How to Use a Battery Bank to Protect Your Home From Outages

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One day, you were sitting in your office and the power went out. But your computer kept running because you have an uninterruptible power supply or UPS. A thought went through your mind. “That was cool. I wonder if I could put my whole home on a UPS?”

Yes. Yes you can. It might even be less expensive than you think. We’ll cover the basics, but a system like this should be installed by a qualified electrician. Stick with it to the end and we’ll see how you might be able to plan out this system, and save some serious money as well.

How Can I Have Battery Backup for My Whole Home?

The concept is simple. Have several batteries connected to form one big battery, more properly called a battery bank or array. Have a way to plug that bank into your home’s power supply to keep the bank fully charged. Then, have a way to use the power from the bank when the power goes out. Simple, right? Yes it is simple, but is it easy? It can be.

What Do I Need?

The hardware required to create this battery backup isn’t a lot. You’ll need batteries, an inverter/charger, some battery cables, an inline fuse, a battery rack, and some electrical wire. Of course there are other little bits and pieces to it, but this is just an introduction to the system. You’re best to have a qualified electrician install this. Consider this a primer so you’ll better understand what your electrician is saying.

battery backup components

When it comes to finding an electrician, try to find one with plenty of experience in this sort of thing. Battery backups and grid-tied power systems are a specialization of sorts. In fact, see if you can find someone who is NABCEP certified first. NABCEP is the North American Board of Certified Energy Practitioners. Don’t be afraid to ask for references and go see some of their other work.

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nabcep accredited

If this looks like something you want to do, the next thing to figure out is the specifications of all the parts needed. This depends on details about your appliances. It also depends on how you currently use electricity. The final piece is how long you want backup when the power goes down.

How Do I Size Equipment for my Home Battery Backup?

You need to figure out some things about how your home and appliances use electricity. What you’re figuring out is the kilowatt hours, (kWh), watts (W), and amp (A) usage of all the appliances and devices in your home. You may want to start writing these numbers down. They’ll help you a lot when you start this project, and are useful for many other energy saving projects. Since homes vary widely across the world, let’s base the calculations in this article on a sample home. In previous articles, we’ve used a 1,600 sq.ft. home located somewhere in the northeast U.S. where power usage is highest in January.

subject-house-streetview

The easiest way to figure out how many kWh your home uses in a day is to look at your power bills. Find the bill that has the most electricity usage. If you’re in the north, it’ll probably be from the first three months of the year. If you’re in the south it will probably be from the peak months of the summer. Your bill should tell you what your average daily electricity usage is for that period. That will be in kWh and is a key number for this project. The average daily electricity usage for this home January is 38kWh.

You’ll also want to know how many watts your appliances use at one time. There are a few ways to figure this out. You could get the information from the nameplates of your devices, write that all down, and then add it up. But many devices don’t have a proper nameplate with all the information. The most accurate would be to get a device like the Kill A Watt EZ Electricity Usage Monitor. The example home’s appliances and devices would draw about 4400W (or 4.4kW).

How Do I Size an Inverter/Charger for My Battery Backup?

The inverter is the device that will take direct current (DC) electricity from your batteries and convert it to the alternating current (AC) that your home’s appliances use. It’s capacity is measured in watts and volts. You’ll want an inverter that has the ability to charge your batteries too. Because your home is still tied to grid power, your inverter needs to be able to safely tie in to the grid. These types of inverters have the ability to automatically detect when the grid power drops and switch you to the batteries. That function is known as an AC auto-transfer switch.

The important number for sizing the inverter is watts. The example home draws about 4.4 kW. You may want an inverter rated higher than that…but not too much. The bigger they are, the more expensive they are.

The next important number is the battery bank voltage (VDC). This means the voltage coming from your battery bank to the inverter. How do you figure that out? A good rule of thumb is to add 12VDC for every 1000W of AC power that you’ll need. We know we need 4400W. Being just over 4,000W you can see that you’ll need 4 X 12VDC. So a 48VDC system is probably best.

We now know we need a 4.4 kW (or 4400W) 120VAC inverter 48VDC charger. That’s a mouthful, but it gives you a good point to start searching for one. Something like the Magnum 4400W Inverter/Charger should work.

How Do I Size My Battery Bank?

A battery bank is just a collection of batteries that are connected together. It’s the battery bank that will determine how long you can run your house while the grid power is down. So you need to figure out how long you want that to be. Take stock of outages in your area over the last few years. What’s the longest outage you’ve had? Was it a freakishly long time like a few weeks due to a hurricane? Try not to count that in to your calculations. Battery backup for a few weeks will require charging from solar panels, wind turbines, generators or a combination of those. At that point, just consider going off-grid.

Solar panels on a house roof

Most people only have an outage for a few hours. . Perhaps you’re in a very rural area where it routinely takes a day or two for the service crews to arrive. Perhaps you live in the city and you’ve never had an outage longer than 4 hours. Events like Hurricane Sandy are rare, and having a backup that would cover you for a couple weeks would be ridiculously expensive. Try not to count exceptions like that into your estimate.

Hurricane Sandy

The example home is in a city, so let’s say you’re never without power more than 4 hours. But you’re cautious, so let’s use 6 hours to build in a safety margin. Let’s figure out how many kWh you need to cover 6 hours. Divide the daily kWh used by 24 to get the hourly kWh used. Then multiply that by the number of hours you need backup. 38kWh divided by 24 gives us 1.58 kWh. Multiplied by 6 hours gives us 6.33kWh.

(Daily kWh / 24h) / Backup Need in Hours = Total kWh Needed

Batteries should not be drained more than 50% in this kind of system. That means you’ll need twice as many kWh, so that becomes 12.7kWh. Yet inverters aren’t 100% efficient, so you’ll lose a percentage of that 12.7kWh to the inverter. Inverter efficiencies vary, but 85% efficiency is a reasonable number to work with. To get your final number, add the 15% inefficiency to your kWh to get 14.9 kWh. Let’s call it 15kWh for the sake of easier math. Here’s the whole formula for this calculation:

Minimum kWh X 1.15 = Total kWh

The final number we need is the Amp hours (Ah). To get that, you divide the total kWh (12.7kWh) by the voltage of the battery system that you want. You figured that out to be 48VDC.

Total kWh / VDC of Battery Bank = Ah

Your Amp hours are going to be 12700Wh divided by 48VDC. That gives you 265Ah. Now you know you need a battery bank that will give you 265Ah at 48V. So how many batteries will you need and how will they be configured? That depends on the voltage and amp hour ratings of the batteries you want to use. For this example, let’s use a 12V 100Ah deep cycle batteries. They’re common, easy to handle, and cost about $200 each.

Using this type of battery, you’ll need 12 of them, costing you at least $2400. You’ll have 3 groups of 4 batteries wired in series. The 3 groups will then be wired in parallel. This will actually give you 48V and 300Ah. The diagram below shows what that will look like.

300Ah 48V Battery Bank

There’s a simple battery bank design tool over at FreeSunPower.com. Go and try different configurations of batteries, system voltages, and amp hours to see what other options you may have.

Can I Do This Less Expensively?

“It sounds like this is pretty expensive.”, you think. Yes, it is. The system we’ve talked about above would be around $6500 to install. For the ability to run needed medical devices, it might be worth it. If you live in a condo or a have a home owners’ association where you can’t run a gas generator, it might be worth it. But just to make sure you have some power in a power outage?

Home Dialysis NxStage

It can be done cheaper. The key is to be strict about what appliances and devices you really need during a power outage. Eliminating non-essential loads could cut your installation cost in half. If you can trim down your power needs, you may consider getting a solar-powered generator like the VSP 3500 watt Generator for around $3,300. The bonus with this system is that it can keep charging even when the power is out.

Trim far enough and you might even be able get away with just needing a self-contained battery back-up solution, like the Energizer 2200-watt Portable Inverter Generator for around $600.

The important point is that you can power your home with batteries during a power outage. Armed with what you’ve learned here, you can continue to research the best way for you with a greater degree of understanding. Be careful though! Once you realize you might be able to get off the grid altogether, you might just develop a whole new energy lifestyle!

What do you think about building such a system in your own home? Will you hire an electrician to build one for you? Have you ever built one? Share your own experiences in the comments section below!

Image Credits: 12V 100Ah AGM SLA Deep Cycle Battery, Magnum Energy 4400 Watt 120/240V Pure Sinewave Inverter/Charger 48V MS4448PAE, MidNite Solar Surge Protection Device MNSPD-300-AC, Whistler IC-2000W 2 Gauge Power Inverter Cables, via Amazon, NABCEP Accredited, via NABCEP, Solar panels on a house roof, via Shutterstock, NASA Examines Hurricane Sandy as it Affects the Eastern U.S., via Flickr, Home Dialysis NxStage, via WikiMedia.

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