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My Home Energy Backup System, by David L.

My home energy backup system was originally conceived to make a little bit of power for a very long time.  Rather than backing up the whole house with a generator for a relatively short power outage of just a few hours or days, I wanted a system that would function in an extended power “grid down” scenario.  I was working from the self declared principle that when the grid is down at night, a single light bulb makes a huge difference in how you feel.  In addition, I wanted to preserve critical refrigeration and freezer functions indefinitely.

So why I am I doing this?  Two words come to mind: Resilience and Instability.  Without turning this into a political manifesto, it doesn’t take a genius to see how dependent we all are on certain “systems”.  Those systems make food appear on the grocery store shelves and plastic junk at Wal-Mart but for the most part, we don’t really know how it gets there.    What happens to grocery store deliveries if diesel prices triple?  Will the dollar always be worth something?  How many more jobs will ship overseas?   Was the President of the United States really serious when he declared that coal-fired electrical plants should be taxed out of existence because of their “carbon footprint?”  So in my mind, resiliency means thinking about how you would accomplish something if the primary or customary way of doing that something were suddenly unavailable.  Instability implies that interruptions to these systems are now so much more likely that they are not insignificant as most people have assumed all their lives, and warrant a second look by everyone.  Why is instability higher than ever?  Our economic system depends on exponential growth of debt to continue that system.  All engineers know that anything that grows exponentially is ultimately unstable and to top it all off, our system is now showing signs of great distress.  Think of it this way, our economic system is like a balloon.  When you start inflating it, you don’t worry about popping it.  However, we have been inflating our “balloon” for such a long time and with so much hot air that it can’t take much more.  Since economic systems are quite complex, no one knows what or when something bad will happen – something to warrant the planning I talk about in this article and the expense it entails – but it seems past the time to be preparing for that something.

Now on with the rest of our program… I had heard of people in Florida who had whole house backup generators fed by 1,000 gallon propane tanks buried under their driveways.  After hurricanes hit the area, these systems were exhausted in a few days – mostly running mammoth central air conditioners.  (Keep in mind that at $3 per gallon, it takes $3,000 just to fill up one of those tanks.) Many of these people were then without power for weeks.  Their systems failed them because their expectations for the length of the disaster were low.

I came to believe that making a small amount of power was my goal and I sized everything around the 2,000-2,500 Watt (W) range.  By that I mean that after spending thousands of dollars, I can only generate between 2,000 and 2,500 watts of continuous power and at 120 VAC [1] that equates to a generated current of roughly 20 Amps (A [2]).  You can walk into a home improvement store and buy a 6,500 watt generator for around $1,000 that delivers about 50A.  Given that most households are supplied by their electric utility with 200A service, have I lost my mind? 

Yes and no.  There are certainly a lot of things that a 2,500 W power system can’t do – like run your central AC (240 VAC), make hot water with your electric water heater, run an electric stove, and you might even be hard pressed to run some powerful hair dryers while operating other electrical devices – so what gives?  Ah, but you can do a lot of other very important things with 2,500 Watts of power, such as, running LED [3] lighting.  At 6 Watts per light, I can light my whole house and not even make a dent in my 2,500 W power budget.  I considered all kinds of fancy refrigerators including those that run on propane, kerosene, and others marketed to off grid folks as super energy efficient.  In the end, I realized that a new model year 2011 nineteen cubic foot upright refrigerator/freezer with the freezer on top is about the most efficient appliance you can buy.  Realizing this tidbit only cost me $700 – delivered- from Lowe’s, and I used the money I saved over some multi thousand dollar device to add some extra photovoltaic (PV) panels to my roof.   I’ve watched this refrigerator run and after the compressor starts up, it consumes 1A AC @ 120V.  That’s 120 watts  or 2,880 Watt Hours (WHr [4]) per day.  However, I would say that being very efficient and well insulated, that this refrigerator is only running its compressor at most half of the time.  Therefore I use about 1,440 WHr max per day for this appliance. 

So lighting and refrigeration/freezing are very much within the 2,500 W limit.  What about air conditioning?  I live in the south and it gets hot and humid here.  I don’t like to sleep in that kind of weather so I have a very generously-sized 3 ton central AC system (15 SEER [5]) to keep me nice and cool 24/7.  However, in a grid down situation, that system will be useless to me unless I want to cover my ¾-acre lot with solar panels – probably not going to happen.  Maybe someday I’ll further investigate a geothermal heat pump.  I see claims that they can run on the equivalent of a refrigerator compressor and actually be viable on solar but with a $20,000 – $50,000 equipment and installation price tag that’s a long shot.  So I decided to try to run a window AC unit off of my alternative energy system so that means first complying with my 2,500 W self-imposed limit.  Let’s see… a ,6500 BTU [6] window air conditioner to cool one good sized bedroom draws about 6A @ 120V when the compressor is running, so that’s 720W – check – still within the limit but there’s another problem…


Starting Appliances
Many appliances have electrical motors.  This includes power tools like circular saws and refrigeration compressors like you find in air conditioners, refrigerators/freezers.  Electrical motors have two power requirements:

  1. The amount of current to start the motor and
  2. The amount of current required to keep the motor running

Items one and two are very different.  Item one can best be described for compressors as the locked rotor amps (LRA [7]).  If you are nosy enough when you go window air conditioner shopping you might be able to view the label on the compressor through the slotted venting on the side of the air conditioner (take a flashlight).  If you can see the LRA number, you may be discouraged – I was.  On my 6,500 BTU window air conditioner that runs on no more than 6A, the LRA is 24.  That means that my system has to provide 24A AC of instantaneous current (2,880 W) for a couple of seconds to start that compressor.  If your power system can’t provide that then you just bought yourself a very expensive fan – the compressor won’t start – ever.

A generator like mine, that surges to 2,500W can produce just over 20A – not enough.  By the way, the LRA on my Trane 3-ton central AC compressor is 83A.
Obviously, you need to buy a bigger generator – one with higher running watts and surging (starting) watts – right?  But bigger, reliable generators cost a lot more money and here’s the kicker – they use more fuel and fuel is something you’re trying to make last a very long time in a grid-down scenario.  And if you’ve seen those “economical” generators at the home improvement stores, just walk away.  I’ve heard them described as disposable as well as fuel hogs.   So, if a generator is on your list of got to have backup items for long term usage, you want one that sips fuel, is quiet, built to last, and that can run your essential stuff.

A note on fuel:  The generators at home improvement stores run on gasoline.  So if you plan to run one of these for weeks on end, you’re going to need a lot of gas – more than 5 gallons per day depending on the generator’s power generation capacity.  Gasoline also has a relatively short shelf life before it goes “stale” and we all know it’s volatile – as in “ka-boom”.  However, almost all gasoline generators can be converted to run on propane.  Propane stores in those nice, cute barbeque cylinders and it lasts for a very, very, long time.  A 20 pound barbeque propane cylinder stores about 5 gallons of propane. 

Moving on… Why don’t we convert that pesky window AC unit to start on less AC current – yes you just might be able to do that.  It turns out that the generator that I have is very popular with RVers because it’s fuel efficient and extremely quiet – 59dBA at load.  It’s so quiet that I can sit next to it while it’s running and talk on my cell phone.  In a grid down situation, that’s a good thing because a running generator says, “I have stuff and you don’t”, “come on over and steal that stuff” as well as irritating you as it drones on for hour after hour.   Continuing, these RVers were having trouble starting their 13,500 BTU roof-mounted AC units with my Yamaha inverter generator.  2,500W of surge just wasn’t enough to do the job so on a web forum discussing the problem, I was introduced to the supplemental hard start capacitor.  You connect this new capacitor in parallel to the compressor start capacitor that your air conditioner already has inside and voila – your AC unit starts on less current.  (I purchased the hard start cap on Amazon for $10 + shipping)  Using a clamp on ammeter capable of reading AC surge current, I measured my window air conditioner drop from 24A to 13A of starting current.  The first of many problems solved but I’m not interested in just long term generator operation because of the fuel issue.  (I should note that when you open your window air conditioner, you could electrocute yourself if you don’t know what you’re doing so if you aren’t used to working with electrical wiring, don’t do this yourself.  I’m a college educated electrical engineer with a master’s degree from a top 10 school, which is another way of saying I’m book smart but prone to electrocuting myself when I work on stuff in the real world – but at least I know the danger.)
 We need to move on to solar.

Building a System
To run indefinitely I would need a fuel source that never runs out – the sun seems like a good choice and while the sun will eventually burn out, scientists still expect the sun to outlast me.  So I decided to invest in some solar panels.  Not so coincidentally, I sized my solar array system in the 2,000 watt range and bought a 2,500 watt inverter.  Inverters have a distinct advantage over generators in that all of the ones that I considered can supply nearly double the rated wattage for surge requirements.  My 2,500W inverter actually surges to 4,000W which is 33A AC at 120V.
I decided to build a system fed by all three energy sources available to me:

  1. Solar
  2. Dual Fuel Generator (Gasoline or propane – propane as a better long term fuel choice)
  3. Utility or Grid Power

The system would have a battery storage component so that I could save the solar energy generated during the day for use at night.  The battery component of the system is also nice because even without solar, you can charge the batteries when the grid is operating and then use the power later when you need it.  This is a scenario that might play out if the grid were being switched off – as in rolling or scheduled blackouts.
Also, I didn’t intend to install enough panels to make tying back into the utility grid to sell my excess power worthwhile.   By my calculations, If I wanted to sell my 6kWHr of power generated each day back to the electric company through a grid tied inverter, I could expect about $0.11/kWHr in my area.  That’s $0.66 per day or around a $20 per month reduction in my utility bill.  Saving $240 per year wasn’t enough in my mind to warrant the additional expense and complexity of the grid tie inverter.  This also made me realize just how much power a modern home consumes since my monthly bill in winter is around $240 and in the summer about $400.

[JWR Adds: Also, keep in mind that grid-tied PV [8] systems are much more vulnerable to EMP than stand-alone systems! This is because of EMP coupling through long utility power lines which act as antennas for EMP. They can carry EMP far beyond line of sight from a nuclear detonation.]

Mode 1 – Solar

In solar mode I have eight 230 watt solar panels feeding a maximum power point tracking (MPPT) charge controller.  I’m using an Outback FlexMax charge controller [9] and its job is to take the DC voltage and current from my solar array (~70Vdc @ 25Adc the way I have them strung) and convert it into the voltage that my battery bank and inverter need – namely 24V.  When the system is running on just solar, the refrigerators and lights draw power form the battery bank during the night and during the day, that usage is replenished by the solar panels and the current needed by the appliances is also provided by the panels.  As long as the batteries can run the appliances all night and with some margin to spare and then fully recharge during the day, you never run out of electricity.  My battery bank uses more expensive gel cells because I didn’t want to fool with adding water to standard lead acid batteries.  Yes, I’m easily distracted and maintenance isn’t my first love.

I don’t want to discharge my batteries more than about 25 – 30% during the night because the deeper you discharge the batteries in between charges, the fewer charging cycles you will get out of your batteries before they have to be replaced.  I have about 14,400 watt hours of battery capacity so the 50% rule would allow me to use 7200 wHr before recharging.  Restricting my usage to only a 25% discharge allows for 3,600 WHr.  That 3600 WHr will run my two refrigerator/freezers and one upright freezer and a number of lights all night long.  My 1920 W of solar panels will realistically produce about 6,000 WHr of power per sunny day given their angle to the sun, our latitude, etc.  As you can see, I have a sizeable margin built in for cloudy days and generally bad weather.  So my panels should be more than adequate to recharge my batteries during the day.

In solar mode, the generator connections and grid power supply connections are shut off.  If I have calculated everything properly, and nothing breaks, the system should run for a long time.
What happens if I want to run that window air conditioner?  It consumes 720 Watts per hour if the compressor is running 100% of the time.  If it is the only AC unit running in my home during a grid down situation, I’ll assume the compressor is running about 80% of the time.  This equates to 576 Whr.  Over a 24 hour period I will need 24 * 576 = 13,824 WHr.  Either I’m not going to run this window AC 24/7 or I need another operational mode because my solar panels are only going to make about 6,000 WHr/day.  Enter the small, reliable and quiet generator.

Mode 2: Generator Power – working with small generators
Let’s say I really want to run that window AC unit – and believe me, I really want to.  This is where the 250 gallon propane tank – professionally installed and plumbed – in my yard comes in.   (Or the other various small sized tanks I have stored outside as well – 20 to 40 gallon tanks that make my generator portable and don’t require me to store a lot of gasoline).   Always store and use propane tanks outside in a well ventilated area. 
My Mastervolt MassCombi inverter [10] is actually an inverter/charger/transfer switch all-in-one unit.  The inverter is intended for marine applications where shore power can be iffy.  It can be set to current limit its AC input to match the shore power (generator) output of roughly 15 amps or any other low capacity AC source.  If the appliances connected to the inverter are consuming less than 15Aac, then the balance of the AC power is converted to dc and used to charge the batteries but here comes the best part.  If an AC motor attempts to start and more surge current capacity is required, the inverter will automatically pull the extra surge current from the battery bank and add it to the power coming from the generator – pretty cool.

During the peak daylight hours, the solar panels will produce enough power to run the window air conditioner, the refrigerators, and a number of other small appliances.  When the sun goes down, I can switch into generator mode and continue to run the window air conditioner, if my fuel situation permits.  This situation lasts for about three months every year when it is so hot and humid that air conditioning feels like a necessity – although a grid down scenario will redefine “necessity” for all of us.

I don’t run the solar charge controller and the inverter/AC-charger at the same time so as to not cause a conflict between the two chargers.  When the sun is out and shining, I run the solar charge controller.  If I need additional power, I run the generator at night and shut off the charge controller.

I could add more batteries and more solar panels and essentially eliminate the need for the overflow generator but to produce 13,824 WHr of electricity per day (just for that window ac unit) and to have some margin for rainy days, I would need about twenty 230W panels and twelve 12V 200 AHr [11] batteries.  The panels cost about $650 apiece and the batteries are about $500.  This doesn’t include additional infrastructure like a bigger battery box, additional charge controller, wiring, fuses, mounting hardware, etc.  The cost works out to an additional $10,000 – more than I want to spend to run a $149 window air conditioner.  And not to mention, I don’t have a good place to put twenty solar panels as I don’t want them visible from the street and the front of my home faces south.

By the way, that little 2000W generator of mine makes up to 48,000 WHr of power each 24 hour period that it runs which is another reason that if you have a small battery bank and solar already, it doesn’t take much of a generator to back it up.  A little Yamaha or Honda 1,000 Watt [12] inverting generator sips fuel (runs 3.8 hours on 0.6 gal of fuel), is very quiet (53-59 dBA [13]), and with a continuous power rating of 900W will produce up to 21,600 WHr of power in a 24 hour period – all for less than $1,000 plus fuel.  Have that generator converted to run on propane by a reputable company and add some solar panels, batteries, and an inverter and you have a small system that can run a lot of stuff for a long time.  Stash a few of those 20 lb barbeque propane cylinders outside to run your little generator and you are now in better shape than probably everyone else in your neighborhood when the lights go out.

Remember, I spent $650 * 8 = $5,200 on solar panels and I only make roughly 6,000 WHr with them on a sunny day.  By the time I add in a battery bank, fuses, inverter, copper wiring, etc., I figure I’m paying about $2 for every watt hour of solar generation and storage capacity.  Of course in a grid down situation, I might make a little more power as I would have more incentive to adjust the tilt angle of the panels monthly to track the sun through the sky.  I might also cut down that pesky tree that is partially shading my panels in the morning.  So in the end, solar is expensive and makes a fraction of the power that a generator can for the same dollar investment – but solar will do it quietly and almost forever – even when the fuel supplies run out.

Mode 3: Utility Mode – Creating an Uninterruptible Power Supply (UPS)
As I mentioned in the last section, my inverter is also an AC to DC charger and transfer switch all in one.  By that, I mean when incoming AC power is detected – and that can be from a generator or your main utility – the inverter runs in charging mode.  This means that it supplies the connected loads with the incoming AC power as a simple pass-thru and converts any remaining AC power to DC to charge the battery bank if the batteries are not already fully charged.  If the AC load of the appliances increases, the battery charging current is automatically decreased.

When my MassCombi detects that AC power has gone away, it automatically switches from AC charger mode to inverter mode in a fraction of a second and starts using DC power from the battery bank to invert into AC power.  In this manner, the system acts like an uninterruptible power supply (UPS [14]) for the devices plugged into the system.  It also is a pure sine wave inverter which means it makes electricity which is just a clean as that coming from the utility.

Even if I didn’t have PV charging capability, this system would buffer the effect of rolling blackouts.  When grid power was present, the system would charge the batteries.  When grid power was absent, the batteries would supply the connected equipment.  As long as the power was on more than it was off and my battery capacity was sufficient for the appliances I am trying to run, this should work.  As the hours of “grid down” increase, the demands on the batteries will increase until the point is reached where some type of supplemental power is required – either a generator or solar or both.

Mode 4:  Bypass
When I wired my system I installed new dedicated electrical outlets to various rooms in my home to deliver the electrical power from this new system.  The lamp in my living room is plugged into one of these new outlets.  When the grid goes down, my lamp stays on.

However, if I am doing maintenance and want to keep the connected appliances running, I can turn off all the solar breakers, shut off the inverter/charger, disconnect the batteries and still route grid power through my system to the new electrical outlets.  This is a handy but non-essential feature.