Three Letters Re: Covert Home Power for When The Grids Go Down

I am not an electrician, and I set as system like this up at my house three years ago. It kept our food cold, and the house tolerably warm for three days when the power was out. But I endorse a couple of important differences.

First, letting your car run at idle will run down your battery. The alternator doesn’t reach full output at low RPMs, so you need to kick up the throttle a little bit. How much will depend on your car. I watch the volt-meter built into the inverter, and set the engine at about 1,500 RPM, because that’s where the voltage stays high enough for the inverter. Also, make sure to check the output of the alternator. (I looked it up at an online auto parts store.) When my inverter runs at full power, it draws 100 amps at 12 volts. If you have a small alternator (smaller car), then 1,500 RPM may not even be enough to power the inverter. That means you’re drawing amperage out of the battery when the inverter runs at higher power.

Second, I would never power the house by using an extension cord with two male ends. JWR was right to point out the danger of potentially back-powering the grid when plugging in a hot extension cord from the inverter. Even with the main breaker turned off, the neutral is still connected to the grid. Can you imagine the liability you would incur if you accidentally electrocuted a local repairman who was trying to get your neighborhood back online? He may even be one of your neighbors. I’ve made mistakes in my life (no one seriously injured because of them) and I can’t justify the risk of injury when it’s so easy to avoid. Because the average 1,200-1,500 watt inverter will only power one or two major appliances (or one furnace blower), I recommend that you plug those items directly into the unmodified extension cord from the inverter. Yeah, you gotta move the extension cord around a bit, but you won’t hurt anyone and you don’t need to worry that you might ruin your inverter when the grid does come back up.

Remember, the main objective is to survive the ordeal, hopefully stay warm, and not accidentally hurt anyone else in the process. – Dave in Missouri


Dear James,
I’m sure Tom H. meant well with his article on using power from a car to power a house, but I have some serious nits to pick.

1. When talks about getting the “largest cable size you can get”, it really opens a door for disaster. Electrical cable needs to be sized for the current it will be carrying. If the cable you find is too small, you risk creating an electrical fire. If it’s too big, you’ve wasted money. What you need to do is to have a handle on the power and current that the cable needs to carry, and size it appropriately.

Remember, Power (watts) = Current (amps) x Voltage (volts).

Add up your power requirements for a given voltage, and then determine your amp load.

Current (amps) = Power (watts) / Voltage (volts).

Here is a link that gives conservative estimates for the current carrying capabilities of various American Wire Gauge (AWG) wire sizes.

2. When adding up the wattage you need to support, don’t go by the tag value on appliance alone. These are maximum values possible, and probably do not represent the power used under normal circumstances. This is a case where there is no substitute for actually measuring the power draw. The good news is that it’s easy to do with a Kill-A-Watt (plug-in power meter). Before TSHTF, test each of your appliances by plugging the Kill A Watt electricity usage monitor into the outlet, and then the appliance into the Kill A Watt. You can see the power being used at that moment, and the power consumed over time. For good measurements, use a Kill A Watt or similar meters.

3. Assuming the proper breakers are thrown, and the proper wires sizes are used, the weak link in this system is not the inverter but the size of the alternator output. The system cannot sustain more continuous power output than the alternator can provide. The battery may act as a ‘surge protector’ by supply the difference when the demand is larger than the alternator can supply, but that will quickly drain the battery.

It would be wise to find out ahead of time how big your alternator is and plan accordingly. The good news is that there are after market alternators available for some trucks and 4x4s which are designed to output more power to run tools through inverters at remote work sites. Best Regards, – Bear in the Sierra

Mr. Rawles,
There are so many problems with the vehicle/inverter backup system presented in today’s article that I felt I had to comment. You are to be commended for pointing out the foolishness of the power cord backfeed method, that can get someone killed in very short order. But there are other reliability/safety concerns the author does not address. These include vehicle alternator issues (overheating from insufficient air flow, diode current ratings, etc.), SLI battery limitations, connection issues, ground bonding, and transient inverter loading concerns.

For the record, I am a degreed electrical engineer and certified reliability engineer, and have over twenty years of experience with power inverters ranging from a few hundred watts up to 300KW (that’s right, 300,000 watts). Best Regards, – John in Colorado