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

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

When you install a home battery, what you are really doing is providing your home with 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, or in areas with Time-of-Use (TOU) rates that make electricity more expensive during peak use hours.

In this article, we’ll show you how to calculate how long a battery paired with solar can power your house during a power outage, and give you some tips for maximizing your battery usage.

First, the big question.

How long can a solar battery power a house?

Without running AC or electric heat, a 10 kWh battery alone can power the basic operations of a house for at least 24 hours, and longer with careful budgeting. When paired with solar, 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 system.

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 story — is one of the main features that influence how long a battery can power a house during a power outage.

Battery storage capacity is measured in kilowatt hours (kWh) and can vary from as little as 1 kWh to over 10 kWh. Multiple batteries can be combined together to add even more capacity, but a 10 kWh home battery is typical for most homes.

Check out this video to know more about Kilowatt hours (kWh).

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. 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 0.5 kWh of capacity in reserve at all times. That leaves you with 9.5 kWh to power your home during a power outage.

On it’s own, 9.5 kWh of battery storage can power essential electrical systems for a day or two. But central air conditioning uses around 3 kWh per hour, which would eat up the entire battery in just over three hours. Let’s see pairing battery with solar can expand your options during a power outage.Sonnen Home Battery

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 electricity does a solar system produce?

Solar panel systems are measured in kilowatts (kW) that 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. But 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. Multiple 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 kW battery to use overnight. But electricity needs vary from home to home, so let’s run through a 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 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 capacity. For many homeowners, the list of priorities includes:

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

Now, heating and cooling is last on the list not because it isn’t important, but because these operations take so much electricity they’ll likely have to be limited regardless of your battery capacity. So the wise move here is to budget your essentials first, and then get a sense of how much battery capacity is left for heating and cooling.

Note: If you have a gas furnace, you won’t need much electricity to keep the heat on.

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 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), microwaved some leftovers for lunch (.12 kWh), and baked a frozen pizza for dinner (0.75 kWh) because your 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 rise your dishes with cold water, and hold off on laundry for a day or two.

And if you have a gas-powered water heater, water heating is a non-issue in terms of battery use.

But for the sake of argument, let’s say you put on some extra deodorant and limit your water heating use to 2.5 kWh per day.

Running total: 5 kWh

Lights: 1 kWh per day

Bulb type Energy use
CFL/LED (8 W) equivalent to 25 W incandescent 0.008 kWh per hour
CFL/LED (15 W) equivalent to 60 W incandescent 0.015 kWh per hour
CFL/LED (27 W) equivalent to 100 W incandescent 0.027 kWh per hour
CFL/LED (38 W) equivalent to 150 W incandescent 0.038 kWh per hour

Compared to larger appliances, lights require very little electricity, especially if you have compact fluorescent (CFL) or LED bulbs. Even at the high end, a 38W LED bulb or 150 W incandescent bulb uses just 0.038 kWh per hour.

So if you budget 1 kWh of battery storage per day to lighting, you could run 26 bulbs for 1 hour each — more than enough to provide light for trips to the bathroom, closet, and getting ready for bed.

Remaining battery budget: 6 kWh

TV and device charging: 2 kWh

Device Energy use
WiFi router 0.024 kWh per day
Modern TV: LED/OLED 4k or 1080P HDTV (20″ to 70″+) 0.014 to 0.18 kWh per hour
TV: Plasma (40″ to 50″+) 0.4 to 0.48 kWh per hour
Desktop computer 0.06 kWh per hour
Laptop computer 0.02-0.05 kWh per hour
Tablet 0.032 kWh per day

Let’s be honest, one of the worst parts of power outages is going without internet and TV. Sure, you try to read a book for an hour or two, but that familiar glow is calling your name.

To keep the WiFi on, you’ll need about 0.024 kWh. If you work from home, you can run your laptop for 8 hours for another 0.4 kWh. And if you want to wind-down with a movie or 3-hour Netflix binge, you’ll need another 0.54 kWh.

That adds up to 1.18 kWh, but we’ll round up to 2 kWh to account for phone charging and multiple devices running at once.

Remaining battery budget: 8 kWh

Air Heating and Cooling: Variable

Appliance Energy use
Heating
Electric furnace (with fan) 10.5 kWh per hour
Portable heater 1.5 kWh per hour
Baseboard heater (6-foot unit) 1.5 kWh per hour
Cooling
Window/Wall AC (8k to 18k BTU) 0.73 to 1.8 kWh per hour
Central AC (3 ton – 12 SEER) 3.0 kWh per hour
Pedestal fan 0.03 kWh per hour
Ceiling fan 0.025-0.075 kWh per hour

As you can see from the table, heating and cooling take a lot of electricity. So if you only had a 10 kWh battery and no solar, you’d have maybe 2 kWh to budget for running fans or space heaters.

But with a battery charged by solar, you would have much more electricity to spare for fans and space heaters — perhaps even enough to run central AC for a few hours, depending on your equipment.

Medical equipment

Device Energy use
Nebulizer 1 kWh per hour
Oxygen concentrator 0.46 kWh per hour
Sleep apnea machine (CPAP) 0.2 kWh per hour

Another thing to consider is powering medical equipment during a power outage. Certain equipment, like a nebulizer, requires a significant amount of electricity and could eat through 10kWh of battery very quickly.

Is getting a solar battery worth it?

Weather-related power outages in the US increased roughly 78% from 2011 to 2021, and are becoming a reality for more homeowners. If you like clean, quiet, and hands-free backup energy that can power your home for several days during outages, then solar and battery are totally worth it.  Not only can battery keep your home powered, it can keep your local grid running, as we saw in California in September 2022.

In addition to providing backup power during outages, solar and battery can reduce your energy costs by shifting your power usage to off-peak hours when electricity is much cheaper (up to 20 cents per kWh!)

And, of course, there are the local and global impacts of reducing fossil fuel that pollute the air and contribute to climate change.

Click here or enter your zipcode above to see how much you can save with solar and battery.

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