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# Understanding WaterLily's Power Ratings

There has been some confusion about the power rating of the WaterLily turbines, and what it all means. We asked our engineering team to help put together a post that will explain the ratings in easy to understand terms.

First off, both the USB output and the 12V output turbines have the same maximum 15W power output, but the actual power is limited by both the flow speed and on the device or battery you are charging.

We’ll start with a basic physics refresher, just so we are all on the same page. Power in Watts (P) is calculated by multiplying the voltage (V) by the current in Amps (I), so that’s P=IV. The other piece of information you need to know is that a millamp (mA) is a thousandth of an Amp, and a miliwatt (mW) is a thousandth of a Watt.

Let’s begin with the WaterLily with USB output. USB output is 5V--that works out to 3A, but a USB device can only use about 2A. Any phone or battery that you plug directly into the WaterLily will therefore max out at 10W. So, why do we advertise it at 15W? Because you can use a USB hub to connect multiple devices to your turbine at once. If you’re in a fast enough flow, and the turbine can output over 10W, using a USB hub will let you get the full 15W output. The other gotcha with USB charging is that “dumb” devices that don’t do any power negotiation at all are only allowed to use 100mA and simple power negotiation only allows for 500mA. Many small devices like headphones and simple battery banks fall into these categories, so those devices will max out at 500mW and 2.5W respectively.

The more complicated issue comes from the WaterLily with 12V output. Again, the maximum output is 15W, and the actual voltage output is 14.6V (it’s not actually 12V, because a 12V battery’s full voltage is 14.4V, and this lets you charge those batteries without the need for a charge controller). So, working from those numbers the current output should be just a hair over 1A--but we advertise a maximum 2A output! This doesn’t add up! We must be lying or cheating you somehow! Not really, but the reason is unintuitive; you see, when you connect the WaterLily to a heavy load (such as a lead-acid battery) it can pull the turbine’s output voltage down to a much lower value. So, if your battery is discharged down to only 11V, the WaterLily’s output voltage will be about the same, and if the flow rate is fast enough, the current will rise to 1.36A. If you have a deeply discharged battery down to 7.5V (or a dumb load like a 12V light bulb), the voltage can drop even further and the current can rise to a maximum of 2A. That’s where the 2A rating comes from--we’re not trying to lie or cheat you, it’s an actual maximum current output, you just can’t get both the maximum voltage and current at the same time.