Thanks to Mother Nature and power grid glitches, American households in Connecticut, San Diego, and many parts in between have been without power for extended periods of time in recent weeks. Now, in the wake of cell phones that can’t be recharged, night after night lit only by flashlight, and obscene amounts of spoiled food, plenty of people are asking: Is it time to buy a backup generator? And how does one figure out which generator to buy?
The purchase is fairly complicated, with questions about appliance wattages and a laundry list of jargony terms like “half load” for the generators themselves. Generators are also potentially dangerous. With the most common type of generator sold (portable), there’s a risk of carbon monoxide poisoning. These units typically run on gasoline, so the general rule of thumb is to place it about 10 feet away from the home. You’ll also need a safe place to store gasoline—because in a blackout, gas stations may not be accessible or operational.
For solid overviews and advice on generators, check out trusty buying guides from Consumer Reports and Popular Mechanics. Big-box home improvement centers Home Depot and Lowes and manufacturers such as Briggs & Stratton also offer primers on which generator is a good match for your needs.
One of the first steps is estimating how much power you’ll want during a blackout. Consumer Reports and a number of the sources linked above list approximate wattages used by various home appliances. A fridge is 600 or 700 watts, while a standard electric light is probably under 100 watts, and a well pump may be 1,000 watts. You probably know this based on your electricity bill in the summer months, but air-conditioning units suck up tons of electricity: roughly 1,000 watts for a small A/C window unit, and upwards of 3,000 watts for a larger unit.
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Now, before calculating the electricity needs of every appliance in your home, think hard about which appliances you really need to get by during an emergency. The typical household will want the fridge to keep running (so food won’t spoil), and perhaps a few outlets for a couple of lights, maybe a TV and a computer, and most definitely for recharging cell phones.
For most households, a mid-size portable unit (5,000 to 6,000 watts) such as the Troy-Bilt 5500 Running Watts Portable Generator (sold at Lowes) or the Stanley 5000 Watt (sold at Home Depot) should suffice. Both sell for about $800, and they’ll allow you to run a half-dozen appliances, including the fridge and either a small A/C unit or portable electric heater.
Bear in mind that you might not be able to buy one right now. When I stopped in at a Lowes over the weekend, there was only one portable generator left. The store was completely sold out of transfer panels as well. Which brings up the issue: What the heck is a transfer panel, a.k.a. transfer switch? Again, CR does a good job explaining that one: A transfer switch allows you to plug a portable generator directly into your breaker box, so that the generator can power whichever breakers you want in the home when the normal power source goes off. Without a transfer panel (from about $500), you’ll need a bunch of extension cords to run from the generator to the appliances you want to keep operational.
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In addition to portable generators, consumers have another option: the stationary generator. Compared to gas-powered portable generators, stationary units are less of a hassle: They usually run on propane or natural gas, come with transfer panels, and may even kick on automatically when the power grid goes off. They’re also more powerful (often 10,000 watts and up), allowing the owner to keep more of a household running, including major electricity sucks like central A/C. Then again, they’re also more expensive, from about $1,500 to over $10,000, and they must be professionally installed (figure another $1K).
Also, a stationary unit obviously stays put in one place, so you can’t bring it camping. That’s what many people who bought portable generators primarily used them for over the years, long before everybody began freaking out about blackouts.
Brad Tuttle is a reporter at TIME. Find him on Twitter at @bradrtuttle. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.
