Duration of Solar Battery Power Supply for a House During an Outage

Duration of Solar Battery Power Supply for a House During an Outage

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How Long Can Solar Battery Power a House During an Outage?

By Isaac Ost | Sep 29, 2023

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When you install a home battery, you’re gaining a backup energy reserve in the case of an outage. Whether you have a solar panel system at your home or not, a home battery can be used to store electricity and deliver it to your home appliances and devices. The benefits of a home battery are clear, especially during short or long-term outages, and in areas with Time-of-Use (TOU) rates or weak net metering policies.

In this article, we’ll show you how to calculate how a solar and battery system can power your house during a grid outage, and give you some tips for maximizing your battery usage. Compare bindings quotes for solar and battery systems to backup your home.

How long can a solar battery power a house?

Without running AC or electric heat, a 10 kWh battery alone can power the critical electrical systems in an average house for at least 24 hours, and longer with careful budgeting. When paired with solar panels, battery storage can power more electrical systems and provide backup electricity for even longer. In fact, a recent study by the Lawrence Berkeley National Laboratory found that when heating and cooling are excluded: “(A) small PVESS with just 10 kWh of storage (at the lower end of sizes currently observed in the market) can fully meet backup needs over a 3-day outage in virtually all U.S. counties and any month of the year.” PVESS stands for photovoltaics and energy storage systems.

But exactly how long you can power your home with solar battery storage varies for each home and depends on three main things:

  1. Your battery storage capacity
  2. The output of your solar system
  3. Your electricity needs during an outage

We’ll show you how to budget your electricity to meet your solar and battery capacity below. First, let’s start with identifying your battery storage capacity.

Home battery capacity

Capacity — the amount of energy a battery can store — is one of the main features that influence how long a battery can power a house during a power outage. Battery capacity is measured in kilowatt-hours (kWh) and can vary from as little as 1 kWh to 18 kWh. Multiple batteries can be combined together to add even more capacity, but a 10 kWh home battery is typical for most homes.

During a power outage, assuming you have a fully charged home battery, you will be able to use most of the 10 kWh of stored energy. However, depending on the battery type, you’ll want to leave a minimum charge of 5-10% on your battery for a couple main reasons:

  1. To maintain the health of your battery
  2. To start your solar inverters in the morning so you can recharge your battery with solar power

For a 10 kWh battery, you’ll want to leave at least 1 kWh of capacity in reserve at all times. That leaves you with 9 kWh of battery capacity to power your home during a grid outage.

Related reading: The 8 Best Solar Batteries (and How to Choose the Right One For You)

Solar system output

The beauty of pairing battery storage with solar is that you essentially create your own miniature utility to power your home. This is especially useful during prolonged power outages because unless you have battery storage, your solar system will be turned off by the local utility to prevent the backflow of electricity from injuring workers trying to repair the grid. But if you have battery storage, your system can operate independently when the grid is down. This is called “islanding.”

How much electricity does a solar system produce? Solar panel systems are measured in kilowatts (kW) which represent the amount of energy the system can produce in an hour of peak sunlight. So a 5 kW solar system can produce 5 kWh of electricity per hour in ideal conditions. However, since conditions aren’t always ideal, we typically assume a performance ratio of 75%. The average number of peak sunlight hours per day varies from 3.5 to 5.5 in the US. Multiply the system capacity by sunlight hours and 0.75 to find the daily output of a solar system. For example, here’s how you would find the daily output of a 5 kW solar system getting 4.5 peak sunlight hours per day equals: 5 kW solar system x 4.5 sunlight hours per day x 0.75 performance rating = 16.875 kWh per day

In many cases, that’s more than enough to power essential electrical systems and recharge a 10 kWh battery to use overnight. But electricity needs vary from home to home, so let’s run through some common appliances and how much energy they use.

Your electricity needs during a power outage

How long solar battery storage can run your home depends on how much electricity you use. And how much electricity you use depends on which appliances and systems you’re running. During a power outage, it’s recommended to budget electricity for the most necessary things first and then make a plan for the remainder of the capacity.

For many homeowners, the list of priorities includes:

  • Refrigerator
  • Kitchen and cooking
  • Water heating
  • Lights
  • TV and device charging

It’s worth noting that heating and cooling take a ton of energy, and typically aren’t configured to be powered by critical battery backup systems. Whole home backup is possible, but it takes a large solar system with around 30 kWh of battery storage.

Let’s run through an example scenario of powering essential systems during a 24-hour power outage to get an idea of how much solar and battery capacity you’ll need.

Use the tables below as an a la carte menu to create your own battery storage budget.

Refrigerator: 1.5 kWh per day
Model Energy use
Older 15-cubic foot unit (1996) 5 kWh per day
Newer ENERGY STAR 17-cubic foot unit 1.16 kWh per day

A refrigerator is among the most important things to power during an outage so that you can avoid having your food and drink go to waste. If you have a modern, ENERGY STAR-approved model, your refrigerator is using around 1-2 kWh of electricity a day. If your fridge lived through Y2K, you might be looking at closer to 5 kWh per day. Refrigerator electricity usage can be minimized by opening the door less and raising the temperature slightly. If you are preparing for a planned Public Safety Power Shutoff (PSPS), set the temperature very low before the shutoff event, and then set it to a higher temperature once the grid power goes out. That way the fridge starts cold and you can devote less battery capacity to power it during the outage. Let’s say you have a relatively modern ENERGY STAR-certified fridge that uses 1.5 kWh per day.
Running total: 1.5 kWh

Kitchen and cooking: 1 kWh per day
Appliance Energy use
Electric oven 2.3 kWh per hour
Oven: surface 1 -1.5 kWh per hour
Microwave oven 0.12 kWh per 5 minutes
Coffee maker 0.12 kWh per brew / 0.4 kWh per hour on warmer
Dishwasher (energy saver cycle) 0.5 kWh per load
Toaster 0.04 kWh per use

Even during a power outage, people gotta eat. And in a large enough outage, it may not be practical to eat out or order delivery. Let’s say you make a pot of coffee and toast in the morning (0.2 kWh), microwave some leftovers for lunch (.12 kWh), and bake a frozen pizza for dinner (0.75 kWh) because you’re trying to clear out the freezer. That amounts to just over 1 kWh throughout the day.
Running total: 2.5 kWh

Electric water heating: 2.5 kWh a day
Appliance Energy use
Electric water heater 4-5 kWh per day (running 2-3 hours)
Heat pump water heater (50-75 gallon) ~2.5 kWh per day

Water heating accounts for an average of 18% of the total energy used in the household, or around 162 kWh per month. On a normal day, a water heater runs for around 2 to 3 hours a day, which means that it will consume roughly 4-5 kWh of electricity a day. Heat pump water heaters are more efficient and can run on around 2.5 kWh per day. But power outages are not normal days. To save electricity, you may want to skip or shorten your shower, wash your hands and rinse dishes with cold water instead of hot.

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