How to Pick a Solar Generator System

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Interested in solar generators, but not sure where to start? In this article, we’ll go step-by-step to figure out what kind of system you need and how to buy it, using my own situation as an example.

There are a lot of options out there when it comes to solar panel and battery systems, but for this article, we’re going to focus on portable solar generators. That means we’re talking about systems that include a battery pack (that you can plug regular devices into, such as refrigerators, fans, phones, laptops, CPAP machines, etc.) as well as solar panels that can charge up that battery pack during the day. The systems would be portable in the sense that they are not installed permanently into a building (I’m renting), and they would generally fit inside a car for transport. However, a lot of the process will actually be the same as for home installations, with the exception of a few missing steps.

If you haven’t already, you may wish to read our previous article that goes over the basics of both solar generators and regular generators:

Intro to Generators and Solar Panels

And if you’re more interested in being prepared for blackouts in general, check out this article:

Preparing for Blackouts

What do I need?

I am in the market for a solar generator to use in case of power outages—specifically, power outages that occur during a deadly heat wave or cold snap. I can go a few days without power if the weather is ok, but my house’s heating and cooling require electricity, and if the outage is widespread, it’s going to be difficult for me to evacuate to somewhere with power. Especially since I don’t actually own a car!

Now, you might wonder why I don’t just get a gas generator… and the answer is that it’s more work than I want to deal with. Generators require regular maintenance that I just don’t want to worry about, especially since I’ll probably barely ever use it. I want something that I don’t have to think about until I need it. That does mean that I’ll have to shell out extra for a solar generator, but perhaps it will be worth it—especially since I’m willing to be pretty sparse with my power needs. I don’t need to power my whole house here, just a couple of basic things in order to stay alive—and at least somewhat comfortable, hopefully. Plus, solar is the future!

Preparing for a heat wave power outage

If there’s a power outage during a heat wave, I’m going to want to keep my fridge/freezer running—both to prevent all my food from spoiling, and also to produce ice and cool drinks that I can use to stay cool. Full-size refrigerators use a lot of power though (e.g. ~700 watts), so I doubt I’ll want to shell out for a solar system big enough to power the one in my kitchen. But I also have a minifridge in my house that I use for drinks, so I could just use that instead!

I also might consider trying to run a small air conditioner in one room. I don’t currently have one, but it occurs to me that it wouldn’t be a bad idea to have one as a backup in case my heat pump gives out during a heat wave—even when there isn’t a power outage. Repairs can’t happen instantly, and good luck trying to buy a new A/C unit during a heat wave. Though I suppose that with the minifridge, I could just pull cold drinks or ice packs out of there to keep cool instead, and it would surely be much more efficient than trying to cool an entire room. So maybe I’ll scrap the A/C idea⁠—it’s probably way too expensive to run one on solar (e.g. over $1000 worth of components).

Finally, I’ll want to be able to run an air purifier (e.g. a box fan with a furnace filter attached) to get wildfire smoke particles out of the air, since the smoke can get quite thick here during the summer.

You’ll note that the minifridge will still probably draw quite a lot of power, even though it’s fairly small. But luckily, summer is also when it’s sunniest out, which means that it will be easier to keep them running on a solar system! Though of course, it’s also worth noting that solar panels perform worse the hotter it gets.

Preparing for a cold snap power outage

If there’s a power outage during a cold snap, I’m not too concerned about powering my fridge/freezer, because the worst-case scenario is that I just move the food outside (or fill the fridge/freezer with ice from outside, using it like a cooler). Nor am I concerned about wildfire smoke, so I don’t need an air purifier.

The main thing that I would want to power is an electric blanket. These use a lot less power than space heaters because it’s more efficient to heat a person directly as compared to heating an entire room. True, I could also use an indoor propane heater, but I’d prefer that as a backup since I don’t want to run a propane appliance indoors while I’m asleep.

It might also be nice to be able to power some sort of food-heating device (like a hotplate, microwave, or electric kettle) to make warm food and drinks with, but that’s going to use a lot of power, and I can just use a propane stove instead (or my gas stove, if natural gas is still running). So let’s stick with just the electric blanket for now… along with basic stuff like keeping my phone and headlamp charged (though that’s almost negligible compared to any heating device).

Note that in the winter, solar generation capabilities are going to be much more limited—there are fewer daylight hours, and the weather is much more likely to be cloudy/rainy/snowy (at least where I live). Even though they might be more efficient due to the lower temperatures, the lack of light means that my panels might only generate 5% as much as I could get during a clear summer day. So if I wanted to run a 60-watt electric blanket on 100-watt solar panels… I might need a couple dozen of them to keep it going 24/7. It might make more sense to just get a battery pack with enough charge to power the blanket for a few nights (until power is restored), and assume I won’t generate any solar power. We’ll say 10 hours for 3 nights, or 30 hours total, is a good target.

So with all that said, here are the minimum stats I’m looking for in a solar generator system:

  • High enough maximum battery wattage to run a minifridge and a fan/air purifier
  • Large enough solar panels to keep the above running throughout the day, as well as charge up the battery for night time
  • Large enough battery capacity to keep the above running throughout the night in summer, or to run an electric blanket for at least 30 hours in the winter

Math time!

Note: If you aren’t familiar with the difference between volts, amps, watts, and watt-hours, you may wish to watch a quick primer video on the subject, such as this one.

Anyway, here are some stats on what each of the devices will use. Note that I used a Kill A Watt to get the numbers.

  • My minifridge uses about 300 watts for a few seconds when it starts up, and about 160 watts when running. But I actually measured its usage (in watt-hours) over the course of 2 days with the Kill A Watt and found that it only used about 1700Wh, which when divided by 48 hours, gives us an average wattage of only 35 watts. This makes sense because refrigerators actually don’t run 24/7; they turn on for a few minutes until the desired temperature is reached, then turn off and wait until the temperature gets too high again.
    • However, this was when the fridge’s internal temperature was 30-40F and the house temperature was 70-80F. If we assume the house temperature is higher (which it would be, during a heat wave), e.g. 100-110F, then the temperature difference is 1.75x greater, meaning that the fridge will probably use 1.75x as much power. I’ll also want to account for constantly cycling out ice packs and such, so to be safe, let’s assume that during a heat wave, it will use, say, about 75 watts on average.
  • My fan uses about 25 watts, and I’ll probably want to use it 24/7 during summer (either as a fan or as an air purifier).
  • My electric blanket uses about 60 watts on average.

During summer, you can see that I’ll be using about 75 + 25 = 100 watts on average. Let’s add 20% to help account for the conversion between the low-voltage DC current (which the battery and solar panels produce) and the 120V AC current (which the appliances need). So we’ll assume that, during the summer, I need about 120 watts on average.

How much solar do I need?

Now to figure out how many solar panels that translates into. If I’m using 120 watts every hour, then every 24 hours, I need to generate 24 * 120 = 2880 watt-hours. Based on this calculator, my zip code gets about 6 peak sun hours per day in summer (i.e. hours when the solar panels are operating with an optimal amount of light). Based on some cursory research, it seems that there’s a general agreement that you can just multiply the number of peak sun hours times the wattage of your panels to figure out how much power you’ll generate every day (on average). Oh, and then multiply it by 0.75 to account for losses between the panels and actually charging the battery (even if you have an MPPT controller). Yes, you lose about 25% of your power just getting it into the battery, and about 20% getting it back out (assuming a ~12V battery system and ~120V appliances).

After rearranging the equation, we end up with a formula that allows us to solve for the required size of our solar system, in watts:

watt-hours needed per day / ( peak sun hours * 75% ) = solar wattage

So if I want about 2880 watt-hours per day, and I have 6 hours of peak sun, then I need about 640 watts worth of solar panels.

To check my work, I used this calculator. Note that it’s specifically used to estimate how much solar you need to charge a particular battery, so it doesn’t actually accept watt-hours as one of the inputs (it takes volts and amp hours instead). But luckily, volts * amps = watts, so we can just pretend we’re using a random voltage and divide our watt-hours by that in order to get our amp-hours. Here are the inputs I used:

  • Voltage: 12 (a random number)
  • Amp hours: 240 (because 2280 / 12 = 240)
  • Battery type: LiFePO4
  • Charge controller: MPPT
  • Peak sun hours: 6

The result? 670 watts, which is pretty close to what I estimated myself.

How big of a battery do I need?

First off, let’s figure out the highest number of watts I’ll use at once. During the summer, I’ll need a battery pack and inverter that can put out at least 300 + 25 = 325 watts in order to handle the fridge turning on while the fan is running. Again, let’s add 20% to account for losses. That means I’ll want a battery that can support a maximum draw of at least 390 watts during summer.

As for capacity, first remember that I’m charging the battery during the 6 peak sun hours, and draining the battery during the other 18. So then if I’m using 120 watts for 18 hours, that means my battery needs to hold 120 * 18 = 2160 watt hours to get me through each summer night.

As for winter, I’ll assume that I can’t generate any solar power at all, because it will likely be cloudy where I live. Luckily, I just have the electric blanket to power, and if I want to use 60 watts for 30 hours, that multiplies out to 1800 watt-hours. Funnily enough, that’s also 2160 watt-hours after losses. So it looks like the battery size we need for summer will also work out for winter too!

Comparing all-in-one solar generators

So in summary, here are the exact numbers on what I need from a solar generator system:

  • 390W maximum output wattage on battery
  • 2160Wh (watt-hour) capacity on battery
  • 640W of solar generation

Doing a quick search yields some relevant devices from a few popular brands:

Wow, those are some pretty hefty prices! I’m not sure that I want to spend $3000 on a solar generator system. Maybe I can get the price down if I build one myself? After all, it can’t be too hard… can it?

DIY systems

I’m very new to the field, of course, but luckily, there are lots of resources on the internet for setting up your own custom solar generator systems. In particular, it seems like Will Prowse is well-known in the space. I’ll be grabbing some reference components off of his website, https://www.mobile-solarpower.com, to estimate how much a full system would cost.

But first off, let’s quickly go over the components you need in order to create a solar generator system.

Batteries

This one is obvious! There are many types of batteries, including everything from old-style lead-acid (think car batteries) to newer LiFePO4 (widely considered to be among the best). You can also use any number of batteries and connect them all together to increase the maximum charge and maximum draw for your system. Often, the batteries run at 12V, and some people prefer to wire them up in series to produce 24V or 48V systems.

Solar panels

Another obvious one!

Solar charge controller

This is a device that takes in power from your solar panels and charges your batteries. No, you can’t just wire the solar panels directly to the batteries!

Many charge controllers allow connecting multiple solar panels and multiple batteries in order to handle a large system. Better ones also include an MPPT feature to help maximize charging efficiency under a variety of temperature and brightness conditions, while cheaper ones will use “PWM” instead.

Inverter

These convert low-voltage DC power (from the batteries and solar panels) into the 120V AC power that you need for most devices. They will generally provide you with multiple regular wall outlets, and many will also provide other ports too, such as USB ports.

When choosing an inverter, you need to make sure that it is rated to handle the maximum draw that you are intending to use. For example, you can’t run a 500W air conditioner off of a 400W inverter! Likewise, the batteries themselves will have a maximum draw as well, as mentioned earlier. You’ll generally want to leave a little buffer room when it comes to the maximum draw.

Also, depending on what you want to power, you’ll most likely want a “pure sine wave” inverter, as this is the type of power needed to run basically any appliance with a big motor, such as a fan, refrigerator, or A/C unit.

Others

There are various other smaller components that may be needed for your system, including wires, adapter cables, fuses, temperature monitors, mounting brackets for the solar panels, and more. A battery charger will also be needed if you wish to ever charge your batteries from wall power instead of solar (which could be very useful if you get rolling blackouts or use your system for camping). All of these components are already included with an all-in-one system, but of course, buying and assembling them on your own allows you to custom-build the system for your own specific needs—and for much cheaper, too.

Pricing out a DIY system

Now that we know what we need to buy, let’s find some examples! Again, here are the exact numbers on what I need from a solar generator system:

  • 390W maximum output wattage on battery/inverter
  • 2160Wh (watt-hour) capacity on battery
  • 640W of solar generation

And here’s a minimal list of components that we could use to build a nice 12V system:

Or a 24V system that saves some cash on the charge controller:

Nice, we cut about a third off of the price! But of course, this doesn’t include any of the extra components that may be necessary, such as cables, brackets, fuses, etc. Also, both of these options only provide 600W of solar generation rather than 640W—I figure it’s not the end of the world if I have to turn off my fan for a couple hours.

But still, that price drop is impressive! Plus, we can always upgrade or repair the system piece-by-piece if need be, and it’s probably also a lot of fun to build! 🙂

…OR I could just get a generator, even though I said I wouldn’t earlier. It would probably be easier, and it would definitely be cheaper, and I would be able to generate WAY more power. Just take this one, for example. 2000 watts for $450! That’s 3x as much power for 25% of the cost. I could run a full refrigerator, an A/C unit, and enough fans to start a tornado! Yeah, I’d have to buy fuel, but if I’m only using it occasionally for emergencies, the fuel cost would really be negligible.

So basically, I’m torn. Stinky, noisy, annoying technology of yesteryear, or ridiculously expensive technology of the future? I guess the decision is a personal one. Good luck making it for yourself. 🙂

I don’t like either option

I decided that I didn’t want to do either of the above. The solar generator system I priced out was way out of my budget, but I still wanted to build one as a personal project. I figured that maybe, at the very least, I could build a system to power my fan during summer. The math here will be abridged, but basically I decided that I would be using about 25 watts on average, and ended up with these new requirements:

  • 720 watt hours used per day
  • 25W maximum output wattage on battery/inverter
  • 540Wh (watt-hour) capacity on battery
  • 160W of solar generation

Here’s a plug-and-play system that meets the new requirements:

And here’s a DIY 12V system:

Though these systems obviously don’t match exactly, they are similar in capacity, generation, and price. So it seems that at the small end, DIY isn’t nearly as good of a deal. Especially if I consider that the plug-and-play system is more compact and easy to carry, and also includes the ability to charge off of wall power if need be (which might cost an extra $100 for the DIY system).

So now I have a different question: Would I rather have a plug-and-play system, or would I rather build my own for fun? Or do I even need one at all? This is still a pretty expensive system, and for what? A minor benefit (running a fan) during an unlikely event. And that fan wouldn’t even necessarily be enough to keep me cool if it’s too hot/humid. Really, I’d probably be better off jumping in the lake or going to a cooling center in my city. It might suck for a little bit, but I save a significant amount of money.

Ultimately, preparedness is all about tradeoffs, and choosing to not have a generator system is a tradeoff that I’m willing to make. So I guess I won’t be buying or making a solar generator after all!

2 thoughts on “How to Pick a Solar Generator System”

  1. Pingback: Intro to Generators and Solar Panels – The Readyist
  2. Pingback: Preparing for Blackouts – The Readyist

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