Why a car is a good backup power source
A typical car battery holds roughly 500 to 700 watt-hours — comparable to a mid-size power station. That on its own is unremarkable.
What makes it interesting is the alternator. Start the engine and you have a generator producing something like 1,000 to 2,000 watts of charging capacity, refilling the battery continuously. A car is not a battery; it is a battery with a generator bolted to it, and you already own one.
For a household with no backup equipment at all, an inverter is the cheapest meaningful step available — an order of magnitude less than a power station and two orders less than a standby generator.
The two limits that decide everything
1. The cigarette socket is fused at 10 to 15 amps. At 12V that is 120 to 180 watts of real throughput, minus conversion losses. Every inverter above about 150W that plugs into a socket is selling you a rating the socket cannot deliver.
2. Above that, it wires to the battery. Heavy cable, short runs, an inline fuse close to the positive terminal. A 1,000W inverter draws over 80 amps at 12V, which is more current than most household circuits carry.
| Inverter size | Connection | What it runs |
|---|
| 150-300 W | Cigarette socket | Laptop, CPAP, chargers, lighting |
|---|
| 500 W | Battery terminals, or a heavy socket | Above plus a small TV, a fan |
|---|
| 1,000 W | Battery terminals with an inline fuse | A refrigerator, a sump pump, corded tools |
|---|
| 2,000 W and up | Terminals, heavy cable, often a second battery | Several appliances — and a real installation job |
|---|
Pure sine or modified sine
The same distinction that decides a UPS purchase, for the same reason.
A modified sine wave is a stepped approximation, and it is fine for anything that just makes heat or spins a simple motor. It is not reliable for switch-mode power supplies, which is nearly everything electronic you would plug into a car.
The specific things to be careful about: CPAP machines, laptop chargers, anything with a microprocessor, and induction motors, which run hotter on a stepped waveform. The battery backup page covers the underlying waveform problem in detail.
Pure sine costs meaningfully more and removes the entire question. On a device you will use rarely and under stress, that is worth paying for.
How long the car will last
Two very different answers, depending on whether the engine is running.
| Mode | Available energy | Limit |
|---|
| Engine off | Roughly 200-300 Wh usable before starting is at risk | A starting battery is not a deep-cycle battery |
|---|
| Engine idling | Effectively unlimited, at roughly 0.3-0.5 gal/h | Fuel in the tank, and carbon monoxide |
|---|
Usable energy from a starting battery is deliberately conservative: discharging a lead-acid starting battery deeply damages it and leaves you unable to start. Idle fuel consumption varies widely by engine.
The practical pattern is a fridge on the inverter with the engine run for twenty or thirty minutes every few hours, rather than continuously. That keeps the battery topped and the fuel bill sane.
The safety rules, which are not negotiable
- Never run the engine in a garage. Not with the door open. A car idling in an attached garage is the same carbon monoxide hazard as a generator, and it is how people die during outages — the CO safety page applies exactly.
- Fuse the positive lead close to the battery. An unfused cable from a car battery can deliver hundreds of amps into a short, and the failure mode is a fire.
- Do not run the cable through a closed window or door. Crushed insulation on a high-current DC cable is the other fire.
- Watch the input voltage. Below about 11.8 volts at rest you are into territory where the engine may not start.
- Modern cars may cut the socket. Many vehicles power down accessory sockets when the ignition is off, which is a feature rather than a fault.
Where this fits against the alternatives
An inverter is the cheapest backup power in existence and it has real limits: the exhaust means it lives outside, the noise is an idling engine, and running your only vehicle's battery down during an emergency is a genuine risk.
A power station does the same job silently and indoors for more money — the 500Wh class is the directly comparable size. A generator does it with more output and more noise.
The case where an inverter clearly wins is the household that owns nothing, needs something before the next storm, and has a car in the driveway. The beginners page covers where to go from there.
The short version
- 150W is the real cigarette socket ceiling. Anything bigger wires to the terminals.
- Pure sine wave for a CPAP, a laptop or anything with a chip in it.
- Engine off gives you a few hundred watt-hours. Idling gives you as much as the tank holds.
- Never idle in a garage, and always fuse the positive lead.
