Let’s assume a cell temperature of 50☌ which is 25☌ above the standard temperature (50-25). On a really hot day, it can get up to 75☌ (167☏). On a warm, summer day when the air temperature is 25☌ and we’re getting full irradiance at noon, the cell temperature is going to be somewhere between 45☌ and 55☌, depending on air flow over the top and bottom of the panel. Because the cells are almost black, they heat up significantly in the sun. If your irradiance level is 500 W/m 2 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☌ (77☏). This would never fly in the world of big-boy glass-and-aluminum framed panels with bankable 25 year power warranties, but I digress. I picked this panel because apparently Sunpower is the only semi-flexible panel manufacturer who can be bothered to publish a datasheet. For that, we will need to look at the datasheet ( PDF). What the label does not tell you is how to figure out the voltage, current and power values at any other conditions. 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/m 2 and 25☌. This is the label from a typical 100 watt solar panel. If math gives you hives, shortness of breath and/or flatulence, take a deep breath and go to your happy place. That’s how we get 100 watt solar panels that only give us 75 watts under relatively ideal conditions. 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. 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. 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. 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. 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. 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.įor decades, this caused relatively few problems. The conditions they picked are called “STC” or “Standard Test Conditions” and are defined as an irradiance of 1000 W/m 2 and a cell temperature of 25☌ (77☏). 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. That 100 watt label is a nominal power rating, variously referred to as rated power, P MP, P MPP, P max, or P nom. Did the seller’s marketing pitch set realistic expectations about the product’s performance under real-world conditions? Probably not. Let’s break it down.ĭid you get ripped off by a dishonest purveyor of solar merchandise? Probably not. 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. 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 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. 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 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. What could be simpler? 100 watts times 5 hours equals 500 watt-hours, right? 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. Solar panels aren’t magic: a guide to understanding the ratings
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