Why is my 100 watt solar panel only giving me 75 watts?

Solar panels aren’t magic: a guide to understanding the ratings

Let’s start with the worst case scenario: you bought a 100 watt solar panel to charge your 500 watt-hour ebike battery with the expectation that it would charge your battery from empty to full in 5 hours. What could be simpler? 100 watts times 5 hours equals 500 watt-hours, right?

You unwrap your shiny, new panel, plug it into your boost solar charge controller, plug that into your watt-hour meter (because you’re a smart cookie) and plug that into your ebike battery. Trembling with anticipation, you focus on the meter’s display but instead of 100 watts, it shows that you’re only pushing 70 watts into the battery. You remember reading somewhere that tilting the solar panel to directly face the sun is a good thing so you try that and the power goes up to 82 watts. That’s better. But almost immediately, it starts to go down again, 81 watts, 80, 79… finally settling in at 75 watts as the panel warms up in the sun. You’re thinking “What’s going on here? Did I get ripped off? This solar crap is a total scam! Somebody owes me an explanation!!” Ok, settle down. Let’s break it down.

Did you get ripped off by a dishonest purveyor of solar merchandise? Probably not. Did the seller’s marketing pitch set realistic expectations about the product’s performance under real-world conditions? Probably not. And that’s the problem.

That 100 watt label is a nominal power rating, variously referred to as rated power, PMP, PMPP, Pmax, or Pnom. A solar panel’s voltage, current and power output varies depending on the temperature of the cells and the irradiance level (sun brightness) so the solar industry had to decide on a standard set of conditions under which solar panels would be tested and labeled. The conditions they picked are called “STC” or “Standard Test Conditions” and are defined as an irradiance of 1000 W/m2 and a cell temperature of 25°C (77°F). These values were selected not because they represent typical outdoor conditions under which the panels will be used but because they are cost-effective when flash-testing each panel as it comes off the assembly line in a factory operating at a comfortable room temperature.

For decades, this caused relatively few problems. Consumer solar devices were limited to things like solar calculators and tiny portable lights where the solar panel’s rating didn’t matter to the consumer. Serious, big-boy solar panels like the ones on used on rooftops were installed by professionals who understood the measurement conventions and no one was upset or confused by the whole thing.

More recently, prices have come down and efficiency has gone up until suddenly small but relatively powerful solar panels can be had for US$ 1-3 per watt. Supply and demand being what they are, we now have a very competitive marketplace full of inexpensive solar charging products and equally full of marketing claims. What a time to be alive! The average consumer doesn’t want to study for an electrical engineering degree before buying a solar panel and the solar industry hasn’t been able to get away from that STC rating scheme because it’s just too well established. That’s how we get 100 watt solar panels that only give us 75 watts under relatively ideal conditions. It’s not a scam. It’s just a B2B standard in a B2C world.

Warning: math ahead. If math gives you hives, shortness of breath and/or flatulence, take a deep breath and go to your happy place. I’ll try to be gentle.

This is the label from a typical 100 watt solar panel. Notice how in tiny print it says “Standard Test Conditions”? This panel was tested to have these voltage, current and power values at 1000 W/m2 and 25°C. What the label does not tell you is how to figure out the voltage, current and power values at any other conditions. For that, we will need to look at the datasheet (PDF). I picked this panel because apparently Sunpower is the only semi-flexible panel manufacturer who can be bothered to publish a datasheet. This would never fly in the world of big-boy glass-and-aluminum framed panels with bankable 25 year power warranties, but I digress.

If your irradiance level is 500 W/m2 because it’s slightly cloudy then you might expect to get 50 watts from the panel (500/1000 * 100) but only as long as the temperature of the individual cells inside the panel is 25°C (77°F). Because the cells are almost black, they heat up significantly in the sun. On a warm, summer day when the air temperature is 25°C and we’re getting full irradiance at noon, the cell temperature is going to be somewhere between 45°C and 55°C, depending on air flow over the top and bottom of the panel. On a really hot day, it can get up to 75°C (167°F). Yes, you could cook an egg on that. I wouldn’t.

Let’s assume a cell temperature of 50°C which is 25°C above the standard temperature (50-25). From the datasheet, we see that our panel has a Power Temperature Coefficient of -0.35%/°C meaning that for every °C of temperature rise above 25°C we lose 0.35% of power. That’s right. Photovoltaic (solar electric) panels lose power as they get hotter. I know, it’s kind of counterintuitive but you get used to it. With a 25°C rise, that’s a loss of 8.75% (25 * -0.35%) or we can call it a 91.25% derate (100-8.75).

So, why aren’t we getting 91 watts (100*0.9125)? Odds are, we’re not getting the full 1000 W/m2 of irradiance. Even with perfect panel positioning relative to the sun’s position in the sky, the atmosphere filters out some light, more so when the sun is lower in the sky or when there’s air pollution. Let’s assume we’re getting 900 W/m2 because we don’t see any clouds in the sky and it’s the middle of the day. That would mean we should be getting 82 watts (100*0.9125*0.9).

Since we’re measuring at the output of the solar controller, we have to take into account any losses there as well. No charge controller is 100% efficient. A high frequency DC-DC converter like our boost charge controller is likely to be 85-95% efficient under typical operating conditions. Don’t be fooled by the marketing copy promising “99% peak efficiency”. We could measure the charge controller’s efficiency with a second watt-hour meter connected inline on the input side but let’s just assume it’s operating at 93% efficiency which gives us 76 watts (100*0.9125*0.9*0.93). The remaining 1 watt loss is in the connectors and wiring.

So, how long would it take to charge our 500 Wh battery using a 100 watt solar panel on a clear, sunny day? Lithium batteries like to be charged using a CC/CV profile meaning that the first 80% of the charge is at constant current and the last 20% is at constant voltage. That last part is much slower which is not the solar panel’s fault so let’s ask instead “How long to get to 80% state of charge?” Using our 75 watt example, that would be 5.3 hours (500*0.8/75) assuming you start a little before solar noon and keep re-positioning the panel to maintain the optimal angle. If it’s cloudy or you’re charging very early/late in the day, it make take quite a bit longer.

I’m leaving out some details here like that the battery’s internal resistance means that you have to push in a little more than 500 Wh to fill a 500Wh battery but I’m running out of steam and I’m craving a cookie so let’s call it done for now.

BONUS: If you’re matching a solar panel to the input and output limits of your charge controller and to your battery, you’ll need to take temperature into account. The VOC and VMP values on the label are just a starting point. You’ll need the Voltage Temperature Coefficient from the datasheet for those calculations. If there’s interest, I’ll cover that in part 2. Let me know in the comments. Don’t forget to like, subscribe and smash that notification bell! Oh, wait, this isn’t YouTube. Yeah, I suck at social media.

How to convert your ebike to a solar ebike

This is a solar bike manifesto primer for anyone who has an ebike and is considering adding solar charging. It’s difficult to write for all levels from novice to expert so this advice is most likely to be helpful to someone who has installed a DIY ebike conversion kit. If your experience is limited to riding a factory, turn-key ebike and you don’t do your own maintenance and repairs then you will want to get help from a friend or family member before trying a solar upgrade. Hint: find someone who has their own multimeter.

About me

I have logged 50,000 miles (80,000 km) on ebikes over the past 13 years. About a quarter of those were touring and road-testing miles with various solar panels for charging the bike. I’ve built several solar ebikes over the years and I’m pleased to report that it is indeed possible to combine these technologies. Whether or not it makes sense to do so will depend on your goals and your budget.

My past successes and failures. The failures are always the more instructive of the two.

Maybe solar panels are not for you?

If you’re only doing a couple of weekend ebike trips each year, my advice would be to skip the solar panels, borrow a friend’s battery and plug-in charger in addition to your own and find electrical outlets along the way. In fact, anyone who has reliable access to electrical outlets at the end of each day will find that carrying solar panels on the bike is less convenient and more expensive.

But I WANT to put solar panels on my bike!

Ok, I get it. You’ve seen photos and videos of solar bikes and you want to get in on the fun. Maybe you’re curious about solar power and want to extend your range? Perhaps you want to use solar panels as decorative plumage to attract a like-minded mate? It’s certainly a conversation starter. If you don’t enjoy being the center of attention wherever you go, this might not be the path for you.

I’ve come to think of solar upgrades as falling into two broad categories:

  • Range extenders: 50-200 watt solar panels to supplement your plug-in charging. Expect to get about 5 miles (8 km) of added range for every hour of charging under ideal conditions with a 100 watt panel. This is a great beginner project because it keeps the cost and complexity low while you learn the basics. You can always upgrade later. Folding panels can be stowed in your panniers and deployed when stopped or strapped to your bike/trailer to collect energy all day. Most trailers will fit a 100-150 watt panel easily.
  • Off-grid touring: 200-400 watt solar panels for ultimate roaming freedom away from electrical outlets. Expect to get 50-100 miles (80-160 km) per day. You will need to mount them on the bike so they collect energy all day which presents some challenges due to the large surface area. Recumbents and cargo bikes are popular in this category but I’ve seen some awkward attempts to attach this much to a conventional upright bike.

These range estimates assume you can manage to pack light and always pedal at a moderate effort. Expect to consume about 15 watt-hours per mile (9 Wh/km) while averaging around 14 mph (23 kph). If you’re riding uphill all day, into a headwind, in the rain, without pedaling, then your mileage will vary. These are long-term average values. You’ll get more on a sunny day, less on a cloudy day. If you’re unwilling or physically unable to pedal, cut the daily range estimates in half.

This small 60 watt panel is great for beginners as a range extender.

You can use any solar panel you want but “semi flexible” panels made with Sunpower cells for boats and RVs (campers/motorhomes) are your best choice in terms of power per unit weight and ability to withstand rough treatment on a bike. You can find them in all kinds of sizes on your favorite shopping site for around US$2 per watt and up. Traditional glass/aluminum frame rooftop panels are too heavy and should be avoided.

One unique exception might be these thin film “floppy solar blankets” made by P3 Solar. At US$5-10/watt they are expensive but it’s hard to beat 88 watts per kg. They’re not a good choice for mounting on the bike because they take up so much space when unfurled. But if you only ride every other day and you absolutely must stow your panels while riding because you’re touring Ethiopia and the local children are throwing rocks at you then this might be the panel for you.


The size of your battery has some bearing on the efficiency of your system but does not determine your range in an off-grid situation. Assuming you’re trying to maximize distance traveled in a day, a bigger battery means you can take longer breaks before your battery is full, at which point you have to choose between getting back on the road or wasting potential solar energy because it has nowhere to go. That’s right, longer charge times are actually a feature. Conversely, an undersized battery (300-400 Wh) coupled with a large solar panel may run into problems with too much charging current for the battery cells or the BMS to handle. In that case, you’ll need to explore getting a bigger battery or using multiple batteries with multiple charge controllers.

Charge controller

Speaking of which, how do you connect your solar panels to your battery? You’ll need a “boost solar charge controller.” Just copy and paste those words into your favorite shopping site. You should find a couple of inexpensive Chinese models with MPPT for around US$30-75. A charge controller is a DC to DC converter which takes the solar panel’s output and converts it to the voltage needed to charge your battery. The “MPPT” business means that it automatically adjusts to finds the “Maximum Power Point” at which the panel’s voltage times current produces the most power. This varies depending on solar irradiance and temperature so we need to have Maximum Power Point Tracking to always extract the most power from our solar panel.

The output can be programmed in 0.1V increments to match your battery voltage. If you have the budget, you should get a Genasun boost controller for US$205. These are not programmable but are available in fixed output voltages. The “WP” version is fully potted which means it is waterproof and vibration resistant. They use higher quality components and run cooler, which should (theoretically) make them last longer. Because they don’t need a big heat sink, they are lighter. I’ve done my own performance tests, measuring watt-hours while stationary (unshaded) and moving (mixed shading) and found that the Genasun controller produced about 2% more over the course of a typical day while touring. Here’s a comparison video created by a fellow solar bike enthusiast.

These are currently the two most popular models among solar ebike builders.

Most solar charge controllers on the market today are made for lead batteries and can only be set to increments of 14.4V, corresponding to nominal 12/24/36/48V lead batteries. You cannot use these safely with your lithium ebike battery. Some of these unsuitable chargers may even state that they support “lithium” but upon closer inspection you may find that they only support multiples of 4 LiFePO4 battery cells in series which happen to like being charged to exactly 14.4V (4*3.6=14.4). Most other lithium cell chemistries need 4.2V per cell so you’ll need 42.0V, 54.6V or 58.8V for your nominal 36/48/52V pack. Your charger will need to be configured to the exact voltage your battery pack needs. If you’re unsure, figure it out before you plug anything into your pack. The labels on the charger that came with your bike and your battery itself are good starting points.

These controllers have PV input ranges which will work with most solar panels — just make sure that the open circuit voltage of your panel (VOC) is less than your battery voltage when empty (around 3.2V per cell) or you may find that you will not be able to charge when the input voltage is higher than the output voltage under some conditions (low battery on a cold day). Understand the specifications of your battery, charge controller and solar panel, keeping in mind that the solar panel voltage is lower than the label value when it gets warm. More on that here.

You may be able to connect two small panels in parallel but with larger panels that will likely exceed the maximum input current so you may need multiple charge controllers. There are trade-offs to be made: for example, a higher input voltage will result in better controller efficiency than a lower input voltage but connecting panels in parallel will give you better partial shading mitigation which matters if any part of your bike or body casts even small shadows on the panel.

If you have a Bosch, Yamaha or Shimano battery… I offer you my condolences. These closed, proprietary systems make it much more difficult to modify and enjoy your bike as you see fit. They’re well-engineered systems, designed to maximize corporate shareholder value and minimize liability and warranty claims. These vendors have no interest in helping you with your wacky solar modification project or supporting inter-operability with equipment from other vendors. If you’re doing pre-purchase research and solar charging is important to you then brands which reject open standards do not deserve your business.

Apparently, the part of Bosch that designs deliberately incompatible plugs does not coordinate with the part of Bosch that writes their website copy.

I’ve read that you can trick the Bosch batteries into accepting a charge from a non-Bosch source by applying +5V to the signal pin and keeping charge current at 4A or less. If anyone knows a similar trick that will work with Yamaha or Shimano, please let me know. I know several solar ebike enthusiasts who charge using AC inverters on the bike using the charger that came with their bike and an intermediate 12V battery but these workarounds are heavy and inefficient. They should only be considered as an option of last resort.

I have written extensively about my solar conversions. I mention this as proof of real world experience in this subject matter and not as an example of a low-cost beginner project. Hopefully, my build will provide some inspiration for what is possible. You can do it. Start small and keep it simple. Add more later after you’ve had your first success. It’s not rocket science. If budget is an issue, you may find used solar panels on eBay, craigslist or your local equivalent. Or reach out to local solar installers or RV/boat supply shops and ask if they have any returned, blemished or damaged panels they’re willing to donate to you. Most of all, stop “thinking about it” and get out there and start doing something about it.

My current build has a 315 watt solar array good for around 80 miles (130km) per day. Just for fun, I recently did a 207 mile (333km) single-day ride using 784 Wh from grid-charged batteries and generating 2266 solar Wh so roughly equivalent to carrying six 500 Wh ebike batteries. Here’s a video.

If you want to learn more, I recommend watching the following presentation by someone who is far more knowledgeable than I.

Happy solar biking.