A few days ago (3/2/2018) I lost power in the recent Nor'easter. I had a battery backup system in place, and thankfully it functioned fairly well. However, I had upgraded my aquarium since I had originally planned out the battery backup and the system did not last nearly as long as I wanted it to. The battery took frequent recharges from my car to keep going. I've since re-evaluated my system and in the process, decided to write a post about general-purpose battery backup designed for reefs.

There are three basic power redundancy options that will cover 99%+ of reefers. First is a generator, powered by natural gas, gasoline, or some other fossil fuel. The second is an inverter that uses a car as its power source. The third is a battery (or battery bank) used to power an inverter. This post will be discussing primarily the battery and inverter backup. While each of the aforementioned redundancy plans has their pros and cons, I feel that for me (and many other reefers) a battery and inverter will make the most sense. While the discussion of generators and car/inverter setups are outside the scope of this discussion, I hope to write a few follow-up posts about the pros and cons of each. Regardless of whether you have a battery backup or not, depending on the length of your outage, you may find yourself in need of a generator or the car/inverter setup. This post will be mostly about the theory behind this specific approach to power redundancy (the battery and the inverter).

I must apologize in advance: this is going to be a long post. There's a lot of information to discuss and lots of nuance to all the parts involved.

Before I get started, I want to clarify that you're going to be messing with some very dangerous things here. At the very least, you'll be wiring 120VAC and you'll be working with high-current batteries. The potential exists for you to destroy your personal property, or even kill yourself and/or others. This stuff is all pretty straight forward and is not difficult if you've wired stuff up before, but still, do not continue if you are not comfortable with wiring and electricity.

System Summary

The solution I propose, even for those who have a generator or other longer-term redundancy plan, is an inverter paired with a battery. An inverter is a device that transforms DC into AC. Batteries produce electricity in the form of DC (direct current). Household appliances use electricity in the form of AC (alternating current). An inverter sits between your DC power source and turns it into AC power that your pumps and various other aquarium gadgets can use. So that's basically it, a source of power (battery) and a way to turn battery power into something your aquarium can use (an inverter). And of course, a charger to keep the battery charged.

My battery (center) and inverter (right). On the lower left, you can see my battery charger, a Battery Tender Junior, plugged into a surge protector.

While this solution can consist of just an inverter and battery, there is one more piece that makes this solution a "killer app." That is an automatic transfer switch or mechanism. When the power is on, the transfer switch supplies power from the wall. When the power fails, the transfer switch draws from the inverter. When the power comes back on, the transfer switch resumes pulling power from the wall. Your battery charger/maintainer should always attached to your battery, so once the power comes back on, the battery automatically starts charging again. Your battery should be good and charged by the time of the next outage.


My DIY transfer switch. From left to right: top down view of the switch, the output side, and the input side. There are basically two inputs, the inverter and the utility. When the utility is supplying power, the switch sends utility power to the output. When the utility stops supplying power (the power goes out), the switch sends power from the inverter and battery to the output.

Even if you have a generator for emergencies or you decide that a car/inverter is the best setup for you, I would still recommend having one of these systems. This system will automatically continue running basic life support (maybe just a pump or two) until you can get your generator running. Generators are great for longer outages, but they're not automatic (unless they're a whole-house solution). Even if you lose power for a few hours one day randomly while you're at work, your tank will continue humming along with a battery backup like this.

While I believe everyone can benefit from this type of backup, this system is especially useful for those who live in apartment buildings or are not allowed to have generators because of homeowner association rules. Battery capacity is not exactly cheap, but if your choice is keeping your reef running or not, this may be the best option that some people have.

This sounds an awful lot like a UPS...

If you're thinking to yourself that this sounds like a UPS (a battery backup that runs your computer when power goes out), you're right. I used the standard UPS as inspiration for this system. Basically, a UPS has a battery, an inverter to turn battery power into AC power, a charger to charge the battery, and a switch to go back and forth between utility power and battery power. While a UPS is a simpler option, there are a few downsides to them.

First, they are not upgradable. You buy a fixed capacity and it has a fixed run-time. If you outgrow that, you need to buy a new UPS. With this system, you can choose how much power you want and for how long. If you decide you need more capacity down the road (like I did) simply upgrade the battery.

Second, if any one piece of the UPS breaks (the battery, the charging circuit, the transfer switch, or the inverter), the whole UPS is worthless (unless you're lucky enough to be within warranty). With this system, you can swap out literally any part with any other off-the-shelf part. If your inverter dies, just get another with similar capacity. If your battery dies, just get a new battery. Same with the charger and the transfer switch.

A UPS is not a bad option. For some people, all this extra work to DIY is just not worth it. Because it is difficult for me to run a generator, however, I need a system like this because I potentially may rely on battery power for days at a time. I can't do that with a UPS.

Potential Parts List

The basic parts of this system are the battery to supply the power, a charger to keep the battery maintained, an inverter to turn 12VDC into 120VAC, and a transfer switch to automatically switch between the utility and battery power during an outage.

I'm going to provide basic guidance on which parts to choose here, but there's a lot of additional information on each one of these topics that could be posts on their own. Perhaps I'll get to that one day. For now, I'm going to skip over a lot of the theory, give some practical information, and some broad generalizations about the different components involved in this system. I would be happy to answer any questions in the comments, as well as discuss the reason for my recommendations there.

How Much Power Do You Need?

Before we discuss the parts of this system, we need to discuss capacity. A battery backup system can run for as short as a few hours or as long as a week. You must choose exactly how much capacity you want. Fortunately, doing so is relatively easy.

First, choose what equipment you want to run in the event of an outage. Your goal should be to run as little equipment as possible in the event of an outage (I run a Koralia 425 Nano in my pump and a Jebao PP-4 in my display). You should at least run one pump in your display. If you can run water through your sump by running a return pump, that would be ideal, but return pumps can use a lot of power. A skimmer would be a nice touch also, although again, they can use a lot of power. The very last thing you should run is your reactors and other media, but they're an option if your battery bank is robust enough. It's best if you get a Kill-A-Watt meter and figure out exactly how much power your equipment uses. You should NOT plan to run your lights or heater on this battery backup. They use way too much power, and your tank will likely be okay without lights and heat unless your outage spans many days.

Now that you know what you want to run and how much it uses, we need to do some math. Let's say you have 50 watts of equipment you want to run in an outage. First, convert that to amps by dividing by 12 (50/12 = 4.16). Next, we should plan to only ever discharge the battery to 80%. Discharging more will greatly reduce the lifespan of the battery. To do so, divide the previous number you got by 0.80 (4.16/0.80 = 5.2). Next, although this is optional, I would add a 10% buffer for some wiggle room. This will help account for inefficiency in the inverter and elsewhere in the system. To do that, multiply the previous number by 1.1 (5.2 * 1.1 = 5.72). This number is how many amps it takes to run your system for one hour.

To see how long a given battery will run your system, simply divide the battery's capacity by the previous number. Let's say you're looking at a 50Ah battery. Simply divide 50Ah by the previous number to get the run time (50Ah / 5.72 = 8.74 hours). For a 100Ah battery, the math would be 100Ah / 5.72 = 17.5 hours.

By the same token, you can take the amps to run your system for one hour (5.72) and multiply it by the hours you want your system to last. Say in the previous example you want your system to last 24 hours. Multiply 5.72 by 24. The answer (137A) is how many amps your battery needs to run your system for 24 hours.


The cheapest type of battery for this type of application is a 12V lead acid battery. The battery you get should be sealed or maintenance-free (they're also sometimes called spill-proof). This is important because batteries produce hydrogen gas as they're charged, and built-up hydrogen gas can cause explosions very easily. Only sealed batteries will not emit significant amounts of hydrogen gas when charged.

One of the better choices for batteries is a technology called absorbent glass mat (AGM). This type of lead acid battery offers a relatively good service life, even when deep cycled to more than 75%.

My new 100Ah AGM battery being maintained at float voltage on a charger. I got this after the last outage and am glad to have it charged and ready. If my main 50Ah battery runs out, this battery will (conservatively) run my backup pumps for 33 hours. A more realistic run time is closer to 40 hours.

For the purpose of this post, I will only be discussing using a single large battery for your power source. You could use a battery bank of smaller batteries, but the nuance involved with that is a bit outside the scope of this discussion.

Here are a few decent options for batteries (I found all of these by searching for "agm battery" on Amazon).

Name: ExpertPower EXP12330
Voltage: 12VDC
Capacity: 33Ah
Price: $80
$ per watt: $0.20
Max theoretical run time for a 25W load: 15.84 hours

Name: UPG UB121000-45978
Voltage: 12VDC
Capacity: 100Ah
Price: $165
$ per watt: $0.1375
Max theoretical run time for a 25W load: 48 hours
(note: this is the battery pictured above)

Name: Renogy AGM 12 Volt 200Ah
Voltage: 12VDC
Capcity: 200Ah
Price: $390
$ per watt: $0.1625
Max theoretical run time for a 25W load: 96 hours


Inverters are the next part of our system. Inverters take the battery power and turn it into AC power that your equipment can use. There are two basic types of inverters, modified sine wave (MSW) and pure sine wave (PSW). The power that comes out of your wall is called AC, which stands for alternating current. The current alternates positively and negatively, and the stuff that comes from your utility has a nice clean sine waveform.

There's lots of theory and information that goes into this topic (Google if you'd like to learn more). Suffice it to say that PSW inverters are better, but they're much more expensive. Additionally, cheap (read: junk) PSW inverters don't always have a perfectly smooth waveform. It looks aliased, like a jagged waveform. May be better than MSW, but you won't get really pure sine waveforms unless you buy a good PSW inverter. Which sine wave inverters are "good?" Hard to tell. For those reasons, and because your equipment will only be running on your inverter for a few hours if you lose power, I recommend a MSW inverter. They're cheaper, and they will likely not harm the equipment that we use in our aquariums, especially on the short-term.

EDIT: 09/15/2018 - After much discussion with people who know a bit more about electronics than I do, I must update the statement I made in this article regarding my recommendation for modified sine wave (MSW) inverters. I had originally said that MSW should be fine even if they're a bit harder on aquarium equipment.

Unfortunately, it's not just as easy as saying that a MSW inverter should be fine. Depending on the quality of the inverter, it should be okay. But, there's no way of knowing that. Cheap inverters usually don't disclose THD (total harmonic distortion) and almost never include automatic voltage regulation (AVR). So, the power could be really good, or it could be really bad. It's hard to tell.

This matters because modified sine wave inverters may be harder on electronics than I had originally anticipated. Devices that run on AC power, such as traditional AC pumps and aquarium heaters, will likely operate fine on MSW inverters, even if the power is not that clean. DC devices, or those with an AC/DC power supply, might not do so well. I had originally thought that AC/DC power supplies would just run hotter, but this might not be the case. Depending on the quality of the AC/DC supply, the unclean AC power from an inverter may actually cause the AC/DC power supply to fail. This likely won't affect the device, so the pump or controller would likely be okay, but still, most AC/DC power supplies cost about $30 at a minimum. More expensive Ecotech supplies cost $50 - $75 or more. This is not an insignificant cost.

If you only have AC equipment, like traditional powerheads, pumps or skimmer pumps, then a modified sine wave inverter is fine. If you want to run electronics, a pure sine wave inverter is probably better. It's worth noting that I'm still running my DC equipment on a MSW inverter, but I am aware of the risks and am prepared to replace my DC power supplies if they fail.

My inverter, a Powerbright 1,100W (2,200W peak) MSW inverter. I've been using this since 2011 with no problems at all.

In most cases, the size of the inverter you choose won't matter too much. To keep power usage low, most reefers would likely only run 100W of equipment at most. Any inverter between 500W and 1,000W will likely work just fine for most reef tanks. If you wanted to get a larger inverter (1,000W or more) so you could use your car as the power source and run a heater in dire emergencies, that's not a bad plan. Whatever size you choose, just make sure it's larger than your power requirements.

Another thing to keep in mind: when choosing an inverter, make sure it has a "peak" or "startup" wattage. When AC motors start, they have a large initial current draw that can sometimes be double (or more) than their standard power use. A 50W return pump may draw 100W or more at startup. Many inverters compensate for this, and advertise a "startup" or "peak" power capacity, but some don't. Just make sure the inverter you buy explicitly states that it has a peak power rating (all the inverters below do).

Here are a few inverters that would work with our setup (again, to find these, I simply searched "ac/dc inverter" on Amazon).

Name: Potek 500W Inverter
Capacity: 500W
Price: $35

Name: Powerbright PW1100-12
Capacity: 1,100W
Price: $75
(Note: I have had and used this inverter since 2011. It has worked flawlessly the entire time)

Name: Ampeak 2000W Power Inverter
Capacity: 2,000W
Price: $155

Transfer Switches

There's actually only one commercial option I'm aware of for transfer switches that do what we want, and that is from Xantrax. It's a bit pricey at $65, but it does the job. I don't actually have one of these, but I'll do my best to describe what you need to do.

First, notice the construction of the unit: it has two inputs, one that plugs into the inverter, and one that plugs into the wall (you'll have to click on the link, I don't think I'm allowed to use Amazon's pictures). It then has one output. You plug your aquarium pumps into the output, plug the "inverter" lead into the inverter, then plug the "AC" lead into the wall.

Unfortunately, we need to do some wiring to get this thing to work in our application. At this point, I need to remind you that improperly wired AC can cause fires, destroy your home, or even kill you. Only attempt this if you're comfortable with wiring and have wired AC power before.

Fortunately, wiring this up is actually fairly straight-forward. All you need is a grounded extension cord, the kind with three prongs. Any will do, even one from Walmart, Target or Home Depot. You just need one that's around 2' - 4'. To wire this thing up, basically cut the cord in half to expose the wires inside. Most extension cords have color-coded wires inside, and most of the time, they are white, green and black:

three wires - ext.png
The inside of a common grounded extension cord I had cut open for something else long ago. The white, green and black wires match those on the Xantrax switch.

The male side (the one with the prongs) gets hooked up to the "from utility" lead on the switch (green to green, black to black, white to white). Next, hook the female end of the extension cord (the one with the socket) up to the AC output part of the switch.

I did a DIY transter switch, and would be happy to provide information on this if anyone is interested. If you've ever worked with relays, this is probably something you could easily do as well. You basically need a DPDT relay with a 120VAC coil. Wire the output to the common terminals, wire the inverter to the NC terminals, and wire the utility power to the NO terminals. When power is on, the relay is energized, connecting the NO terminals to common (read: utility power flows to the output). When the power is out, the relay is closed, and NC flows to common (read: inverter flows to common).

See this post for a diagram of how I wired my transfer switch.

The inside of my DIY transfer switch. It's tough to tell what's going on here because I crammed it into a tiny junction box. Basically, two wires come in, one from the wall and one from my inverter. The output goes out the other end, and the switch is wired as above (note the grounds are all tied together).

EDIT: check out this post for a new and more sleek looking transfer switch that I built.


The last part of our system will be a battery charger. This is something you can plug into the wall that charges the battery. There are a few basic guidelines for choosing a battery charger. First, get one that's designed to be connected to the battery at all times and maintain the voltage. These are sometimes called maintainers, smart chargers, or three (or more) stage chargers. Second, since this is going to be connected to your battery at all times, you may want to get one that does not have a cooling fan, to reduce noise.

My charger that I use to keep my 50Ah battery topped up, a Battery Tender Junior. It's passively cooled and has no fans. I actually got it originally for my motorcycle 10+ years ago. It's only a 0.75A charger and takes forever to charge my 50Ah battery, but it works just fine to keep it topped up and maintained. And yes, it came from the factory with salt creep all over it.

It's also worth mentioning charger capacity at this point. Usually, chargers will be sold with a certain amperage rating (1A, 2A, 6A, 10A, etc). This will ultimately decide how long the battery takes to charge. To find the charge time, divide the battery amps by the charging amps. If you had a 50Ah battery and a 2A charger, the battery would take 25 hours to charge (50Ah / 2A = 25 hours).

You might be asking why not get the largest charger possible so your battery can be ready as quickly as possible? In general, it's better to charge batteries slow than fast to prevent wear. At most, you shouldn't charge your battery at more than 25% of its total capacity. So if you have a 100Ah battery, you wouldn't want to charge it with more than 25A (100 * 0.25 = 25A). If you have a 35Ah battery, you wouldn't want to charge it at more than 8.75A (35 * 0.25 = 8.75A). The chargers I'm going to recommend are all in the 1A - 6A capacity range. The reason for this is in most cases, the charger is not supposed to be charging your battery as quickly as possible. Your charger is supposed to just sit there and keep your battery ready until it's needed. In other words, if you only have power outages every few weeks, and only in the winter, it doesn't matter if your battery takes 25 hours to charge. It'll be ready by the time you need it. There are circumstances where you may want a larger battery charger, but unfortunately this post is already way too long. Perhaps I'll follow up with another post on that later.

Here are a few cheap chargers that will work just fine for our purposes (these were found by searching "battery charger" on Amazon).

Name: Black and Decker BM3B
Voltage: 6VDC and 12VDC
Capacity: 1.5A
Price: $14
Theoretical time to charge 50Ah battery: 33.33 hours (1.38 days)

Name: Potek 2/6/10 Amp Smart Battery Charger
Voltage: 12VDC
Capacity: 2A/6A/10A
Price: $50
Theoretical time to charge 50Ah battery: 25/8.33/5 hours

Putting It All Together

Unfortunately, I don't have too many pictures of a system up and running. The pictures scattered throughout this post are of my own system. Basically, hook your inverter up to your battery and turn it on. Hook the charger up to the battery and plug it in. Plug the "from utility" wire from the transfer switch into the wall. Plug the "from inverter" wire from your transfer switch into your inverter. Plug what you want to run on batteries into the output of the transfer switch. You can test that your system is working by unplugging the "from utility" wire from the wall (this will simulate a power failure). Or, if you're feeling really adventurous, kill the breaker! There's something really satisfying about watching your pumps continue to run when everything else is shut down.

Please let me know if you'd like to see any up close pictures of my system. I'd be glad to share.

Here is how my system is set up. The white surge protector is powered by the battery backup system. In the event of an outage, everything plugged into it will keep running. The rest of my pumps are plugged into the black power strip, which only runs when the power is on. Please ignore the relative mess of my wiring.


There's nothing more anxiety provoking than when your power goes out, especially during a massive storm when the power company has no ETA for restoration. When will it come back on? Will my tank be okay? How many of my wonderful pets are going to die before my power comes back? My hope is that for a few hundred dollars ($200 - 300 on the lower end), most reefers will never have to ask themselves these questions again.

Please let me know if there are any questions or clarifications I can offer on this system. I'd be glad to help in any way I can. Similarly, please let me know if you spot something incorrect in this post. I'm no expert on this stuff, by any means. I just had a burning desire to learn to help protect my reef.