Factors that determine battery system size

First, let's talk about the ways in which battery size is calculated: storage capacity and load capacity.

Storage capacity: This indicates how much energy a battery can hold and is measured in kilowatt-hours (kWh). A kWh is a measure of how much energy you use. It's equal to the amount of power you'd use if you kept a 1,000-watt appliance running for an hour. So a 2,000-watt oven would use one kWh of energy in 30 minutes of operation. A typical battery system has about 13 kWh of storage capacity.

Load capacity : This measures how much power a battery can send out and depends on the conditions of the power grid. Capacity is measured in kilowatts, which is different from a kilowatt-hour! A kilowatt is a unit of power equal to 1,000 watts.

• Continuous capacity refers to how much power a battery can keep sending out under normal conditions, and tells you how many appliances you can expect to run at once.
• Peak capacity refers to how much power a battery can send out during moments of heightened demand, such as when your air conditioning kicks on.

The size of the battery system you need depends on a number of factors.

Your electric load

To size your battery system appropriately, you need to understand how much energy you use. According to the US Energy Information Administration (EIA), American households used 886 kWh a month on average in 2021. That can vary a lot depending on the size and location of your home, though. Homes on the Pacific coast used an average of 655 kWh per month, compared to 1,147 kWh for homes in the central Southeast.

Understanding your average energy usage will help you judge how much electricity you might need during a power outage, for instance.

Your solar system output

If you pair battery storage with solar panels, you need to know how much electricity your panels produce. That will give you an idea of how much electricity you need to be able to store in your battery system. You can also install battery backup without solar panels. In this case, your battery will charge from the grid, so solar system output won't matter for you.

Your energy goals

People install battery storage for different reasons. Your personal goals will help determine the size of the system you need. We'll talk more about this in the next section.

Haven will help you navigate the battery backup process from start to finish. Get a quote today.

What are your goals for your battery system?

From backup power to lowering your carbon footprint at home, batteries serve a variety of functions. And the size of your batteries will vary depending on the primary goal of your storage system.

Resiliency

If your main goal is keeping your lights on during a power outage, a single battery will likely be able to provide enough backup. If you're concerned about longer power outages (e.g., multi-day outages), you would likely choose more backup storage.

Energy savings

On the other hand, if your main objective is saving money on your electric bills, you'll need more storage capacity. To maximize savings, you should consider a solar-plus-storage system, which would allow you to generate power on sunny days and then tap into your stored energy in the evening, when rates are high, and at night, when no solar generation is possible. During these latter periods, you'll need enough storage capacity to cover all of your energy needs.

Total energy independence

In order to go totally off the grid, you'll need to cover all of your energy needs through solar panels and battery storage. That said, even with a robust number of batteries, complete energy independence usually requires having backup power for backup power, like generators.

How to calculate the amount of battery storage you need

As we've already discussed, the amount of required battery storage will vary depending on your needs and goals. But there are two factors to consider that can help you estimate how many batteries to install.

What appliances do you want to power?

This is the electrical load mentioned earlier – it's the power required to run the appliances in your home. You'll see this on your monthly utility bill as the energy you consumed during a billing period.

Whatever your goal for battery storage, you need to know the electrical load that you must meet with battery storage:

• If your primary goal is saving money, determine what appliances you want to run when you're not drawing power from the grid and your solar panels aren't generating electricity.
• If your primary goal is resilience during a power outage, decide what critical appliances you want to keep running while you wait for power to be restored.
• If you want to go fully off the grid, figure out the electric load you need to support when your solar panels aren't generating electricity.

To estimate your electrical load, you need to determine the demand of the appliances you want to power. In other words, you need their wattage.

The US Department of Energy has an energy use calculator that can help you estimate your total electricity demand. You can select an appliance from the dropdown menu to get the average wattage for that appliance, then add it to the wattage for your other appliances.

In reality, you're not going to turn on all of your appliances at once. Your washing machine, for instance, doesn't run constantly like your refrigerator does. But you can calculate your personal peak demand including those sometimes-used appliances to know the upper limit of the electrical load you need to cover.

How long do you want to power those appliances?

The amount of time you need to power your electrical load will depend on your primary battery storage goal:

• If your primary goal is saving money, you need to cover the time that you don't want to draw power from the grid. This could be the times when your solar panels aren't producing electricity and/or the times of peak demand if you're on a time-of-use (TOU) rate.
• If your primary goal is resiliency, you need to be able to make it through a power outage. According to the Energy Information Administration, the average power outage lasts around two hours. It's important to remember that you don't need to power everything during an outage, just your critical appliances – think your refrigerator, garage door opener, cell phone charger, etc. And if you have solar panels, they can recharge your battery system during an outage.
• If your primary goal is going completely off-grid, you have to cover all the times when your solar panels aren't producing electricity. It's probably a good idea to overestimate here because you won't have the grid as backup.

To estimate how much storage capacity you'll need,** multiply your electric load by the number of hours you need to power that load.** This will give you a ballpark figure.

Let's calculate an example using the Department of Energy's calculator. The average refrigerator has a wattage of 225. It runs 24 hours a day, 365 days a year, but let's say that you have solar panels to cover 8 of those hours every day. So you'd need battery storage to cover 225 watts of power for 16 hours a day, every day of the year. That means you'd need to cover 410.6 kWh of electricity for your refrigerator every year, or 1.1 kWh a day.

Can a single battery power a house?

Yes. You can power your house with a single battery, but not for an extended period of time. You could keep your lights on during a power outage, for example, but you couldn't go completely off the grid on a single battery.

How long your battery backup protection will last during a power outage relies on several factors. If you charge your battery from the grid, for instance, you have a finite amount of back-up power. If you have a solar-plus-storage system, you essentially have an “energy island.” Your solar panels continue to generate electricity, which recharges your battery and lets you keep drawing power.

The number of appliances you use will also affect how long your backup power lasts. If you try to run everything just as usual, you'll deplete your stored power in a few hours. If you run only the most critical appliances, you can make your backup power last a lot longer.

Let's talk

To learn more about sizing your battery system or to better understand the benefits of home batteries, get a quote here or give us a call to speak with a Haven consultant. We'll help you figure out the best option for your exact situation and be there to support you every step of the way.

NOTES

Your storage needs could depend on the type of electricity rate you have:

Flat rates

On a flat rate (i.e., you pay the same for electricity no matter the time of day), you should aim to use as little energy from the grid as possible – that means covering more of your energy needs with solar and storage.

Time-of-use (TOU) rates

On a variable, or time-of-use (TOU) rate, you'll want to make sure you have enough storage to get through peak pricing times. You may also want to check whether your utility offers arbitrage, which means that you can sell extra stored energy back to the grid during peak times at a higher price than usual. If so, it could be worth installing a little extra storage. Just note that there will probably be an arbitrage cap that limits how much stored energy you can sell back to the grid.