Buying a giant battery for the next blackout or off-grid adventure can be daunting, especially when hundreds or even thousands of dollars are on the line. Get it wrong and you’ve either spent too much money for something you’ll never use or discover that you didn’t buy enough capacity to keep your most important devices running. To get it right, you need to become intimate with the watt-hour (Wh).
The watt-hour is a measure of capacity, or how much electrical energy a battery stores. If you know how much power — measured in watts — your devices consume, then the Wh rating of a battery lets you quickly calculate how long those devices will run. For example, a typical LED light bulb requires about 10W to illuminate. So a 1000Wh (1kWh) battery can run that bulb for 100 hours because Wh divided by watts gives you the time.
For context, the average US home consumes about 889kWh per month, or about 29.2kWh per day, according to the US Energy Information Administration. During a blackout, you’d need about $15,000 worth of batteries on hand to keep it running for just one day (based on a rough average of 50 cents per Wh of battery capacity). But that’d be silly because you really only need to keep your critical devices running for as long as the power is out.
We capitalize the “W” in Wh and kWh because the watt is named after Scottish inventor and engineer James Watt. Sadly for James, not all batteries or devices sold list the Wh spec; some opt for amp-hour (Ah or mAh) instead, where the capitalized “A” is for French physicist André-Marie Ampère. No offense to André-Marie, but the amp-hour is a dumb spec for consumer devices because it requires knowing the voltage. Nevertheless, amp-hours multiplied by the voltage gives you watt-hours.
According to the most recent data available, US homes average about five to eight hours of electricity interruptions per year. But natural disasters like hurricanes and wildfires can extend outages over days. Buying a battery is therefore like buying insurance: do you spend more to cover all possible outcomes or just protect yourself against the basics? Either way, you need to calculate the watt-hours.
There are many publicly available watt-hour calculators available including this one from EcoFlow and this one from Unbound Solar. What follows is a list of common household devices and a very rough estimate of how much power they could consume over a single day. To determine your battery needs, identify which electrical devices are critical to you and how long they’ll need to run, and then total up the watt-hours. That’s how much battery capacity you’ll need to survive without power.
Table above based on a chart originally compiled by battery maker EcoFlow that we’ve heavily modified with additional appliances and data.
In the real world, the actual watt-hours available to power your devices can be lower than the capacity written on the battery due to factors like age and operating temperature. And batteries aren’t very useful if you can’t easily access that stored energy. That’s why they’re often sold in the form of all-in-one power stations (aka “solar generators,” if equipped with a solar charge controller) with built-in AC and DC outputs for USB devices and household appliances. Just know that they divert some of that battery capacity to convert DC power to AC, to keep Wi-Fi and Bluetooth radios operating, to maximize solar energy collection, and possibly to heat the battery so it can be charged in cold weather.
As such, it’s always a good idea to hedge a bit and purchase at least 10 percent more capacity than you actually need. Most battery systems designed for RVs and homes can be easily expanded with more capacity as your needs grow. Batteries built for RVs tend to start around 1kWh and max out at around 15kWh due to space and weight concerns. Those same batteries can be used in the home, where they can be scaled almost without limit.
Vanlifers have modest needs and tend to be keenly aware of every watt they consume. Personally, I go through about 1.6kWh per day when traveling in my modified Sprinter van. I usually keep my Starlink Mini running day and night, make coffee and tea throughout the day, and cook a few meals using an induction cooktop. My battery system also keeps my 12-volt devices operating, including LED lighting, a water pump, a rooftop fan, and a portable electric fridge, while also charging laptops, phones, and watches daily, along with a variety of USB-C devices like headlamps, action cameras, and drones as needed.
Right now, I’m testing an EcoFlow Delta Pro 3 solar generator with 8kWh of total battery capacity, giving me the ability to live and work comfortably off grid for at least four days at a time. I use a combination of fixed and portable solar panels as well as an alternator charger to juice up those energy packs. That means I can go weeks without ever having to plug in so long as I’m driving through sunny climates.
Charging solutions for homeowners are similar, just bigger and more expensive. Homes can be fitted with solar roofs and fully integrated battery backup systems from companies like Tesla, Jackery, EcoFlow, Anker, Bluetti, and others. They can also plug a car into the home to power it off their EV’s giant battery. Otherwise, you can always keep it cheap and old school by firing up the ol’ diesel generator.
Regardless, it all starts with the Wh — get that calculation right, and you’ve taken the first step toward energy independence.
