Understanding The Math To Calculate Your Solar Requirements For Camping & Hiking


As solar power technology become cost effective for more mainstream users, there’s a metric shit ton of online solar products, from as many different sellers. Most of them are mono-crystaline panels of Chinese origin, which are actually pretty decent.

Solar For Camping & Hiking

Figuring out your requirements for portable applications like camping is a little different than for a fixed installation that’s never going to move. You’re not going to be carrying around a 400 watt solar array with a few hundred pounds of batteries when you’re out hiking. At least I hope not.

So, what size portable solar panel do you need, and what kinds of batteries should you buy to store that solar power you’re not using? Aside from a few factors like solar panels being rated at noon on a summer day in the Sahara, and battery manufacturers universally overstating their product’s capacity, the calculations themselves are fairly simple.

Watts & Watt-Hours

In this article I’m mainly going to be using watts and watt-hours, so that the calculations will be the same regardless of using a 12 volt fixed solar panel or a 5 volt mobile USB panel. Same with batteries. In fact, we’ll be using a few different formulas:

Watts = Amps x Volts
Watt-Hours (Wh) = Watts x Hours
Amp-Hours (Ah) = Amps x Hours

For example, if you run a 100 watt bulb (like the old incandescent ones) for 1 hour, then you just used 100 watt-hours of energy. A small microwave oven, heater, or A/C system uses about 1000 watts. Run one of these for an hour and you just used a kilowatt hour. You get the idea.

Now, the portable solar panels and batteries you carry in your backpack aren’t generally going to power your blender, though the equipment you carry for your car camping could, when paired with a 12 volt power inverter.

Calculating Your Power Requirements

Requirements are the driving force behind any design. How long do you plan to be off the grid, and what devices do you want to run? Most electronic gadgets like phones, tablets and GPS units internally use some form or lithium polymer battery that runs at 3.7 volts, but uses a 5 volt USB interface to charge the device.

Every device manufacturer lists the charge capacity of its device, which is basically the capacity of the battery inside it. What I’m going to do for the purpose of this article is to ignore the slight difference in charging voltage, and the fact that most battery capacity is overstated (and also degrades over time,) and a couple other factors like the overhead of the circuit which charges the battery.

So, rather than try to squeeze every milliwatt-hour out of my gear design, I’m just going to do the rough calculations, and factor in a little extra under the heading of “real world performance is never as good as the calculations” and plan for a little more than I need.

What I do to calculate my power requirements is to add up all the stated capacity of my devices. I might have an awesome phone with a 3,000 mAh battery, and a decent tablet with a 2,500 mAh battery in it. Since every device discharges at a different rate, especially depending on how you use it, it’s very hard to do the calculations based on this. What I’m going to do is assume that every device I carry will need to be charged fully every day, and I’m going to assume that I’ll be storing all the power from the solar panel into a battery pack, which will then be used to charge my devices.

Calculating Your Solar Panel Output

Most of the portable solar panels you’d actually carry with you hiking are going to be in the 20 watt to 50 watt range, and the output gets better as the panels get bigger. You can carry larger solar panels for car camping, like this 100 watt unit, but now you’re looking at carrying around an extra 12 volt battery. Either way, you’ll want to take the voltage of your solar panel into account when doing your calculations, and I’ll give a few examples.

Portable Solar Panel: I have a 40 watt solar panel in my emergency bag. This 40 watt rating is made for the best possible circumstances. In practice you’ll be lucky to even be in the ballpark of its stated output, even in direct sunlight on a summer day.

Now, watts = volts x amps. I know that my portable panel is rated at 40 watts with a 5 volt standard USB output. Which means that under ideal conditions, it should put out a whopping 8 amps!

From my own calculations, I know that on the hottest, brightest day of the year, my panel puts out about 2 amps at 5 volts, which means the effective output is only 10 watts! But it’s easy to lose sight of the fact that most USB wall chargers are only 2 amps. The cheap ones are even 1 amp!

So, again, under ideal conditions in a northern latitude, my portable solar panel puts out about the same as a wall charger–2 amps. In indirect sunlight it jumps below an amp, and it just gets worse from there.

You don’t have to carry portable power packs around. I know ultralight backpackers who don’t. Just be aware that if you plug your 2,500 mAh phone straight into the solar panel, and it’s only putting out .25 amps (250 milliamps,) then your phone is going to take 10 hours to charge! Which means that your solar panel will give you less than a full charge of your phone on an overcast or rainy day.

Now, carrying around battery packs to hold your extra charge isn’t going to store power that’s not being put out by your solar panel, but it will let you store the extra power during sunny days that you can use for rainy days.

For example, hooking my 10,000 mAh power pack up to my 40 watt panel on the sunniest day in Spokane will give me a full charge in about an hour and a half-ish, if I assume the panel is putting out about 2 amps. This way I can carry around and store about 4 full charges for my phone. And if the battery pack is full, then I can just plug in the phone itself to keep it topped off. And assuming my phone needs a full charge about once a day, I might be able to go hiking with just the battery pack and leave the solar panel at home!

Travelling light has never really been my thing, so I assume about 6,000 mAh per day usage. On the best day my panel has ever seen, that’s about 3 hours a day with my solar panel in direct sunlight. And as long as I store the extra power in a power pack, then I should be OK with a rainy day here and there.

12 Volt Solar Panel: Usually I carry a 12 volt power inverter when I go car camping. At that point, my car is generating the power I need to power my laptop and other small 110 volt AC devices. If I was going to use solar on extended car camping trips, then I would hook it up to its own battery, and not risk hooking it to my car. Most 12v panels come with their own charge controller, and unlike most portable panels which also include their own charge controller, the 12v panels usually come with the controller as a separate unit.

Now, the calculations become a little more difficult, especially factoring in being able to run your laptop or blender. How do you intend to charge your mobile devices? Wall chargers waste a lot of power, but they’re convenient if you’re already planing to run an inverter. But you could also hook a 12v-to-5V USB adapter up to your battery and charge your mobile devices while wasting less power. Like elsewhere in this article, I’m going to just do the rough calculations.

So let’s assume you have a 75 amp-hour deep cycle marine battery connected to your 100 watt “suitcase” solar panel. That means the panel on its best day will put out about 8.3 amps at 12 volts on the longest day, at the equator. Unless you live at the equator, I’ll make this easy and figure that you’ll be lucky to see 4 amps at 12 volts. Which means that it’s going to take almost 20 hours to charge your good quality deep cycle battery to full. But at least for larger setups, you can leave your solar panel hooked up to the batter around the clock.

Going back to the example of charging your 2,500 mAh smart phone, it’s important to keep in mind that your 12 volt battery doesn’t have to work as hard to charge a 5 volt USB device. You’ll get about 2.4 times the mojo (12/5) from a 12 volt battery as you do from a 5 volt battery.

So, a 75 amp-hour battery would give you about 30 (75,000 / 2,500) charges of your smart phone if they were both 12 volts, but we’ll multiply that number by 2.4, which gives us approximately 72 full charges of your smart phone. That’s probably overkill if your whole setup is geared towards charging your electronics.

But when you start running your laptop, TV, etc. off the grid, the picture doesn’t look so rosy. A small mini-fridge might draw about 200 watts continuously. Converting the 75 amp-hour battery to watt hours, means that 75 x 12 = 900, which puts the battery at about 900 watt hours. This means that the same battery that looked overkill to charge your mobile devices can only run your fridge for about 4 1/2 hours.

Now, you could take 2 batteries with you, wire them parallel and get about 1,800 watt hours, but in the example I used above, it’s already taking 20 hours just to charge the single battery. Adding more batteries won’t help unless you dramatically increase your solar panel output. At least for car camping, it can quickly spiral out of control.

Other Factors

Battery manufacturers almost universally overstate their capacity, just like solar manufacturers rate their panels at a wattage that’s only seen in their labs. All battery chargers waste power, and the capacity of all batteries degrades over time.

For custom built 12 volt setups, even a less-than-perfect connection on one of the cables can take away power that should be going from your solar panel to your deep cycle battery.

What does all this mean? To me, all this means that I usually like to double what I think for solar output and battery capacity. I’d rather see all my batteries full than my devices dead!

Putting It All Together

Usually I bring my 40 watt panel and a few good battery packs with me when I go camping, and that’s plenty for running my phone and tablet to read my books at night by the campfire. And if I need to plug in my laptop for car camping, then I usually just plug it into my truck with a small, 100 watt power inverter I have. My opinion is that the 5 volt USB style portable solar panels are the sweet spot for portability. I can charge my phone for a week with a couple of battery packs before even needing to break out the solar panel. And it all fits nicely in my electronics bag.

Something between total portability of it all fitting into a bag, and fixed solar where everything’s permanently connected is less than ideal in my opinion. Though a couple suitcase-sized solar panels and a couple hundred pounds of batteries isn’t too outrageous.

The main trick for 110 volt devices from solar/battery power is budgeting your watts. It’s not uncommon to be running 3,000 watts continuously in an average sized house. But to run that output for just an hour would mean about 3 75 amp-hour batteries, and you still have to charge those batteries!

In the next article I’m going to focus more on fixed solar installations, where you have a bank of solar panels and batteries. But hopefully you can start to get an idea of how much equipment you need just to run a little cabin in the woods, not to mention the average sized house. Microwaves, vacuum cleaners, and small A/C units all run about 1,000 watts. To be able to accurately calculate how much wattage you need over time, having a device like this one helps to get a feel for your power usage in real time. It’s usually surprising to see how much power all your devices use. For example, my desktop computer with monitor draws about 200 watts just sitting there doing nothing.

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