Do the watt-hour math before you buy any backup power

Quick answer: Call your equipment supplier and your utility first. Then read the wattage on the device, multiply by the hours you need, and buy to that number with room to spare. If a machine keeps someone alive, the plan can't depend on a single battery or a single gas can.

The usual mistake is walking into a hardware store the day before a storm and buying whatever generator is left. Sometimes that works. Often you end up with a loud machine that's too big for the job, too small for the job, or sitting in the garage with no safe way to hook it up.

Do it in order.

Make two calls before you spend a dollar

Call the company or clinic that supplies the device. Ask what they recommend for an outage: how long the internal battery lasts, whether they'll provide backup oxygen cylinders or a spare battery, and what the device needs in terms of clean power. Write the answers on an index card and tape it near the device.

Then call your electric utility. Most utilities keep a list of customers who depend on powered medical equipment. Ask to be put on it. Know what it does and doesn't do. It can get you advance notice of planned shutoffs and sometimes a phone call. It doesn't put you first in line for restoration, so don't treat it as a guarantee.

Also look at the HHS emPOWER Program. It's a federal effort built for people who rely on electricity-powered medical and assistive equipment, and its planning resources are worth twenty minutes of your time.

Then ask your doctor the question nobody likes asking: how long can this person safely go without the device? That answer drives everything else. A person who can manage a few hours without a machine has a different plan than someone who can't manage fifteen minutes. Your doctor can tell you which one you are dealing with. A website can't.

Read the label, then multiply

Every device has a plate or a line in the manual showing its power draw in watts. Find it. If it only lists volts and amps, multiply the two to get watts.

Now the arithmetic. Watts times hours gives you watt-hours.

Say a device draws 60 watts and you want 24 hours of cover. That's 1,440 watt-hours. A battery pack rated at 1,000 watt-hours won't make it through the day. And no pack delivers its full label rating, because the inverter wastes some of it. Knock about a fifth off whatever the box says and plan on that.

Add everything else that has to run beside it. A humidifier on a CPAP, a pump, a charger for a mobility scooter, a phone, a lamp. People forget the humidifier more than anything. It can draw more than the machine it's attached to.

Two more numbers matter.

• Running watts is what the device pulls steadily.
• Starting watts is the brief spike when a motor kicks on. Compressors and some pumps spike hard. The generator or battery has to handle the spike, not just the average.

Check the manual for one more detail. Some devices want "pure sine wave" power, which is the clean kind that comes from the wall. Cheap inverters produce a rougher wave. If the manual or supplier says pure sine wave, believe them and buy accordingly.

Battery or generator

Neither one wins every time. Here's the honest trade-off.

Portable power stations (the big rechargeable battery boxes) are silent, have no exhaust, and can sit next to the bed. There's no fuel to store and no carbon monoxide to worry about from the unit itself. The limit is finite capacity. When the watt-hours are gone, they're gone until you recharge from the wall, a car outlet, or solar panels. They suit short outages and low-draw devices very well.

Generators keep going as long as you have fuel. That's the advantage for a multi-day outage. The cost is noise, gasoline you have to store and rotate, and a hard rule that it runs outdoors, well away from the house. Our piece on twenty feet from the house and other generator rules covers placement, cords and carbon monoxide in detail. Read it before the first time you pull the starter cord, not after.

For a device where interruption is dangerous, many families end up with both. The battery covers the first hours and the night. The generator recharges the battery during the day. That setup also means you can shut the generator off at 10 p.m. and keep the neighbors friendly.

Two warnings the sales rack won't give you:

• Never plug a generator into a wall outlet to power the house. It can send power back up the line and injure the utility workers restoring service. A licensed electrician can install a transfer switch or inlet the right way.
• Have battery-powered carbon monoxide alarms in the house before the generator arrives. Fumes don't announce themselves.

What the box doesn't tell you

Practice once. Run the device from the backup for a full night while the power is on and everyone is calm. You'll learn the cord is too short, the beep is louder than you expected, and the battery reads lower than advertised. Better to learn it on a quiet Tuesday.

Keep it charged and label it. A power station left in a closet for a year may not be at full charge when you need it. Put a reminder on the calendar every three months. Tape a note on the unit saying what it powers and how long it lasts.

Medications that need cold. Insulin and some other drugs require refrigeration. Many have labeling for how long they can sit at room temperature once out of the fridge, but it varies by product. Check the package insert or ask your pharmacist now, and keep a small cooler and ice packs ready. Don't guess on this one.

Know your fallback. Oxygen users should ask their supplier about backup cylinders and how long each lasts at the prescribed flow. Ventilator users should know the internal battery runtime and have a plan for when it ends. The plan should include when you leave for a hospital or shelter, who drives, and what you take. Write down the address of the nearest facility with power. In a real emergency, call 911.

Cash and fuel. Gas stations can't pump without power either. Fill the tank and your approved fuel cans before a storm, not after. Keep some cash on hand. Our piece on the emergency fund and the cash you can actually reach explains how.

If it's your parent, or you live alone

Different situations, same method. An older parent may not remember the steps at 2 a.m., so print them in large type and tape them to the device. See what your parents need before the next three-day outage for the wider checklist.

If you live alone, the problem is that nobody notices when something goes wrong. Give two people your plan and agree on a check-in time. Our guide on what to do before the power goes out when you live alone builds that out.

Look next door

You've done the math, bought the unit and rehearsed the night. Now think about the house two doors down, where an older man runs an oxygen concentrator and doesn't have a plan.

Knock on the door before the storm. Ask if he has a backup. Offer an outlet, a charged battery, or a seat in your living room if it comes to that. Bring a thermos when you go. Don't make a speech out of it.

Nobody forgets who came by before the lights went out.