dawn to dusk and sleeping in trees, you own a panel and collect nothing.

Where solar genuinely earns its place: long remote trips without resupply, basecamps in open country, group charging, cameras and drones with several batteries each, and any vehicle trip running a fridge, which is a different scale of problem entirely and is covered in 12 volt power and camping fridges.

Check this before you buy, and test it at home#

You can characterise a panel in a weekend with domestic equipment.

1. Get an inline USB power meter that totals watt-hours. Put it between the panel and the bank.
2. Set the panel out on a clear day, angled towards the sun, and reposition it three times. Read the total watt-hours at dusk.
3. Divide that total by the panel's rated watts. The result is your equivalent full sun hours for that day and place. Compare it with the table above.
4. Repeat with the panel lying flat, and again strapped to a pack for a walk. The three numbers tell you what repositioning is worth to you specifically.
5. Deliberately shade one cell with a finger and watch the reading. Most people are startled by how much they lose, and the demonstration changes how carefully they site the panel afterwards.

Two buying checks are worth making before any of this. Confirm the panel charges a bank without dropping the connection every time a cloud passes, because a panel that resets the charging handshake on every shadow can spend a whole day achieving nothing. And weigh it yourself, with the cable, since the mass in the break-even table is the mass in your pack, not the mass on a specification sheet. If you are counting grams at this level, our base weight guide explains where the panel belongs in the total.

Common mistakes#

Multiplying rated watts by hours of daylight. A 10 W panel does not make 80 Wh in an eight hour day. It makes 20 to 35 Wh in good conditions, and less after conversion.

Buying a bigger panel instead of moving the one you have. Angle beats area. Repositioning three times a day is worth roughly 50 to 75 percent more harvest at zero grams.

Charging a phone directly from a panel. Passing clouds interrupt the charge and some phones stop and restart repeatedly. Charge the bank from the panel, then the phone from the bank, and you buffer the variability.

Ignoring shade from your own kit. Guy lines, poles and pack straps put thin shadows across whole strings, which is the shape of shadow that hurts most.

Leaving the bank in the sun with the panel. Heat ages lithium cells and reduces the charge they will accept. Panel in the sun, bank in the shade, cable between them.

Taking a panel for the fridge without doing the sums. A fridge changes the numbers by an order of magnitude, needs panel and battery sizing done together, and should be compared against simply running a cooler, which we cost out in coolers and ice.

Frequently asked questions#

Are solar panels worth it for camping?#

For trips under about four days, no: a larger power bank is lighter and needs no management. Beyond four days without a socket, and especially with several devices or a group sharing, a panel starts to pay. For vehicle trips running a fridge, solar moves from optional to close to essential.

How many watts of solar do I need for camping?#

Work backwards from consumption. Divide your daily watt-hours by realistic equivalent full sun hours for your conditions, 2 as a default and 3 to 3.5 with open sky and repositioning, then divide by 0.85 for charging losses. A hiker using 15 Wh a day needs about 15 divided by 2 divided by 0.85, near enough 9 watts.

What are peak sun hours or equivalent full sun hours?#

They are a way of compressing a whole day of varying sunlight into a single number: the hours of full rated output that would produce the same total energy. Multiply the panel rating by that figure to estimate daily harvest. Open sky with repositioning gives 3 to 3.5, a fixed pitch about 2, forest or heavy cloud 1 or less.

Why does a small shadow stop my solar panel working?#

Because cells are wired in series and a series string carries only as much current as its weakest cell. A shaded cell throttles the whole string, so a thin shadow across a panel costs far more than its area. Bypass diodes help at group level, but the fix is siting: keep every part of the panel clear.

Can I charge my phone directly from a solar panel?#

You can, but it works badly. Output varies with every cloud, and many phones respond by stopping and restarting the charge, sometimes ending the session entirely. Charge a power bank from the panel and the phone from the bank. The bank absorbs the variability and the phone sees a steady supply.

Do solar panels work in cloudy weather?#

They produce, but far less. Overcast conditions leave only diffuse light, and daily totals fall to around 1 equivalent full sun hour or less. Panels do not stop working in cloud, but no panel you would carry in a pack will keep pace with a real device load through several overcast days. Plan battery capacity for the cloudy case.

Should I strap the panel to my pack while walking?#

Treat it as a small bonus, not a plan. On a pack the panel sits at a poor angle, is shaded by your head and shoulders, swings on the strap and gets no repositioning, so expect around 2 equivalent full sun hours at best and much less in trees. Panels also wear at their mounting points if they flap.