Post #1020: PriUPS

Posted on February 20, 2021

This post is prompted by a recent article on Texans using their hybrid vehicles as electrical generators.  This being Texas, of course the vehicle in question is a pickup truck, in particular, the Ford F150 hybrid pickup.

And so, in 2021, Texan F150 hybrid owners are finding out what Prius owners have known since at least 2005:  A hybrid car makes an excellent backup generator.  In this post, I’ll lay out the simplest approach to using your Prius (or similar full hybrid) as an emergency generator.

Definition first:  An inverter is a device for converting DC power (such as a 12V car battery) to AC power (such as what comes out of the sockets in your house).  So this is all about adding an inverter to your car.

In a nutshell, for Prius owners:  Buy an inverter (1 KW or less, pure sine wave preferred), clip it to your 12 volt battery, and run an extension cord out the back of your trunk and into your house.   Leave the car turned on, outside.  The Prius will run the gas engine periodically to keep the battery charged.  And you can (e.g.) keep the fridge running and the lights on.

This post gives little history, then describes what I use with my wife’s Prius, and explains why I use that.  (Although, in fact, you can get this guidance from any number of internet sources, so you don’t have to read it here.)

A little history and the PriUPS.

With the hybrid F150, Ford now builds hybrid generator capability into the vehicle.  It’s all user-friendly plug-and-play technology, with outputs up to about 7KW.  (This idea isn’t new, but the mass-marketing of it is.  There was a limited-edition Chevy Silverado hybrid with built-in backup-generator capacity.)

But the granddaddy of hybrid-as-generator concept has to be the 2005 creation of  the PriUPS.  That’s a Prius wired up to be an uninterruptible power supply (UPS). I still get a chuckle out of that website and discussion.

The PriUPS was the opposite of plug-n-play.  That was a pioneering system set up by a guy who tapped into the high-voltage side of the Prius.  This is, to say the least, somewhat dangerous, and voids your warranty to boot.  But by doing that, he was able to access the roughly 20KW of peak generating capacity built into the Prius.  And he ended up with a system that can run his entire house, on demand.  In essence, he converted his Prius to a 20KW peak on-demand emergency generator.

These days, you can buy an aftermarket kit to does that something like that, for most Toyota/Lexus hybrid vehicles.  These are plug-n-play systems that safely tap into the high-voltage side of your hybrid, and produce 3KW to 5KW of AC power.  That’s just modestly less than the highest option available for the Ford F150 hybrid.  Near as I can tell, the sole drawbacks to these aftermarket kits are the price ($2K and up) and the need to have enough moxy to go anywhere near the high-voltage side of your car.  Even if it’s just to insert a plug in a socket.


Why buy an inverter for your hybrid?

Each of the solutions above is great in its own way, but what I suggest here is far simpler:  Clamp an off-the-shelf inverter to your 12V battery.  Turn the car on and leave it.  You can’t draw as much power that way, but it doesn’t cost much and it takes almost no effort.

I’m sure this would work with any full hybrid, but you’d have to look up some discussion or specs to see how much power you could safely draw.  Why am I so sure?  Because as far as the car is concerned, there’s no difference between drawing current to run the lights and stereo in the car, and drawing current to run the lights and stereo in your house.  All electrons are created equal.  Just as long as you don’t draw enough current to blow a fuse and/or otherwise overtax your car’s systems.

That said, I see virtually no examples of people actually doing this, other than for the Prius.  I’m sure there’s nothing unique about the Prius.  It’s just that there are so many of them, it’s more likely that some are in the hands of technically savvy individuals.

The consensus of informed opinion is that if this works with a Prius, it will work with any Toyota hybrids using the Hybrid Synergy Drive system.  Beyond, that, the data are sparse.  Here’s a Chevy Volt example.  Otherwise, most of what I’ve come across is a lot of tut-tutting, without any factual basis for it.  E.g., here’s a guy who actually did this with a Honda MDX, and all he got on a car-specific forum was a load of grief for doing it.

You can do a little thought experiment to get some idea of why modest current draws via an inverter are safe.  Imagine sitting in the car running every 12V device in the car.  Turn it all on:  Lights, brake lights, defrosters, heater, wipers, stereo.  In round numbers, that has to be around 500 watts of power.  (I don’t include the AC because on the Prius, at least, that’s hard-wired to the high-voltage side of your system, not the 12V side).  Your hybrid is designed to be able to do that indefinitely, with power to spare.  Now turn off all that stuff, hook up an inverter, and draw 500 watts of power off the inverter.  Your car won’t know the difference.

In a nutshell, the advantages of this are that you get small, cheap, reliable, zero-maintenance emergency power.  There are plenty of ways you can have some emergency electrical generation capacity on-hand.  But for my money — and after trying several approaches — if you have a Toyota hybrid, this is the best solution.

I wouldn’t do this routinely.  It is, after all, kind of nuts to use a $30K automobile in place of a $400 stand-alone generator.  But for the once-in-a-decade extended power outage, I think this is a good solution.

You can’t tap off enough electricity to run your entire house using this approach.   But you can keep the fridge cold and the lights on until the power is restored.  All for about $200, and a little shelf space in your garage. 

For me, the real draw is zero maintenance.  I bought a 1000 watt pure-sine-wave inverter in 2005.  It looks and works just like new.  Probably 99.99 percent of the time I’ve owned it, it has rested on a shelf in the garage.  When I need it, it takes about five minutes to set up.  And as long as our ancient Prius is still running, we’ve got some emergency power.

Compare that to taking your disused 10-year-old gas-powered generator off the shelf and getting it going.  In the dark and the cold.  Maybe it’s just my prejudice, but for me, it’s vastly easier to hook up what amounts to a set of jumper cables, than to coax some long-out-of-service small gasoline engine back to life.

So, I used to own a gas-powered portable generator.  I gave it away once I figured out I could just hook an inverter up to a Prius.

Let me just walk through the details of what I do, and why.

Why 1000 watts?  That’s about the limit of what you want to draw from the low-voltage side of the Prius.  There is a key fuse that protects the 12V portions of the Prius, and that will blow at about 100 amps (or 12 x 100 = 1200 watts).  You want to keep your draw well below that, because if that fuse blows, the car won’t run.

Why pure sine wave?   A pure-sine-wave inverter is one that accurately reproduces the AC voltage that you have in your house.  The voltage varies smoothly.  Cheaper inverters will produce 120 volts AC, but with much more abrupt changes in voltage. Inverters of that sort will either not specify the output waveform, or describe themselves as “modified sine wave”.  You don’t want that.

Inverter manufacturers may sometimes refer to “total harmonic distortion”, instead of saying “pure-sine-wave”.  My crude understanding is that, if that number is small, that’ll work fine.  So you want to see THD of no more than a few percent.  Or a description of “pure sine wave”.

A lot of electronic devices won’t work on anything but a pure sine-wave electricity.  And a lot of other devices, such as a motors, will perform poorly if not fed pure sine-wave AC or something close to it.

This is not just a bit of theory.  At the time I bought this inverter, I had a key piece of equipment that I wanted to use that absolutely refused to run on anything other than pure sine wave AC.  It wasn’t something exotic — it was a vented kerosene heater.  But the electronics for it required pure sine-wave AC.

If you absolutely, positively want your equipment to run, and run well, get a pure-sine-wave inverter.   (But if you’re just going to run (e.g.) a 600-watt space heater, it doesn’t matter what you use.)  If you see an inverter with a price that seems too good to be true, it’s almost a certainty that it’s not a pure-sine-wave inverter.  If you see “modified sine-wave”, then ditto.

How do you hook it up?  With things that look like jumper cables, more or less.  But you have to know that the 12V battery for the Prius is located in the back, on the passenger side.  As long as you can remember that, and know that red = positive, you won’t have a problem.

How do you use it?  Put the car outside.  Clamp the inverter to the 12V battery terminals.  Run a heavy-duty extension cord out the back of the Prius and plug it into the inverter.  (The hatch will close over it.)  Start the car.  Leave it running.  Use the physical key to lock the car.  And that’s it.  You can find detailed instructions for this all over the internet.

And this is a caveat:  Many (most?) inverters are sold without those jumper-cable type clamps.  In most cases, you’ll need to buy those separately.  So check on that, and figure the cost of those in if you go this route.  You need short, thick (3 or 4 American Wire Gauge) cables with clamp ends, like these.  So plan on another $30 for the cables if they are not included.

You can attach these with cables that have ring ends on both ends of the cable, if your battery terminals are set up to accept those.  But in my experience, if you end up using this, it’ll be cold, and dark, and it’s just a lot easier to have jumper-cable-type clamps, rather than have to fiddle with wingnuts and such to get these attached.  YMMV.

Will it run the fridge?  Yep, runs mine.  In fact, I ran both my fridge and a separate freezer, although that’s a bit of a crap shoot.  Both devices have high starting current, and if they both decided to switch on at exactly the same time (within about one second of each other), you’ll exceed 1000W.

Mostly what I need this for is to run the fan on my wood stove insert.  I burn about two cords of firewood each year (mostly to balance out heat injection and withdrawal of my ground-source heat pump).  It’s doesn’t completely heat the house, but it takes the edge off.  So I’m a lot less concerned about heating the house than most people would be.

This is nowhere near enough power to run the main heating or cooling systems in my home.  Just the air handlers alone would draw about 1500W if they were both running at once.  (Two units, each with a 1 HP motor.)  Let alone the heat pump compressors.  Arguably, it might be enough to run a smaller home heating system, particularly a hot-water baseboard system, if you could isolate that electrically from the rest of your house.

I never connect this to the house wiring.  Not that you can’t, if you know what you’re doing.  But I don’t.  Not even if I throw all the breakers (which then, in theory, disconnects each house circuit from the others, and from the grid.)  There’s a reason that real whole-house backup power uses a transfer switch.  I just run extension cords where I need a little power.

One reason not to do this is that, for some types of inverters, you will instantly burn out the inverter if you connect it to standard three-wire (hot-neutral-ground) house wiring.  And you probably won’t know if your inverter is that type until you do it.  (The reason is that while your house current is one hot (120V) line, and one neutral (0 V) line, some inverters achieve the same 120V differential by having two 60V lines 180-degrees out-of-phase.  Which works just fine if you use it as a stand-alone unit.  But in your house system wiring, the neutral and ground are shorted together at some point.  Which means that if you plug in that type of inverter, it instantly shorts the second 60V line directly to ground.  And instantly fries  your inverter.  Just another reason not to plug these into your house wiring.)

Let me see if I can find anything on Amazon that looks like single-purchase plug-and-play for this use.  These look inexpensive to me.  In real (inflation-adjusted) terms, I paid about $450 for my inverter, back in 2005.  And that was likely the cheapest one I could get.  Now, there’s plenty of choice under $200.  In real (inflation-adjusted) terms, inverters cost well under half of what they cost fifteen years ago.

So I’m looking for sine-wave inverters that include jumper-cable-type cables as part of the package.  Under 1000 watts.  This isn’t an endorsement.  It just shows that you can easily get what appears to be an adequate inverter for this purpose for under $200.

600W, pure sine, clamp-type cables included, $90.

1000W, pure sine wave, ring-type cables included, $150.

1000W, pure sine wave, ring-type cables include, $150.

1000W, pure sine wave, ring-type cables included, $190.

Other portable backup power.  Maybe I’d go for a quiet gas generator if I had to do this all over again.

The idea of having a 120V power outlet in a car or truck is hardly new.

At the least, you’ve been able to buy a cigarette-lighter-plug inverter for decades now.  (Like this one, say).  Those are limited to about 300 watts.  That won’t do much for running your appliances, but is more than enough for a laptop computer and some lights.

Just as an aside, my kids didn’t believe me when I said that the power outlet in the car was a “cigarette lighter socket“.  And that all cars used to come with built-in cigarette lighters and ashtrays.  The lighters have been gone for more than two decades but the name lingers on.

Even the idea of some kind of built-in 120V AC plug in your vehicle is not new.  Here’s a 2016 list of all the vehicles that had a 120V outlet built in.  For those where I could find the wattage, they all appeared to be low-wattage outlets.  More-or-less, those are just a built-in version of cigarette-lighter-plug inverters, with the same limitations on the amount of power you can draw from them.

Above 300 watts, you pretty much need to get an inverter that will connect directly to the car battery.  That bypasses the relatively thin (low-capacity) wiring of the cigarette lighter socket or typical built-in 120V inverter.  One you get up to around 1000 watts continuous power, the connection to the car battery ends up looking like a set of jumper cables.  Because from a power-draw standpoint, that’s pretty much what it is.  Like these.

Then why isn’t your house wiring that thick?  It’s just Ohm’s Law.  Briefly, power flow = watts = volts x amps.  But heating losses in the wires are determined solely by the (square of) the amperage.  To deliver the same power with 12V and with 120V, the amperage in your car’s 12V system has to be 10X the amperage in your house wiring.  The #3 AWG cables above have a cross-sectional area that’s just about 10x that of standard 14-AWG U.S. house wiring.  Upshot:  You need much bigger wires at 12V than you do at 120V, to deliver the same power.  The same phenomenon explains why power transmission lines run at extremely high voltages.  That minimizes power (heating) losses and reduces the size of the wires needed to transmit the power.

To be clear, you could hook up a 1000W inverter to any car’s 12V battery.  But there are some drawbacks to doing that with a non-hybrid vehicle.  So let me run through why this “works” in a Prius but not your standard non-hybrid car, in the sense of “works easily and well”.

A typical lead-acid car battery holds about 1 kilowatt-hour of energy.  In theory, if you draw 1000 watts, you’ve got about an hour until the car battery is completely dead.  But in practice, a) the inverter would shut off well before then, and b) you can fully discharge a standard car battery maybe a dozen times before you kill it.  (And let’s not forget c), if you leave it on too long, your starting battery is dead and you can’t restart your car.)

And so, with a traditional vehicle, it’s a hassle.  You either have to leave the engine idling, or go out and start the car every-so-often to recharge the battery.  Neither one is a very good solution.  And if you are using a smaller vehicle, the 1000W draw will be close to or over the capacity of the car’s alternator.  It’s never a good idea to run equipment like that at capacity for an extended period of time.  So with a smaller vehicle, you’d have to scale this back accordingly.

Smarter users of traditional cars (and RVers and boaters) will step this up a notch, and work a “house battery” into the system.  One battery is used to start the car, and the engine alternator then charges a separate bank of “house batteries” that are used to run the inverter for an extended period of time.

A “house battery” system can extends the length of time that you can draw electricity without running the engine. But this comes at increased cost, complexity, and maintenance.  This is perfectly fine for a crude off-grid system, a boat, or an RV, but it’s a lot of investment to make to deal with a once-per-decade extended power outage.  In particular, you have to keep the house batteries trickle-charged between uses, and in my limited experience, they’ll die in a few years anyway.

Which, if you don’t use the system, you will probably only discover when the power goes out. I used to keep an extra car battery (with attached low-wattage inverter) on trickle charge, thinking that I could use it to run the blower motor of my wood fireplace insert in the event of a power outage.  I eventually gave up on that, because every time I really needed it, I’d find out that the battery had died some months to years before.  And I got tired of buying new car batteries, only to sacrifice them on the altar of my preparedness paranoia. 

If you’ve followed along this far, it should dawn on you that a Prius plus inverter is just a fully-integrated house-battery system.  The big traction battery of the Prius functions as a 2KWH house battery.  If you leave the car on, the engine runs occasionally to charge the big battery.  And you slowly draw that down, via the inverter clamped to the 12V battery.  In effect, the car provides you with a top-notch house-battery system for free.  All you need to do is add the inverter.

In short, with some effort, you can produce a practical AC power system that uses your standard non-hybrid car as the generation source.  But I think relatively few people do that.  And mostly, I think that boils down to money.  If you’re going to invest hundreds of dollars in some high-maintenance and potentially unreliable equipment that you rarely use,  you might as well get a little gas-powered generator. 

Portable gas-powered generators are vastly better now than they were twenty years ago. 

Back in the day, what you typically had for a small portable generator was a Briggs-and-Stratton engine directly connected to a generator unit.  There was a governor to keep the gas engine running at constant speed, because engine speed directly determined the 60-cycle-frequency of the output current.  The output of the electrical generator element was fed directly to the AC socket on the face of the generator.

Those worked, for sure.   I used to own one.  But a) they were easily as loud as your average lawn mower, b) they had a nice sine-wave output, but unstable frequency under heavy load (as it took time for the governor to adjust the engine speed), and c) they were inefficient, because the generator had to run at constant speed, all the time, regardless of load.

The noise issue was a particular problem for neighborhood use, because neighborhoods get very quiet when the power goes out.  So, while you basically can’t hear a Prius starting and stopping to serve an inverter-based system, you can most definitely hear an old-fashioned direct-drive generator run by a small gas engine.

Modern portable gas-powered generators — at least the “quiet” generators — take a different approach.  They are combination generator-inverters, and will typically be labeled that way.  The generator isn’t directly connected to the AC output.  Instead, the generator feeds an inverter, which then produces the 60-cycle-AC voltage that you need.  Among the advantages of this is that the engine doesn’t need to run at full speed all the time, because the 60-hertz output frequency is determined by the inverter, not by the speed of the crankshaft rotation.  And because pure-sine-wave is what everyone needs, I think you’d be hard-pressed to find one that didn’t generate pure-sine-wave output, or at least advertise a low total-harmonic-distortion percentage.

These days, for maybe $400 or less, you can get a seemingly-acceptable quiet 2KW portable generator-inverter.  Sure, at that price, it’s a Chinese copy of a name-brand setup.  But all things considered, at that price, I’m not sure I’d go for the Prius-plus-inverter approach if I had to do this today.


I guess it all boils down to how prepared you want to be.  I’m not the sort of person who requires a whole-house generator so that my life can continue unchanged in a power outage.  I expect a little hardship when the power goes out.  But not freezing is a pretty good idea.  Not having the food in the fridge spoil, ditto.  And being able to run (e.g.) internet boxes and computers during a power outage is a plus.

If you already own a Toyota hybrid, you can set yourself up for 1000W of zero-maintenance backup power for under $200.  On the scale of preparedness, laziness, and cheapness, that was my sweet spot.  That’s the setup I currently maintain.

You can almost certainly do this with other brands of full hybrids, but you’d have to do the research to determine how much current you can safely draw.

But technological improvements and cost reductions in small portable generators make them a viable alternative.  Really, the only problem I have with them is that they aren’t made for long-term inactive storage.  That, and and many bad memories of small gas engines that would not start.

Either way, ongoing reductions in the cost of inverters and inverter-generators have made these, well, downright cheap.  Take a look in your fridge/freezer some time, and just roughly total up the value of the food that you’d lose in an extended power outage.  For me, at least, one fridge full of food would more-or-less pay for the generator setup.  At that point, having your own backup power on hand might be both prudent and cost-effective.