Choosing solar equipment

Finding solar panels for solar generator: Tips and tools

If you think that a standard solar panel system is too big of a commitment, maybe it is a solar generator that you need. PV generators are portable, safe, reliable and unlike gas generators, they don’t smell or make a sound. The problem is how to choose solar panels for a generator. In this article, you’ll find tips as well as a tool that will make picking equipment easier.

Use a solar generator at home or on the road

A solar generator is a power station and solar panels together. A power station combines in itself an inverter, a battery and a charge controller. The kit is made to be portable and you can use it at home or on the road.

Solar generator for camping

Your power needs on picnics, camping and hiking trips are likely to be small — maybe you’ll need to charge a phone or use a laptop or GPS device. A small portable PV system would be enough and for this pick a solar panel that is lightweight, foldable, and is easy to set up and take down.

Vendors such as Ecoflow, Bluetti, and Goal Zero have worked to improve their portable panels in recent years. Traditionally portable panels have maxed out around 100W, but there are models now that provide 400W or more. The weight of one panel can range from a few pounds to 20-30 pounds, depending on the size and design. The modules often come with kickstands and you can easily set up your array on any flat surface.

What you gain in portability and weight you lose in durability and price. Portable panels are not designed to spend weeks and months outside in the elements, and they will begin to break down over time if overused. You also pay a premium for portability, with prices as much as five times that of a non-portable setup.

Solar generator for RV and vehicles

RVs and vehicles are mobile applications of solar generators, but unlike camping there is often an opportunity to permanently mount panels on the roof. Folding panels can still be used when parked, but a permanent panel mounting is usually preferred.

For smaller vehicles, it's common to use 100W or 200W rigid or semi-rigid panels. For larger vehicles such as RVs, you can use panels that are 300W or higher, the same models that are used on homes and businesses. The dimensions and weight of solar panels are primary considerations, along with compatibility with your solar generator.

Solar generator for home

Home power backup power can also use portable folding panels when the solar generator is only used during emergencies. It's more common to use permanent rigid panels, mounted on the roof or ground-mounted, to make use of solar power year-round. For home power backup, the primary consideration is the power output of the panels, and the compatibility with your solar generator.

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Not all solar panels work with all solar generators

Each solar generator has electrical compatibility limits for connecting solar panels. These limits are included in the specs for each model, and there are usually three key figures:

1. Maximum input voltage (Volts)
2. Maximum input current (Amps)
3. Maximum input power (Watts)

Single or multiple solar panels hooked to a solar generator will need to account for these three figures. At the end of an article, you’ll find a tool that will save you a lot of hard work!

Getting solar panel specs wrong can damage your equipment!

Voltage is arguably the most important spec to get right. For many solar generators, hooking up solar panels that exceed the maximum input voltage will permanently damage the circuitry and break the solar input, so you want to stay below the limit. See the guide below for detailed steps on calculating panel voltage.

On many charge controllers, maximum input current is more forgiving than voltage. If your solar panels can provide more current than the generator's max, and it's a sunny day, some solar energy goes unused. It's not uncommon for people to hook up slightly more amps over the stated limit to provide more power when it is cloudy or the sun is at a bad angle. This is known as overpaneling. Still, you should confirm with the generator manufacturer that overpaneling is safe for your model. If you can't get an answer from the manufacturer, it's best to stay under the limit.

Lastly, maximum input power usually indicates a hard limit on what the solar generator will use to charge, and you shouldn't expect to see it use more than this regardless of which panels you use.

Voltage changes with wiring and temperature

So how do you account for those limits when choosing solar panels? You want to maximize the power available from your panels, keep voltage well under the damage limit, and of course, minimize the costs!

Wiring multiple panels together

To maximize power, solar panels can be wired together in "series" or in "parallel". Let's assume we are only using one model of panel. When panels are in series, their voltages add up and current is whatever the current is from the specs of one panel. In parallel, voltages stay the same as one panel but currents add up. And finally, sometimes panels are wired in a combination of series and parallel to get as close to the allowable voltage and maximum current of the solar generator as possible. See this article on connecting solar panels together for more details.

Understanding Voltage Open Circuit (Voc)

This is a spec that each solar panel model publishes. It means the voltage when you measure the panel unconnected to anything, at certain standard environmental conditions. Voc is affected by ambient temperature, so you also need to know the "Beta Voc" stat from your panel. It is often indicated in the datasheet as the “Temperature coefficient of Voc” and given as a percentage like -0.29%. This is how much the voltage changes per Celsius degree away from 25C. Here's the relevant equation:

V array = Panel count in series × (Voc + ((βoc ÷ 100%) × (Low Temp — 25) × Voc))

For example, for three panels in series with a Voc at STC of 40.1V, and a Voc temperature coefficient of -0.29%, and a low temperature of -5C, the Voc would be

3 × (40.1V + ((-0.29%/100%) × (-5C - 25C) × 40.1V)) = 130.77V

Make math easy with WattBuild matching too

That was a lot of scary math. Luckily, you don’t have to do this work by hand — use the WattBuild tool. It allows you to pick a power station model and gives you a list of compatible solar panels, sorted by which panels will maximize your power at the lowest cost. You can either enter your zip code or input the lowest temperature area by hand so that the tool gives a proper estimate of how high the voltage in your array can get. 

Josh is the founder of WattBuild, a helpful webiste for everyone who wants to go solar by themselves. He has been interested in solar power for years and built his first system in 2016.

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