Best case, you're in the middle of a movie when the screen goes black. Worst case: a storm hits and you're without power for hours, if not days. If these scenarios are cause for concern, a portable generator can give you peace of mind and be an invaluable device in the event of a power outage.

A portable generator is not as straightforward as being just a backup power source, though, so deciding what model to buy can be frustrating. Read on to find out all the key considerations, features, and safety tips you should carefully consider before buying a portable generator.

Main purpose and power needed

What will the primary use be? For an emergency power source in your home, camping or RV travel, or will it be on a site with no power supply? Obviously, you'll want a workhorse for powering a large home that needs electricity after a storm, but a less powerful model for occasional use and minimal electricity needed.

Look at each item that you'll need power for, from your heat or cooling sources to cooking appliances, lights in each room, and other crucial electric devices. You won't be able to power everything in a large home at once on even the best portable generator, so only consider the most important rooms and items.

Then, you need to add up all your needs in wattage it takes to power them. A typical home needs between 10,000 and 20,000 watts. (You can check your electric bill to see approximately what your house will need.)

Don't only consider the wattage it takes to power something when it's on. Many devices require a bigger burst to turn it on, then run at a lower wattage. The highest wattage needed, or starting watts, is what to take into account. However, you'll rarely need to start everything at once. For example, You can restore power to the lights before the air conditioner.

Here's a breakdown of what common household devices need:

Gasoline is the most common and requires less fuel to run for the same amount of time, but portable generators can also run on propane. So, usually it's more economical and practical to use gasoline.

Portable generators use a gasoline- or propane-powered motor to convert mechanical energy into electricity.

Portable inverters work similarly, but because they have unique mufflers and use fuel injection, they are more compact, lightweight, and less noisy. Power inverters are great for recreational use. They also auto-adjust well to changes in wattage. They produce a steadier flow of power, which makes them safer for sensitive electronics. Two can also run in parallel to up your power (if you need it) and they usually have an eco mode. However, they usually don't generate as much power as portable generators and they are expensive.

How long will you want it to run, and how often can you refill its fuel? Be skeptical of manufacturer claims on these capabilities, because they often quote the absolute maximum for efficiency. It's not meant to be deceptive — portable generators just can't run at maximum efficiency the whole time they are in use.

If having an easy-start feature is important, look for a model with a push button or key start. Most models feature a ripcord start.

This lets you check how much fuel is left without needing to remove the cap.

Prevent damage and get a model that warns you when oil, an essential component of any portable generator, is too low.

These increase efficiency and run-time by automatically adjusting performance levels when power needs fluctuate.

Not all portable generators come with wheels built in, so if you'll want to move it easily, get a model with them or buy them as an accessory. If the unit is under 45 pounds, wheels aren't usually necessary.

Portable generators have at least one 120V AC outlet. Some also have a DC outlet (to charge batteries), an RV outlet, or a 120/240V 30A (twist lock).

Although you can find a portable generator around the $100 price point, these aren't often good quality and we recommend spending closer to $300 at minimum. A good one should not cost more than $800 unless you need a very powerful one.

First, read the instructions booklet included with your portable generator. But beyond that, here are some common safety tips to avoid the hazards associated with the fumes, use, and maintenance of portable generators:

Keep a minimum of five feet of space clear around your generator.

Use them outdoors only — and inside a garage with windows open does not count.

Don’t fill the gas tank when the unit is hot and don’t add too much fuel (it will expand).

Let it completely cool before storing it away.

Don’t overload it beyond its power capabilities.

Q. How many watts do I need from my portable generator?

A. It depends what you use it for. Every electrical device has a watts rating marked on it somewhere. If you want to run a coffee maker and laptop in an RV, you'll need around 1,200 watts. A drill and angle-grinder? 2,000 watts. A microwave, freezer, and a couple of household lights? Over 3,000 watts. Do some quick math and you'll come up with a figure. Bear in mind a portable generator is excellent in places where no power is available, or for emergency use, but can't run an entire house on its own.

Q. What's the difference between starting and running watts?

A. Every electrical appliance and tool needs more energy to start than to keep it running. That initial demand is called the "surge" or "starting" watts. Moments later they settle down to "running" watts. Portable generators generally give both figures. The trick to avoiding overloading your generator is to not start everything at once. Turn things on one at a time. A delay of just a couple of seconds is enough. But, no matter when you turn them on, you can't go over the machine's maximum rating.

Q. Can I bring my portable generator indoors if it's raining?

A. Absolutely not. Portable generators are fueled by gasoline or, occasionally, propane. Both produce carbon monoxide gas which is deadly. It's extremely dangerous because carbon monoxide is invisible and has no odor. You won't know about it until it's too late. Never run a portable generator indoors – not even in the garage.

Q. What are some important things to know before starting a generator for the first time?

A. Most important is to never run a generator inside the house — not even in the garage. Plan on setting it up in an outdoor location that you can easily reach with extension cords, and that is protected from the weather. Always test the generator immediately after purchasing. Put oil in it first, as instructed, then fuel it up and test start. Some generators need to be "burned in" by running them for a few hours; check the instruction manual to find out if this is necessary.

Q. Can electronics be charged safely from a generator?

A. Today's electronics can be charged safely, but most generator-repair professionals recommend that you use a surge protector or power conditioner with sensitive electronics like smartphones, computers and televisions.

Best of the best: Generac GP5939 Portable Generator

Our take: Delivers the right amount of power to keep key home systems running in a prolonged power outage, but does run loud.

What we like: Provides 5,500 to 6,875 watts, and like most generators of this size, it's pretty loud. The recoil start is very reliable, usually starting up within the first couple of pulls. It can keep a refrigerator running cool for at least 12 hours.

What we dislike: Pricey and very heavy, but the wheels help.

Best bang for your buck: DuroStar DS4000S Portable Generator

Our take: Ruggedly built mid-range model, with good range of outlets. Offers tremendous value.

What we like: Thoughtfully designed. Delivers competitive performance and has plenty of features – without breaking the bank.

What we dislike: Wheel kit costs extra, but a great option for those on a budget.

Yamaha EF2000iSv2 Portable Inverter Generator

Our take: If you want power when camping, or for your RV or boat, there's simply nothing better.

What we like: Extremely portable and quiet inverter generator rated for 1600 watts. It's CARB compliant and has a 'smart' throttle that adjusts itself according to load, thus reducing fuel consumption.

What we dislike: Pricey, but for recreational use it's unbeatable.

Sportsmans Series 4000-Watt LP Generator

Our take: While its recoil start may be a little frustrating, this propane-fuel generator delivers enough juice to easily power small appliances and charge electronics, at a very reasonable price.

What we like: Runs pretty loudly for a four-stroke engine, but starts up within a couple of pulls, and can run continuously for hours. Uses up 20 pounds of propane in seven to 10 hours.

What we dislike: Higher-wattage appliances, like microwaves, may tax this generator.

Katie is an editor at BestReviews. BestReviews is a product review company with a singular mission: to help simplify your purchasing decisions and save you time and money. BestReviews never accepts free products from manufacturers and purchases every product it reviews with its own funds.

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