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Installing And Using a Power Inverter in a Prius

Updated on October 30, 2016
A power inverter installed in the hatch of a Toyota Prius.
A power inverter installed in the hatch of a Toyota Prius. | Source

Inverters for Power in a Blackout

Losing power is always a drag, especially if you don't have a generator or other source of backup power to keep the lights on and the fridge running and food from spoiling. Generators can be noisy, smelly, and a pain in the rear to maintain for those few occasions when the power goes out. Now a car is basically a generator on wheels, with a steady supply of fuel; it can power an inverter feeding off the 12 volts DC (direct current) generated by the alternator. The Prius is just a different kind of generator on wheels.

I'm not arguing here in favor of buying a Prius or any other hybrid solely for this purpose, or saying that using non-hybrid vehicles for backup power is a poorer choice, but just to show how to install and operate an inverter in a Toyota Prius and explain how this affects the vehicle and how it operates.

Examples of Devices That Can Be Run on Power Inverters

Appliance
Typical Wattage
Incandescent Light Bulb
60-75
CFL Light Bulb
13-21
LED Light Bulb
7-13
Refrigerator
150-300
Freezer
400-600
Cell Phone Charger
4-6
Box Fan
60-90
Window A/C Unit (5000 BTU)
500-700
Personal Space Heater
200-750

Powering Devices With an Inverter in a Prius

Preparing to test the inverter powering a small (200 watt) personal space heater.
Preparing to test the inverter powering a small (200 watt) personal space heater. | Source

What Kind Of Inverter To Get?

When picking an inverter, there are many options to pick from.

  • Some inverters can be connected directly to the cigarette lighter or 12-volt power outlet in a vehicle. Inverters that connect this way are generally limited to supplying 100 to 150 watts of power.
  • Other inverters that connect directly to the 12-volt battery in a vehicle; usually these will come with power cables that clamp onto the positive and negative terminal posts on the battery. These are capable of supplying more power: 400 to 750 watts, give or take. The limiting factor with these inverters is the gauge or thickness of the wires connecting them to the battery. The thinner the wire, the less power the inverter can actually draw from the battery before the voltage sags or dips below the minimum required for it to operate.
  • Larger inverters can supply power in the range of 1000 or more watts. As a cost-saving measure, these inverters are generally without the positive and negative cables as a cost saving measure, so the customer has to purchase them separately or fashion a set themselves.

Besides differing in power capacity, inverters differ in the shape of the electric wave they produce.

  • Pure sine wave inverters, which generally cost more, produce power in a smooth wave form like what is produced by the electrical utilities, and are recommended for sensitive or delicate electronic devices.
  • Square wave inverters, which generally cost lost, produce power in a wave form that looks a bit choppier than a perfectly smooth wave, but tries to match it closely. In some devices this results in a buzzing or humming noise.
  • Modified square wave inverters (also called modified sine wave inverters) are ok for powering lights, most devices that use motors (like refrigerators and freezers), and devices that use power adapters (like laptops and cell phones). For emergency uses, the modified square wave inverter would be fine for most uses, unless you absolutely need power just like from the utility grid.

1200 Watt Modified Square Wave Inverter

A 1200 watt continuous inverter sold by Harbor Freight.
A 1200 watt continuous inverter sold by Harbor Freight. | Source

The inverter I chose for my own use is a modified square wave inverter carried by Harbor Freight and sold under the brand name of CEN-TECH, item number 69659 (unfortunately discontinued it seems), which I snapped up for $67.49 with a 25% off coupon, originally $89.99. The unit is rated to crank out up to 1200 watts continuously. This is more than enough to run a refrigerator and some lights at night as well as make coffee and toast for breakfast and a small 5,000 BTU window a/c unit and some fans. It's not enough to run an entire house, but it is much better than having no power at all and can come in handy any time of the year.

The inverter I picked out didn't come with any positive or negative power cables, so I got a pair of 4-gauge starter cables from Advance Auto Parts. I got a slightly longer positive cable than the negative one since the positive cable needed to be a little longer to reach from where I was installing it in the hatch area to the positive terminal on the 12-volt battery. The negative cable could be shorter since I decided to attach it to the body itself where the negative cable from the battery connects for the ground point.

Before doing anything, make sure your Prius is turned off. Wrap one end of the positive power cable in duct tape or some other material to prevent it from making direct contact with the negative power cable, the body of the vehicle, or any nuts or bolts attached to the body.

The procedure as I describe it involves electrically disconnecting the battery from the vehicle, which will cause radio presets to be lost and will require the automatic window rollup/rolldown feature to be reset.

I am not liable for any mistakes, accidents or damage that result from following this procedure. PROCEED AT YOUR OWN RISK!

Installing an Inverter in a Prius

First, the rear mat, cover and cargo tray had to be removed along with the side cover that goes over the battery area. I also removed the right rear taillight bulb access cover as I would be routing the power cables for the inverter through there into the hatch area.

Remove the red plastic cover over the positive terminal.
Remove the red plastic cover over the positive terminal. | Source

On the battery itself, I removed the red plastic terminal cover by slipping it off the clasps or hooks that hold it on and sliding it out of the way. Depending on the dexterity of your fingers you may also have to remove the brake-control power assembly, which is the black box bolted down beside the battery.

Remove the bolt beside the positive terminal clamp, attach the positive power cable and tighten the bolt on top of it.
Remove the bolt beside the positive terminal clamp, attach the positive power cable and tighten the bolt on top of it. | Source

I connected the negative terminal to the ground connection on the vehicle where the battery's negative terminal cable was connected. I did this by unbolting the negative battery cable from the vehicle body, mounting the inverter's negative power cable directly against the body of the vehicle, and then bolting the battery's negative power cable on top of the one for the inverter. When you do this, you may see sparks, but don't be alarmed, as the voltage we are dealing with between this battery and the vehicle and inverter is only in the 12-14 volt range and won't cause you any harm. Just be sure to NOT short the positive and negative power cables.

Connect the negative power cable where the negative from the battery connects to the body of the vehicle.
Connect the negative power cable where the negative from the battery connects to the body of the vehicle. | Source

Once the red plastic cover was off, I removed the bolt that is beside the positive terminal clamp (not the one that actually holds the clamp to the terminal), bolted down the end of the positive cable there, and then fed the cable through the right rear taillight access opening.

Route both the positive and negative power cables through the right rear taillight access hole.
Route both the positive and negative power cables through the right rear taillight access hole. | Source

After I fed the negative power cable through the right rear taillight access opening, I was able to fit the cover back into place over top of the cables, albeit with a bit of a gap around them.

Put the rear taillight access cover back on.
Put the rear taillight access cover back on. | Source

I then attached the end of the negative cable to the negative power post on the back of the inverter and screwed the terminal cap on top of it to hold it in place. The photos here show that I wrapped the end of the positive power cable inside an old t-shirt since there was no duct tape handy when I did this. I recommend using tape, since it isn't as likely to come off and risk a short, but the shirt worked in a pinch.

Keep the positive power cable from making contact with the negative power cable or any metal contacts within the vehicle to avoid shorting the battery.
Keep the positive power cable from making contact with the negative power cable or any metal contacts within the vehicle to avoid shorting the battery. | Source

I then uncovered the positive cable end, attached it to the positive power post on the rear of the inverter, and secured it tightly with the terminal cap screwed in place. Before starting the Prius, I flipped the power button on the front of the inverter and made sure the green power light lit up.

Securely attach the power cables to their appropriate post on the rear of the inverter.
Securely attach the power cables to their appropriate post on the rear of the inverter. | Source

Voila, flip the switch and let 'er rip!

Just don't leave the inverter powered on while not in use, or when the vehicle is turned off, to avoid draining your 12-volt battery.

Testing the Power Inverter

I tested the newly installed power inverter by running a few devices—the refrigerator, a coffee maker, toaster and small 200 watt personal space heater—individually to avoid overloading the inverter. To do this, I turned the Prius on and made it READY, so that the car could start and run the engine as needed to keep the high voltage battery sufficiently charged. Using the Torque app on my Android smartphone and a bluetooth OBDII adapter connected to the diagnostic port, I was able to get a baseline reading for how many amps were being pulled from the high-voltage (200+ volts) battery to run the car, and by extension any 12-volt devices connected, like the inverter.

Baseline Current With Inverter Turned Off

I found that with the vehicle at rest, with the radio, interior lights, and display screen turned off, the current coming out of the high voltage battery fluctuated around 1.16 amps. This is enough current to keep the car on with the engine off.

Inverter powered off.
Inverter powered off. | Source
1.16 amps is my baseline reading for how much current is being pulled from the high voltage battery to power the car while the engine is stopped.
1.16 amps is my baseline reading for how much current is being pulled from the high voltage battery to power the car while the engine is stopped. | Source

Baseline When Inverter Turned on

When I flipped the power button on the front of the inverter, the current number jumped to about 1.21 amps, meaning the inverter itself draws some power when it is on but not powering anything.

Note that the power switch for the heater is turned off, but the inverter's power switch is turned on, as evidenced by the green power light.
Note that the power switch for the heater is turned off, but the inverter's power switch is turned on, as evidenced by the green power light. | Source
1.21 amps being pulled from the high voltage battery.  This figure, a little bit higher than my previous resting rate of current, shows the inverter does have some standby losses.
1.21 amps being pulled from the high voltage battery. This figure, a little bit higher than my previous resting rate of current, shows the inverter does have some standby losses. | Source

Testing the Current Load From a Space Heater

I took a 200-watt personal space heater to use as a test load on the inverter. I plugged it in, turned it on and let it warm up for a few seconds. I took a reading of approximately 2.43 amps coming out of the high voltage battery. The Prius, like most hybrids, doesn't have an alternator like non-hybrid cars do. Its 12-volt power comes from a DC-to-DC converter that steps down power from the high voltage DC battery to roughly 14 volts, to keep the 12 volt battery charged and power any 12 volt electronics.

Heater and inverter both turned on and allowed to stabilize for 1 minute.  It's hard to see in this photo, but the inverter has a "power level" indicator, which has the bottom-most LED lit up with the heater on. Each LED appears to equal 120 watts.
Heater and inverter both turned on and allowed to stabilize for 1 minute. It's hard to see in this photo, but the inverter has a "power level" indicator, which has the bottom-most LED lit up with the heater on. Each LED appears to equal 120 watts. | Source
Drawing 2.43 amps one minute after turning on the heater.
Drawing 2.43 amps one minute after turning on the heater. | Source

How Does The 12 Volt Battery Stay Charged?

If you use your Prius to supply electricity, you keep the high-voltage battery charged by occasionally starting the internal combustion engine to generate electricity by using the electric motor. This way, the Prius never drains its battery too low and can run for a long period of time on a full tank of gas. How long depends on how much energy is being demanded from the inverter.

You might wonder how it's possible to use an inverter on a hybrid without killing the 12 volt battery. As long as the vehicle is "started," or turned on so to speak, electricity is taken from the high voltage battery pack, and passed through a DC-DC converter. The DC-DC converter simply takes electricity from one source of direct current (DC) and modifies or converts it, stepping down the high voltage DC from the larger battery into low voltage DC to recharge the 12-volt battery and run any devices connected to the car that uses 12 volts DC for operation.

I haven't directly tested how much energy, in terms of amps or watts, the DC-DC converter can crank out at 14 volts DC, but a quick look at that red cover over the positive battery terminal will show a 120-amp rating. Now that doesn't mean you can or should be pulling 120 amps or 1440 watts constantly. Others who have installed an inverter in a Toyota Prius say they imposed a limit on themselves of no more than 80-100 amps, to avoid causing any damage to the inverter or the car's electronics.

One last thing that should be said: the amount of current or amps taken out of the high voltage battery may seem very low compared how much power is actually being used by the inverter. That's not a trick or false reading. Since the high voltage battery has a much higher voltage (200+ volts) than the 12-volt battery, it takes fewer amps at a higher voltage to equal the same amount of energy.

For example, let's say I need 200 watts, which I can get that from either a 200-volt source or a 12-volt source. Actual voltages can fluctuate but we'll stick with these two for the sake of simplicity. Watts are determined by voltage times amps.

  • At 200 volts, that 200 watts requires 1 amp of current (200 watts divided by 200 volts = 1 amp).
  • At 12 volts, that 200 watts requires 16.67 amps of current (200 watts divided by 12 volts = 16.67 amps). Therefore, we aren't draining any more power from the vehicle than it can provide at that level since what the DC-DC converter does is reduce voltage and increase current to compensate.

What To Do After Installing an Inverter

After completing the installation for the inverter and testing it, I turned it off. I put the battery cover panel, cargo tray, deck cover and mat back in. I also reinitialized the driver window automatic up/down feature by rolling the window all the way down and all the way back up, holding the button down and up manually to do so. If you use the radio, you'll have to go in and reprogram your stations. You will also temporarily lose the fuel gauge level reading until driving around for a bit for it to re-register. Any miles accumulated or your current fuel economy average will also be lost. All of this is a result of disconnecting the battery from the car temporarily, which resets any settings or temporary information.

No matter what inverter you choose, if it has a manual on/off switch, always make sure to turn it off when you are done testing or using it to avoid draining your 12 volt battery, which would prevent you from starting your vehicle. Also, any time that you are using an inverter, always make sure that your Prius or whatever vehicle you are using it in is started to avoid running the battery too low when actively using it to power devices or appliances. And, lastly, with any vehicle, prevent carbon monoxide poisoning by making sure that you don't allow it to run in an enclosed space like an attached garage. Park with the exhaust pipe facing away from your home.

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