Learn how to size charge controllers, inverters, and solar panels already on hand with a goal of future expansion.
Welcome to our series answering reader questions and concerns about how to cut the utility- company cord.
I just purchased eight 200-watt solar panels that I plan to pair with two 200-amp deep-cycle batteries (that I’ll probably run at 12 or 24 volts). However, I’m in the dark for what size of charge controller and inverter I should buy for my current setup if I hope to add more panels and batteries in the future.
-Ron Clement
In country lingo, I might say you’re building your system “bass-ackward.” Over the years, I’ve met many folks who took the same approach: They picked up what appeared to be good deals on solar panels or batteries and attempted to build systems around the equipment they’d scrounged. This order of operations can yield fine results, but I prefer designing a system first – allowing you to buy your equipment as an interoperable set. But let’s see what you’ll need to make a suitable system using the equipment you already have.
Inverters operate off the voltage of solar panels and batteries. Because you have eight solar panels rated at 200 watts each, you can expect a maximum of 1,600 watts of direct current (DC) input power under optimal conditions. Temperature, time of year, your location, and a host of other variables will influence actual power production – plus, you’ll experience losses when converting from DC to alternating current (AC). Expect to never achieve the full 1,600 watts of power. (Be wary of salespeople who try to sell you solar systems based on nameplate ratings, which inflate output numbers.)
Your 12-volt batteries and 12-volt solar panels should work well with the OutBack Power FXR series of inverter-chargers or Renogy Pure Sine Wave inverters. You’ll get 120 volts of AC output from a single FXR inverter module, 240 volts with at least two FXR modules, and more output with more FXR modules (for when you add solar and battery capacity). In other words, the FXR series is built for the purpose of letting you start small and grow your system. In fact, because you currently have eight solar panels, you’ll immediately have room to grow; two more solar panels – for a total of 10 in your system – should do the trick.
As a 2,000-watt, AC-output system, the FXR is the smallest fixed system I’d go with, whether your setup is grid-tied, grid-tied with battery backup, or entirely off-grid. Smaller inverters available at the local hardware store are intended for portable uses, such as running power tools.
Because I don’t know your intended uses, let’s say you’re hoping to forgo any AC output and are instead hoping to stick with a straight DC-output system. You can do this if you’re planning to only operate DC-voltage devices. In this case, you’ll only need a charge controller. Just make sure the charge controller is specifically intended for a solar-plus-battery system; one such controller is the OutBack Power FLEXmax 60/80 Charge Controller. For the FXR series discussed earlier, the charging function is built into the inverter.
You can see that knowing how big you eventually want to go with your solar system – and what devices you’re intending to power – makes a big difference. I’m making a whole lot of assumptions about your system on limited information. No matter your choice, combine my advice with a conversation with the inverter manufacturer or a solar professional, and take the time to read the OutBack Power FXR owner’s manual.
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May all your days be filled with sunshine – even when it rains!
Hoss Boyd is founder, president, and CEO of TeraVolt Energy and a recognized solar and energy-storage expert.
