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Solar Battery Backup: A Homeowner’s Guide

Energy storage (battery backup) is becoming increasingly popular for homeowners considering “going solar.” According to a projection by SEIA, 30% of behind-the-meter systems will include battery backup by 2027, up from just 10% in 2022. Battery backup is in a boom. 

Homeowners add battery backup to their solar systems for all kinds of reasons. Some want to ensure they can keep their lights and essential appliances on during a power outage. Others want to run almost everything as usual during an outage. Some want to completely disconnect from the grid and live off only the sun’s energy. 

We’ve put together a guide to help you better understand battery backup systems. Whatever your situation, there’s a system that will serve your needs. 

Understanding Your Energy Needs and Goals

As you’re deciding whether battery backup is for you, think about what is essential in your home during an outage. 

There are two critical questions when it comes to battery system sizing: 

  1. How often do you experience power outages where you live? 
  2. What do you want to power in your home, and for how long? 

The best way to find the right system for you is to sit down for a consultation with a reputable solar installer. An experienced sales engineer who’s seen your situation before can build a system that suits your needs. 

If you’re just curious about battery backup and not quite ready to book a consultation, read on! 

One of the most important terms with battery backup is “days of autonomy.” DOA measures how long your battery backup can power the loads you choose without electricity-generating sources. If there’s no power from a solar system (because it’s too cloudy for the system to charge fully) and no power from the utility, what’s the number of days your battery bank can power your house? 

You can get by with a small battery bank if you only want to power lights and essential appliances. You’ll need a much larger battery bank to run your entire house and all your appliances as normal for a multi-day outage. 

When it comes to sizing battery backup, we serve three main “customer profiles” at Exact Solar:

  1. People who seldom lose power, and if they do, it’s for an hour or two at maximum. 
  2. People who lose power frequently, sometimes for more than a day. 
  3. People who want to disconnect from the grid entirely. 

Each person has very different needs, and the amount of battery storage that works for one will be too much for another. 

It depends heavily on what appliances you consider “essential.” If you go with the smallest possible battery system but want to blow-dry your hair, run your AC, and cook a Thanksgiving turkey during an outage, you’ll run out of power very quickly. 

Considerations for Sizing Solar Battery Backup

How much battery backup you’ll need and your potential days of autonomy will depend heavily on how much energy your household uses. 

According to the U.S. Energy Information Administration, the average Pennsylvania home consumes 10,402 kWh annually (more than half of that is space heating). The average household in New Jersey consumes 8,942 kWh annually

When you book a consultation with a solar system designer, they’ll look at your: 

  1. Daily kWh usage (average amount of energy you use). 
  2. Peak power usage (the maximum amount of power you use at your highest-use times of day).

There are two primary variables that you’ll need to consider when sizing battery backup for your home: 

Storage Capacity

Storage capacity measures the total amount of energy that can be stored in your battery bank. Storage capacity is measured in kilowatt-hours (kWh). 

An easy way to think of kilowatt hours is as a measurement of how much energy a 1000-watt appliance uses if left on for one hour. Most vacuum cleaners tend to fall in the 1000-1400 Watt  range, so as an example, running a typical vacuum cleaner for one hour will use roughly one kWh of electricity. 

Load Capacity 

Load capacity refers to how much power your battery bank can send out at once and is measured in kilowatts (kW). Kilowatts are distinctly different from kilowatt-hours. A kilowatt (i.e 1000 Watts) is a measure of how much power an appliance takes to run at any moment. 

Our example vacuum cleaner from before is a 1000-watt appliance, meaning it takes 1000 Watts (1 kilowatt) to run at any given second. 

A kilowatt-hour refers to how long it takes to run an appliance over time. If you use that vacuum cleaner for an hour, you’ve used one kilowatt-hour of electricity. 

Within your load capacity, two factors determine how much available power you’ll have during an outage: 

Continuous capacity is you how much power your battery bank can discharge continuously under normal conditions. It gives you a rough idea of how many appliances you can run simultaneously during an outage. 

Surge capacity is you how much your battery system can send out at once to meet high demand, e.g., if a sump pump and AC unit kick on simultaneously. 

One of the best ways to ensure you’ll have enough power during outages is to make your home as energy-efficient as possible. If your fridge and dishwasher are efficient and all your lights are LEDs, your battery bank will sustain your home for much longer. 

You’ll want to ensure that in the event of an outage, you’re very selective about the appliances you use. 

Pairing Battery Backup with Solar Panels

One of the best arguments for pairing battery backup with your solar system is that if you only have a standard, grid-tied solar system, your system will shut down when the power goes off. 

If you have a solar system without batteries and the power goes out, then you won’t have power. 

Grid-tied solar systems are subject to net metering, so when there’s an outage, the power shuts off automatically to protect line workers from electricity flowing in both directions. Since your solar system isn’t back-feeding the grid, there’s nowhere for the power to go, so your power flow is cut off. 

If you have a battery backup system, then during an outage, your solar system will continue feeding electricity to your batteries, and the system will send that power into your house. It’s important to know exactly how much energy your solar system generates because it will tell you how much it will offset your power consumption during the outage. 

Working with your system designer up-front to determine how many days of autonomy you’ll have is critical. 

Since you’ll have solar power continuously recharging your batteries, you’ll always have a power supply. But on the rare occasion when it’s heavily overcast, and your system isn’t producing enough power to run everything you need, knowing that you have the battery power to run your home for a day or two will give you peace of mind. 

Battery Backup Considerations

Batteries are expensive. Because we have yet to reach peak adoption of lithium battery backup technology, batteries are the most costly part of a backed-up solar energy system. The technology is newer and has yet to be widely adopted. 

The numbers make sense for some households, but battery backup isn’t right for everyone.  

When considering adding batteries to your solar system, consider the warranty length and cycle life. 

Batteries are generally warrantied for ten years and will work for 10-15 years (for context, your solar panels are warrantied for 25 years and will most likely produce power for 30). 

Because lithium battery technology is so new, we’re still learning their upper limits on how long they’ll function well. 

Every battery has a “cycle life,” referring to the number of times a battery can be drained and recharged. Like the battery in your phone or an EV, your solar system’s battery will lose a small amount of its ability to hold power each time it’s charged and drained. 

Every time there’s an outage, and your battery power goes into powering your house, that’s a charge cycle. If you experience frequent outages, it can strain the battery and cause it to lose its potency sooner. 

Work with your system designer to ensure that your specific home’s power backup needs are worth spending the extra money on a battery bank. 

The Decision is Yours

Integrating a solar battery backup system into your home is a significant step towards energy independence that offers peace of mind and reliability during outages. 

The initial investment might seem daunting, but the long-term benefits—from reduced reliance on the grid to the environmental impact of maximizing renewable energy—make it a worthwhile consideration for many homeowners.

Battery Backup technology continues to advance, making batteries more efficient, accessible, and compatible with a range of solar setups. 

Remember, the path to energy resilience is unique to each household. Whether you’re looking to keep essential appliances running during outages, aiming for a completely off-grid lifestyle, or somewhere in between, there’s a solar battery backup solution for you. 

With 19 years of building custom solar systems behind us, we’re ready to help you make the best choice for your home at Exact Solar. Book your free consultation with one of our solar experts today!

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