Solar Charging an Electric Bike

The appeal is obvious. An electric bike carries a battery, the sun is free, and portable panels have got cheap enough that a 100W folding unit costs less than a decent set of tyres. Somewhere in the middle of that reasoning most people conclude that a panel strapped to a pannier will keep a touring ebike topped up indefinitely.

It will not, and the gap between expectation and reality is roughly an order of magnitude. The problem is not that solar charging does not work. It works fine. The problem is that a panel's nameplate rating is measured under laboratory conditions you will never see, and that every watt hour it does produce has to cross three or four energy conversions before it reaches a cell. Each conversion takes its cut, and they multiply rather than add.

This piece does the arithmetic in full, because the arithmetic is the entire answer. Once you have the real numbers, the question of whether solar makes sense for your riding becomes easy, and for most people it resolves quickly in one direction or the other.

The number people get wrong

Start with the load. A common mid-market ebike carries a 48V 14Ah pack, which is 48 multiplied by 14, or 672 watt hours. Bigger commuters and cargo bikes run 720 to 1,000 Wh, and the dual battery machines in the longest range electric bikes go past 1,500 Wh. Take 672 as the working example.

Now the source. A panel rated at 100W is rated under Standard Test Conditions: 1,000 watts per square metre of irradiance, a cell temperature of 25 degrees C, and an air mass of 1.5. Those conditions occur in a test lab and, briefly, on a cold clear day at high altitude with the panel aimed perfectly at the sun. They are not what happens on the roof of a tent.

The industry handles this with peak sun hours, which is the daily solar energy at a site expressed as an equivalent number of hours at full 1,000 W per square metre. Across the continental United States the annual average runs roughly 4 to 5.5 peak sun hours. A good summer day in the southwest can reach 6 or 7. A December day in the Pacific Northwest or New England is 1.5 to 2.5. That seasonal swing is larger than most people building a touring plan account for, and it compounds with everything in the way cold weather already cuts your range.

So a 100W panel at 5 peak sun hours is nominally 500 watt hours a day. If that number were real, one panel would charge a 672 Wh pack in a day and a half. It is not real, because peak sun hours describe the light arriving, not the electricity leaving.

Where the watt hours actually go

Four categories of loss sit between the sunlight and the cells, and they stack multiplicatively.

Panel derating

Silicon photovoltaic cells lose output as they heat, with a temperature coefficient typically between minus 0.35 and minus 0.45 percent per degree C above 25. A panel sitting in full sun runs 25 to 30 degrees above ambient, so on a 30 degree day the cells are near 55 to 60 and the panel is down 12 to 15 percent before anything else has happened. Add soiling, which is 2 to 5 percent on dust and pollen, and cable losses of another 2 to 3 percent on the thin leads portable panels ship with.

Then the big one: angle. Output falls with the cosine of the angle between the panel and the sun. A folding panel propped at a fixed angle and left alone all day, which is what everyone actually does, loses 10 to 25 percent against a tracked panel. Lay it flat on a tent or a rack and it is worse. Shade a single cell and, depending on the bypass diode layout, you can lose a third of the panel's output from a tent guy line.

Multiply those and a realistic figure for what a portable panel delivers is 70 to 80 percent of its nameplate over a day. Call it 75 percent.

The charge controller

A panel's maximum power point is a specific combination of voltage and current that moves constantly with light and temperature. A cheap PWM controller simply clamps the panel to the battery voltage and throws away the difference, which on a mismatched panel costs 20 to 30 percent. An MPPT controller actively tracks the maximum power point and converts, and good ones run at 93 to 97 percent efficiency. This is the one place in the chain where spending money is unambiguously worth it.

Buffer storage

You cannot charge an ebike straight from the panel, for reasons covered in the next section, so the harvest goes into an intermediate battery. Lithium round trip efficiency in a decent power station is around 90 to 95 percent.

Inverter and bike charger

To use your stock bike charger you need mains AC, which means an inverter. A power station's built-in inverter runs 85 to 90 percent efficient at moderate load, and worse at very light load because its own idle draw is fixed. The bike charger itself is another switching supply at 85 to 90 percent, as covered in how ebike chargers work.

That final line is the honest answer, and it is why so much online advice on this topic is wrong. The mistake is almost always the same: someone divides pack watt hours by panel watts, gets seven hours, and stops. The real ratio between a panel's nameplate and what lands in the battery is around 0.5, and it drops further in weather, in winter, and with a cheap controller.

Why you cannot plug a panel into a bike charger

Two versions of this idea circulate and both fail, for different reasons.

Panel into the bike charger. Your charger has an input line reading 100 to 240V AC, 50 to 60 Hz. It contains a rectifier and a switching front end expecting a stiff mains supply. A solar panel produces DC at 18 to 40V depending on the configuration, and its voltage sags under load and jumps when a cloud passes. There is no wiring arrangement that makes one acceptable to the other. You need an inverter, which is a device that manufactures AC, and then you are back to the chain above.

Panel straight into the battery. This is the more dangerous idea because it superficially works. Connect a nominally 36V panel array to a 36V pack and current will flow. What is missing is the entire charge profile. A proper charger holds constant current until the pack hits its target voltage, then holds that voltage while current tapers to a cutoff. A panel does neither. It delivers whatever current the light allows at whatever voltage the pack drags it to, and it has no idea when to stop.

The failure modes are specific. Panel open circuit voltage rises as temperature falls, so a panel whose maximum power point voltage is 18V may show 22V or more on a cold bright morning, and an array sized to look right at noon in July can overshoot in October. Nothing terminates the charge, so the pack sits at its top voltage as long as the sun holds. And the battery management system, which people assume will catch all of this, is a protection circuit. It opens the charge path when a cell group exceeds its limit and it will do that repeatedly and indefinitely if you make it the primary control, which is not what it was designed for. The division of responsibility between charger, controller, and management board is worth understanding properly, and the guide to ebike controllers and electronics covers it.

The three arrangements that actually work

Swipe sideways to see all columns →

The power station route is what almost everyone should use. A portable station from Jackery, EcoFlow, Bluetti or similar contains the MPPT controller, the buffer battery, and the inverter in one box with an AC socket on the front. You plug the panel into one side and your normal bike charger into the other. It is the least efficient sensible option and by a wide margin the most robust, because none of the voltage matching is your problem. It also charges your phone, lights, and everything else, which matters more on a real trip than the efficiency gap.

The house battery route is the same thing unpackaged, and it makes sense only when the bank and the panels already exist for another reason. If you have a van with roof solar and a 100Ah house battery, charging a bike off it costs you nothing extra in kit. This is quietly the best case in the whole article.

The direct DC route skips the buffer and the inverter by using a charger that accepts DC input and produces the pack's exact CC/CV profile. The Grin Satiator is the well known example, and Genasun and Victron make boost controllers that can be configured for a specific pack. Cutting two conversions is worth 15 to 20 percentage points, which is real. The catch is that setting the target voltage wrong here damages the pack directly, and you have removed the buffer, so the charge stops and starts with every cloud. Cells do not love that, and neither does the balancing behaviour of most management boards. This is the right answer for someone who has read the pack's specification sheet and understands it, and the wrong answer for everyone else.

What a real touring rig weighs and costs

The theoretical discussion collapses the moment you have to carry the hardware. Portable solar is not heavy per watt by household standards, but a bicycle is a very unforgiving platform.

Thirty five to fifty five pounds is not pannier weight. It is a trailer, and pulling one has its own effect on consumption, which is worth reading about in ebike trailers and towing before you commit. There is a genuine irony here: the extra rolling resistance and mass of hauling a full solar rig eats a measurable fraction of the energy the rig produces.

What most bikepackers actually carry is a 30 to 60W panel bungeed to the top of a rear bag. Run that through the same chain and it delivers roughly 75 to 150 watt hours a day, which is 11 to 22 percent of a 672 Wh pack. That is a very good phone, headlamp, and GPS charger with a slight bicycle side effect. Sold as an ebike charging solution it is misleading; sold as a device charger that occasionally adds four or five miles of assist, it is honest and quite useful.

Panels mounted on the bike itself

Every few years a bike appears with panels integrated into the rack, the panniers, or a fairing, and it gets written up as the future. The physics has not changed.

A rear rack is roughly 0.3 square metres. At the 20 to 22 percent module efficiency of good consumer panels, that area intercepts about 60 to 65W under perfect noon sun and delivers rather less. Now put it on a moving bicycle. The panel is horizontal rather than aimed, which costs a large fraction of the remaining output outside the middle of the day. Your body shades it whenever the sun is behind you. It cannot be cleaned or repositioned while you ride. And it adds weight and, if it is any bigger than the rack, aerodynamic drag, at speeds where aerodynamic drag is already the dominant force resisting an ebike.

The rough result is a system that might contribute 15 to 30 watt hours over a full riding day, against a consumption of 300 to 600 Wh for that same day. It is a one to five percent contribution paid for with weight, cost, fragility, and drag. Bike mounted solar is a demonstration, not a system, and it should be evaluated as one.

Solar on a stationary bike is a different and better proposition. If the bike is parked at camp while the panel works all day, you get the full harvest without any of the mounting compromises, and that asymmetry is the basis of every setup in this article that actually functions.

When it is genuinely worth it, and when it is theatre

That last line is the comparison that settles it for a lot of people. A spare 672 Wh pack weighs about 8 lb and costs somewhere between $400 and $700. It gives you a guaranteed second full charge, instantly, in any weather, at night, with no setup. A solar rig capable of producing that same 672 Wh in one clear day weighs four to six times as much, costs more, and does nothing at all under cloud. If your requirement is "more range on a three day trip", a second battery wins outright, and the section on pack sizing in volts, amp hours, and watt hours is the place to work out what you need.

Solar wins when the trip is long enough that carrying enough batteries stops being feasible. Somewhere around day four or five, depending on how hard you ride, the cumulative energy demand exceeds what you can reasonably carry in cells, and a panel that keeps producing every day starts to pull ahead. That crossover is the real decision point, and it is a duration question rather than an efficiency one.

If you are working through the practicalities, start with the charger you already own, because it sets the AC requirement for the whole system and its efficiency is one of the multipliers above. Ebike chargers explained covers voltage matching and the CC/CV profile. If cold weather is part of the picture, note that you must never charge a pack below freezing regardless of where the energy came from, which is one of several traps in riding an ebike in winter. And if the underlying goal is simply more miles between charges rather than off grid capability, the longest range electric bikes is a more direct route to it.

Frequently asked questions

Sources and further reading

• UL 2849: Standard for electrical systems for ebikesUL Standards & Engagement
• Bosch eBike Systems: drive units and batteriesBosch eBike Systems
• Lithium-ion battery safetyNational Fire Protection Association