They are called all kinds of things. Solar Generators. Portable Power Stations. Portable Battery Backup. Portable Power Stations. Solar Battery Banks. Whatever. These are just words without context, and these products’ manufacturers make sensational claims that have to be true – after all, they’re on the Internet!
If It’s Too Good to be True, It Probably Is
Have you read something like “This thing can power your whole house” or “It puts out enough power to run your air conditioner?” Does that make sense? Can a little portable box that you roll around your garage really do that? If so, why doesn’t everyone have one? Here’s why – it’s not true. Or at least it’s a wild stretch and needs some serious context.
I’m not saying these aren’t good products or they don’t have a place in the market. They do store energy, and they can provide backup power. The question is, how much and for how long. And can you really connect solar panels to them for “endless power?”
One of the early players in the market was Goal Zero. Others have emerged, like Jackery, Bluetti, EcoFlow, and most recently Anker, the popular electronics manufacturer you can find on Amazon (more on Anker later). Even Harbor Freight has a portable solar backup generator. And there are tons of other smaller brands flooding the market. Clearly, there is demand and people are buying these things, and some of them get great reviews. When people buy them for the right reasons, they work great. Charging laptops and phones, powering small household electronics, and running LED lights and portable fans are well within the capabilities of these systems. But what happens when you try to apply this to larger loads in your home, and want to connect it to your home’s electrical system?
That’s where the promise and practicality of these units break down.
You Can Recharge With Solar Panels
Ok, I’m getting the the crux of the matter. Without a charging source, these things are extremely limited in a grid outage scenario. You could charge them up with a generator or take them somewhere that has power, of course. But wouldn’t it be great if you could charge them up with solar panels? What would be even better would be recharging them with solar panels that are already on your roof! Well, I have news for you – you can do it – but it’s not that simple.
Most of these units have some sort of input for solar panels, and panels may even come as a kit. The solar panel input is designed to match the battery voltage, and there is usually an integrated charge controller that limits the current and shuts off the solar power when the battery is full. This is an obvious safety requirement. There is a limit to how much solar power that can be connected. For example, if you have a 1,000 watt-hour battery, you can’t connect it to 10,000 watts of solar because the battery can’t charge that fast. Imagine trying to refill a battery from empty to full in six minutes. That’s the kind of discrepancy we are talking about here. Today’s battery technology cannot accept a charge rate like that.
Solar panels on your roof are typically many times the capacity of these battery systems, but that is changing as capacity grows and these systems can be paralleled together to increase capacity. Over time, we may be able to pair more solar power with limited battery capacity, and there are fixed systems on the market that have essentially no limit to the ratio of solar panels to battery capacity. That does not exist with portable battery systems. And there are other issues here.
Issues With Solar Panels and Portable Battery Systems
First, we need to understand how these systems get recharged. The solar panels that come with these batteries put out DC electricity, which is what the battery is – a DC energy storage device. Solar panels go through a charge controller and recharge the battery. This is a very traditional way that solar panels and batteries work together. But you can also recharge these batteries by plugging them into an AC power source like an outlet in your home. That AC power is converted to DC via an onboard converter, which is technically called a rectifier. The electronics regulate the charge rate.
Rooftop solar panels put out DC power. Obviously, we somehow use them for AC power in our homes. There are multiple ways that is accomplished these days. The DC power from the solar panels goes through an inverter or inverters to convert the DC power to AC power. These inverters are designed to work in parallel with the utility company to seamlessly provide power to your home and even “sell” the excess solar power back to the utility grid. This all happens through the “magic” of a UL Listed device that shuts off instantly when utility power is lost. This standard is known as UL1741. Without a voltage source, the inverter must shut down and cease output of AC power to prevent backfeed of electricity when the grid is down. This protects utility workers from electrocution.
Solar panels shut down UNLESS you have a battery and/or transfer gear to disconnect your home from the grid. Your solar energy system can continue to provide AC power if you have the right automatic or manual transfer switch to fulfill this need. Well, that sounds easy enough, right? Not so fast…
You can’t just hook up ANY battery. The systems need to interoperate. And that brings us to the next issue – rapid shutdown. This is a requirement of the National Electrical Code and Fire Prevention Code that requires solar panels to shut down via an accessible switch to help first responders when they are called to your home.
Rapid Shutdown Requirements Complicate Things
Rapid shutdown, per se, doesn’t prevent the use of portable battery systems. It’s the way that rapid shutdown is implemented for rooftop solar panels that creates complications. There are two main ways this is accomplished today – power optimizers and microinverters.
In the case of power optimizers, there is a single traditional inverter that accepts somewhere around 150-450 Volts of DC power, but a signal system causes power optimizer output to drop to just 1 volt DC when the signal is lost – a safe level for first responders. These devices are proprietary to the inverter or the rapid shutdown system. They do not “talk” to portable battery banks, so even with a switch of some kind, you couldn’t just connect the output of the solar panels to a portable battery. And even if you could, the voltage output would not be compatible with the battery without some kind of converter and regulator.
Microinverters are now the norm and have built-in rapid shutdown capability, making them a popular choice, among other advantages. This completely changes the way that solar panels are connected to your home. The DC power from solar panels is immediately converted to 240 Volt AC power at the panel level. There is no DC power coming off your roof. Each microinverter is a UL1741 inverter that immediately shuts down when there is no outside AC source (there are caveats to this, but not relevant to this discussion). So, you need an AC source of voltage for the solar panels to work.
Well, that’s no problem, you say, because the portable battery system does output AC power, and that could “trick” the microinverters into turning on. Sure – that is a possibility, but how would that be implemented in a way that safely regulates the output of the microinverters? That brings us to…
Ah ha! This is the Gotcha moment where you are going to tell me that there is a system out there that can do exactly that. AC Coupling is the method where the output of microinverters (or string inverters) is connected to the AC output of an energy storage system (which is a battery with an integrated inverter to keep things simple here). There are plenty of systems on the market that have AC Coupled architecture. The most popular are systems like the Tesla Powerwall and Enphase’s battery system, which pairs perfectly with their microinverters.
Recently, Anker, one of the brands I mentioned earlier, released a portable battery that claims to do AC coupling with your rooftop solar panels. Can this really work? Sure it can, with the installation of additional gear. We’re not talking about a plug and play system here. You need to make permanent changes to your home’s electrical system, carve out a critical load panel where circuit breakers are installed for backup loads, and introduce a transfer switch to separate these loads from your house in a grid-outage scenario. Then, you need to AC couple your solar panels to this backup load panel.
Easy, right? Um, no.
First, most rooftop solar panel systems these days are too large to connect on the load side of your house’s electrical system. They are connected on the utility side of your main panel in most cases. And if they are smaller and connected on the load side, there are complicated rules and safety standards on how and where they can connect. Introducing a transfer switch and backup load panel complicates matters.
And that gets us back to rapid shutdown. You could carve out some solar panels to be connected to your backup system and have some solar panels that are just grid-interactive. But you have to comply with the rapid shutdown requirements, which means you need a single switch to shut everything down, or multiple switches if they are considered separate systems. But there may also be utility requirements that necessitate a single switch to shut everything down. That may not be possible if you have multiple grid-interactive systems on site.
Everyone is getting in on the game. Goal Zero also advertises a manual transfer switch that allows you to connect your battery backup to your home’s electrical system, but does not outright advertise the possibility of AC Coupling your existing solar panels – yet. That is probably coming, but the same problems exist.
Limitations of Portable Backup Systems
“You can back up your whole house!” Sure you can… for an hour or so. Ok, I’m being a little facetious here, but the average consumer is wildly misinformed about how power and energy work (and even the difference between the two). It’s not your fault – it’s not something that is taught in school, but it should be, like how to balance a checkbook. Sorry – now I’m on my soapbox.
These portable backup systems are extremely limited, but they are catching up. I’ll pick on the Goal Zero brand for a moment here, because they were one of the first, and now they are dabbling in whole home backup systems with the promise of powering all of your essential needs. They have a 4,000 watt-hour battery now, which is no small amount of energy. That will run a 40-watt lightbulb for 100 hours – for $4,000. Does that sounds like a good value? If that’s what you need to do, it could very well be the best way to achieve your goals. You can save the cost of integrating your battery with your home’s electrical system and just run extension cords. But don’t expect it to recharge with your grid-interactive roof-mounted solar panels. You need a separate solar panel system to recharge it.
I’m not saying these batteries don’t have their place in the market. These are good companies with good products that do meet some people’s needs. But the promises made can be grossly overinflated. It is important to know what you are getting and the system’s limitations.
One good thing about modern battery backup system is the instantaneous power ratings. Inverter systems have much better surge handling capability than generators, but there are still limits. If you are running a coffee pot and a hair dryer, your refrigerator starting up could easily overload one of these systems, causing it to shut down. The smaller your battery, the smaller the inverter it will have, and the lower the instantaneous power output it can handle.
To be clear, 4,000 watt-hours is actually quite a lot of energy. But a large refrigerator will use around 2,500 watt-hours per day, and a central air conditioner in Florida in September can use upwards of 40,000 watt-hours per day. So, we need to put things in perspective and deal with reality. These portable batteries are designed for very limited needs.
Speaking of a lot of energy, I chuckle (and kinda cringe) at the sight of Goal Zero’s website showing their 4,000 watt-hour battery in someone’s kitchen. Do not – I repeat – do not do this. Although there are no fumes and LiPO battery safety has improved dramatically over the years, the fire risk of having this amount of potential energy inside your house is not to be taken lightly.
Building codes prohibit the installation of batteries inside of a dwelling except in storage locations, garages, outdoors, or in detached structures. And there are limits on how much battery capacity you can put in each place. These codes are there to save lives and prevent catastrophic fires. Electricity is no joke, and the energy density of these devices is very high. Unlike old ugly lead acid batteries (like the kind in your car) these new battery systems are neatly packaged in a consumer-friendly way, but don’t let them fool you. I won’t say these are ticking time bombs, but imagine the amount of energy your refrigerator uses in a whole day being discharged in an instant.
To be concise, there are capacity, output, practicality, and safety limitations that need to be considered with portable battery backup systems.
What Is The Solution?
Reinventing the wheel seems to be a goal of the Internet. All you have to do is fry your brain on TikTok for a few minutes to see how people make things harder than they need to be. My favorite is this woman who decides to walk under a hose rather than just stepping over it.
You can have battery backup at your home for a limited subset of critical needs in an outage using your existing solar panels using batteries that are built for this purpose. It won’t be portable, but do you really need it to be? You can even back up your entire home with enough batteries or install a smaller battery that is still connected to your entire home. That way, you can choose what to run in a power outage and for how long based on how much capacity you have installed and what recharging capability you have.
This all comes at a hefty price, for sure. But when you look at the energy capacity and power output of portable battery systems, they are way more expensive than fixed battery systems that are purpose-built for your home. If you look at the price per watt-hour (or kilowatt-hour) of portable battery systems, it can be over double the cost of a home battery system, and when you take into account all of the other gear you need to connect it to your home safely and properly, fixed batteries are much cheaper.
Oh, and then there is the tax credit. You can take a 30% tax credit on batteries permanently installed at your home that are charged exclusively with solar panels. You can’t do that with a portable battery backup system that is intended to be mobile and recharged multiple ways, most typically from an AC outlet.
For the same money, you can easily get twice the backup capacity with a fixed battery system in your home. With both systems, you need additional gear if you want to integrate batteries with your home’s electrical system. That’s the part these portable battery system manufacturers gloss over. No matter how you look at it, battery backup is not cheap, but when done right, it can be a cost-effective backup solution that is on par with or even better than generator systems.
Essentially, these portable “solar generator” manufacturers are trying to get in on the lucrative home backup game by sidestepping the requirements that exist for permanently installed systems, which are admittedly onerous. But they are doing it at a much greater cost than the traditional ways. Certainly, they have the advantage of being portable, and if that is what you want, they may be right for you. But don’t expect them to easily connect to your home, and definitely not to your rooftop solar panels.