How to prepare your family to survive and thrive in todays uncertain world

How To Build an Emergency Power System

A large bank of batteriesThis post is a hands on how-to which will show you how to size and build your own emergency power system.  We’ll talk about what we want to power with the system, and then design an appropriately sized power system.  We’ll talk about how to use it in an emergency and how to keep it charged up.  This is all oriented around emergencies where you are tucked in nicely at home, and not running around through the woods like Rambo.

An introduction to electricity

If you know the ins and outs of electricity feel free to skip this section.  For the rest of us mere mortals I’ve written an introduction to electricity and concepts that are important to us.  To help us understand a bit, I’ll compare it to water going through a hose.  Warning:  This is a simplistic but sufficient explanation!

There’s three things that are important for us to know about:


Electrical current measures the  rate of flow of electrons in a circuit and is measured in Amps.  In our water analogy, it’s the flow rate of the water.


Voltage measures the amount of electrical potential.  In our water analogy, it’s a measure of how much water pressure you have.  For most applications, voltage doesn’t change much.  Common voltages for emergency power systems are 220/240, 110/120, 24, 12, and 6.


Power is how much electricity you’ve used up.  When you see Watts or VA referred to on an appliance tag, it’s talking about Power.  In our water analogy, think of the total volume of water transferred.

Ohm’s Law

A fine gentleman named Georg Ohm discovered a relationship between these three measures.  Nowadays we call this Ohm’s Law.  Simply put, the equation is P = VI.  In non algebraic  terms, it means that Power = Voltage * Current.  The main thing to take away from this is that for any given load, as you reduce voltage your current goes up.

Putting it All Together

If you have an appliance that uses 1100 watts of power at 110V it will pull 10 Amps of current. If you run it off an inverter hooked up to a 12V battery,  it will pull 91 Amps.  YOWZA!

In our water analogy, think of your common 1/2″ garden hose.  The water pressure (Voltage) doesn’t change.  If we grab a stopwatch and figure out how much time it takes us to fill a five gallon bucket, you get a rate of water flow (Current).  If you swap that hose out for a 1″ diameter hose and measure the fill time now, you’ll find out that your water flow rate (Current) has changed even though pressure at the spigot (Voltage) and the size of the bucket (Power) haven’t changed at all. If it doesn’t make total sense yet, that’s ok … read on, we’ll have some practical application down below!  If it STILL doesn’t make sense after that, then ask in the comments or email me and I’ll clarify.

First, we need to think about what we want to use power for

Lights, heat, cooking, tv, fridge, power tools, all that stuff, right?  Nope, you need to pare it down.  Think about what you need to have power for vs what is just nice to have power for.  Heat, for example, isn’t a great use of stored power.  Lighting can be, but you can also get light from lanterns, candles, and the like.  Cooking can be done on a propane grill, camp stove, rocket stove, and other such things.  Here’s what I would use power for.

Short Term Appliance Usage

For short to medium term emergencies you’ll want to be able to keep your fridge and freezer going every few hours to keep stuff cold and frozen.  You may want to wash a load of clothes from time to time.  Or you want to use the Kitchenaid to process some food.  You get the idea.  The key here is to remember to be frugal with appliance usage because these things can eat up power like there’s no tomorrow.

Rudy’s Tip: During an emergency you can run your freezer or fridge for a few hours a day and things will keep nice and cold as long as you try not to open it too much.  Chest freezers are far better at staying cold, and so are full freezers.  The amount of time you need to run your fridge will vary based on the weather.  If it’s summer in Arizona you may want to forget this plan completely!


I wouldn’t suggest running standard incandescent lights off of an emergency power system.  Using LED based lights can significantly reduce your overall power usage.  Look for bulbs and fixtures that are designed for RV use.  Using candles and lanterns for lighting is also a good idea.

Charging Batteries

You’ll want to keep batteries charged up as best you can.  You DO have a stash of rechargable batteries, right?  These can be used in lots of different things.  Flashlights, radios, GMRS type walkie talkies, you name it.  Don’t forget to charge IPods, portable video game units, etc to keep entertainment options open.

Rudy’s Tip: Some batteries can’t be safely charged on cheapo inverters.  Some chargers require a proper sine wave to function.  Most inverters have a modified sine wave.  More about this later, but keep it in mind…


Using power tools can seriously improve your effectiveness when you’re working around the house.  Using a circular saw to cut plywood to protect your windows is far more efficient than using a hand saw.  Well worth the power usage.  Be prudent, but smart.

Components to an emergency power system

There are three main components to an emergency power system that we need to consider:

Power Source

During a grid up situation we obviously have grid power.  If the grid goes down, our power can come from our power storage system, a generator, solar power, or any combination of the above.  Since the power storage system can’t replenish itself, we need a power source that can fill up the storage system and ideally also be used for a power source directly.

Rudy’s Tip: For me, a good generator fits the bill here.  I don’t have enough sun year-round to count on solar for anything but extra candy on top.  Things I look for in a generator are remote start, efficiency, and either diesel or propane operated.

Power Storage System

This isn’t an absolute requirement, but it’s definitely a recommendation.  You should have a way to store power for later use.  Generally speaking you’re looking at a bank of batteries with an inverter to convert DC to AC and a charger to top off the battery banks from our power source(s).  You need to know what you want to drive off the storage system in order to properly size it.  More about this later!

Rudy’s Tip: You should avoid using auto batteries for your battery banks.  Look for deep cycle batteries which will stand up to hundreds of charge cycles before losing capacity, vs tens for a car battery.  Your best bang for buck will normally be with 6V batteries designed for golf carts and the like.

Power Distribution

This can be as easy as a few extension cords and as complicated as secondary electrical wiring in the walls of your house complete with emergency lighting.  Go as hog wild as you want here.  I recommend using extension cords for most applications, but if you have an opportunity to run dedicated wiring it’s well worth having emergency outlets in every room.

Rudy’s Warning: Stay away from transfer switches that will switch your entire house between grid power and your own system.  It’s really easy to drain your storage without even noticing until it’s too late.  The average house has so many different things plugged into the house that we don’t even think about or notice.  This warning doesn’t apply if you are doing a major power storage system or if you’re doing whole house solar, etc, but if you’re doing that then you probably don’t need this how-to!  Feel free to email me or ask in the comments if you want more information on this!

Sizing your system

First, you need to decide what you’ll run off of your storage system vs what will only run when there’s an available power source.  For example, I would recommend you not try to run major appliances off of your battery banks.  Plug the freezer and fridge directly into the generator and operate them that way.  Major tools and such I would recommend doing the same thing with.

That said, let’s take a super simple system that my reader was asking about.  She wanted to be able to run her crockpot for ten hours a day, have some 24 hour lighting available, and a bit of extra just in case.    All run off of the battery storage system which is replenished from a generator.

First things first, we need to know how much power these things burn up.  This is easy if we’re provided with the power draw by the manufacturer, but if they only provide us with current draw we can calculate power as well.

For lighting, let’s figure she’ll want 10 bulbs.  If you buy CFLs you can get the equivalent of a 100W incandescent bulb that only uses 23W.  Fantastic.  So for lighting, your overall power usage is 230W.  Realistically you won’t have them all on at once but let’s be conservative and say we will.

Unfortunately the crockpot doesn’t tell us how much power it uses, but it does say that at 120V it will use 2A.  Since we know P = VI, we can calculate the used power as 120V * 2A = 240W.  Fantastic.

Rudy’s Note: Battery storage capacity is measured in something called Reserve Capacity and is defined as the length of time in minutes that the battery can power a 25A load without the available voltage dropping too low.

Since we know how long we want to power these loads, we can calculate the amount of Reserve Capacity (RC) we need for each load.  The formula used for this is RC(in minutes) = Run Time (in hours) * Wattage / 4.545 … the 4.545 is a conversion factor, just in case you were wondering.

Lighting:  24 hours * 230W / 4.545 = 1215

Crockpot: 10 hours * 240W / 4.545 = 1268

Total RC Required:  1215 + 1268 = 2483 minutes

If we go get a bunch of 12V batteries with a RC of 200 each, we’ll need 13 batteries total.  As always, we want to buffer this a bit, so call it 15 batteries.  Even a small system like we’ve outlined can be a large footprint and can start getting rather expensive.

To keep these charged, we’ll want to get a trickle charger that can keep them topped off while on grid power and that can be plugged into our generator for top off during emergencies.  We’ll need to have a power inverter to convert the DC power off the batteries into AC power that our appliances can use.  Power inverters have an input voltage (in this case we want 12V DC) and an output voltage (in this case we want 120V AC) and are rated for certain loads.  Generally speaking inverter specifications talk about peak and sustain loads, and they are measured in Watts.  In our case, we need an inverter capable of at least 500W, better to get one that’s bigger than that.

Rudy’s Tip: Get a charger that is a multistage smart charger.  This will prevent your batteries from being damaged by overcharging and will optimize the speed of charge.

Finally, you’ll want to size your generator appropriately.  We’ll assume that we want to run a fridge (600W) and a freezer (800W) off of it, charge our battery bank (800W) for a total of 2.1kW.  Buffer it a bit and you want a generator capable of at least 3kW sustained load.

To recap, to cover the needs of this system we need a good charger, an 800W inverter, a 3kW generator, and 15 200 RC batteries set up in a single battery bank.  Estimated street price as I write this right now is about $2500 for the major components, not incuding wiring.  Seems a little expensive to run a crockpot!  But you get the idea.

Rudy’s Disclaimer: This is not intended as a recommendation on exactly what you need to build.  You need to design a system to your own needs, and not to the needs of one of my readers.  I’ve intentionally left out technical details and in some places cut some corners.  You should research further before building your own system and potentially seek expert assistance.  The goal of this post was to get you thinking about it, and to prod you to decide if it’s something you really want to do.  That said, I’m willing to talk a bit more about this in depth if it’s something that you guys want … let me know!

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6 Responses to How To Build an Emergency Power System

  1. can you talk more about inverters? I have an oxygen concentrator that doesn’t work on an inverter and I’d love to know why!

    • You probably need a full sine-wave inverter. They are expensive, but you could get a good UPS (uninterruptible power supply) intended for a computer and run your concentrator off that. Keep it charged by plugging it into a MSW (modified sine-wave) inverter that is running off the storage batteries. This has a double benefit of giving you a true sine-wave, and a short term battery backup of your battery powered system.

      For a full discussion of this, you might want to visit:

  2. Hi Rudy,

    Thanks for the post, its very informative. I live in Kenya and we use a standard 240V. I wanted to put up a wireless hotspot base station and needed to provide an emergency power supply for about 4-6hrs to run my wireless equipment and server. My sever is a desktop that uses a CRT monitor and i presume uses about 200W of power. My Wireless devices and switches consume about 155W in total. I am particularly concerned about the inverter rating, the type of charger and the number of batteries i will require. Please email me ur response.

    • Send me an email … rudy @ with some more details on what you want to do.