What Size Solar Battery Do I Need?
As grid electricity gets more expensive and unreliable, homeowners are using solar and battery systems to reduce their energy costs and keep the lights on when the grid goes down.
But while sizing a solar system is pretty straightforward, choosing a battery size takes a bit of nuance and largely depends on how you plan on using it. In this article, we’ll explore the nuances of sizing a solar battery and lay out a process for determining the ideal battery size for your needs.
How to size a home battery
Home batteries are sized based on how many kilowatt-hours (kWh) of electricity they can store. There are two measurements to be aware of:
- Nameplate capacity is the maximum amount of electricity a battery can hold
- Usable capacity refers to the maximum amount of electricity the battery can discharge at once without exceeding the manufacturer’s recommendations for depth of discharge
For example, the SunPower SunVault 13 has a nameplate capacity of 13 kWh, but a usable capacity of 12 kWh after factoring in that only 92% of its full capacity can be discharged without affecting its lifespan. So, when choosing a battery size, make sure to focus on the usable capacity.
Next, follow three steps to figure out how many kilowatt-hours of electricity you want your solar battery to hold.
Step 1: Establish your energy goals
The first step to sizing your solar battery is determining which function(s) you would like it to perform. There are three basic roles battery storage can play:
- Critical loads backup: Powering a select number of essential electrical systems during grid outages
- Whole-home backup: Powering the entirety of your home electrical systems during grid outages
- Self-consumption: Storing excess solar power produced during the day to avoid buying expensive electricity from the grid at night
Historically, home battery systems are most associated with critical loads backup. However, with time-of-use rates becoming more common and net metering policies eroding across the US, using battery storage for solar self-consumption is gaining popularity as a cost-saving strategy.
There may be cases in which you can use a single battery system for two purposes. For example, if you have a 10 kWh backup battery you may also be able to use it for solar self-consumption (with the understanding that you won’t get much or any backup power if the grid goes down shortly after your battery has been discharged).
So, let’s say your primary goal is to power critical systems during grid outages. What’s the next step to determining the right battery size?
Step 2: Determine the size of the load
The next step to sizing your solar battery is determining the maximum load you want it to power. In other words, figure out:
- Which systems you want to back up
- How much electricity they consume
- How long you want to be able to power them
For example, if your goal is to power critical systems during grid outages, then the first step is to identify those systems. These systems typically include:
- Medical devices
- TV, Wi-Fi, and device charging
- Kitchen appliances
- Water heating
Air conditioning and electric heat consume a ton of electricity and therefore typically aren’t included in essential battery backup systems. But smaller systems, like the ones listed above, are routinely backed up by home solar and battery.
Next, you’ll make a “loads list” that adds up how much electricity each system uses. This will start to give you an idea of how much capacity you’ll need to power these systems on battery power alone.
Pro tip: Google “(refrigerator model) wattage” or check the labels on your appliances to determine the power needs of your critical backup loads.
Example critical loads list for battery backup
|System||Power*||Daily usage (hours)||Power per day|
|Refrigerator||625 Watts||24||1.5 kWh|
|Sleep apnea machine (CPAP)||200 Watts||8||1.6 kWh|
|LED lights||38 Watts||26 bulbs @ 1 hour each||1 kWh|
|Tower/Box fans||50 Watts||2 fans @ 6 hours each||0.6 kWh|
|Wi-Fi||10 Watts||24||0.024 kWh|
|TV (60 inch OLED)||100 Watts||5||0.5 kWh|
|Device charging (laptop + phones)||30 Watts||12||0.36 kWh|
|Coffee Maker||120 Watts per brew / 40 Watts on warmer||1||0.16 kWh|
|Microwave||120 Watts per 5 minutes||15 minutes||0.36 kWh|
|Electric oven||2,300 Watts||30 minutes||1.15 kWh|
|Water heater||1,250 Watts||2||2.5 kWh|
*The figures above are averages and are meant for example use only. Check the power rating for your specific devices when creating a loads list.
In this scenario, the battery is responsible for around 10 kWh of critical backup loads over a 24-hour period.
Step 3: Choose how long you want to power your loads
The final step is to determine how long you want to be able to power these systems with battery storage alone – known as “days of autonomy.” Ideally, your solar panels will charge your battery during the day, but it may be worth planning for scenarios in which snow, cloudy weather, and short winter days limit your solar production.
For what it’s worth, the average utility customer in 2021 experienced 1.42 power outage events per year that lasted more than 7 hours on average (up from 3.5 hours per outage in 2013), according to data from the EIA. But there have been 28 outages that lasted 10 hours or more in the first 9 months of 2023 alone.
The size of your battery is essentially a product of your critical backup needs and your desired days of autonomy, as shown in the chart below.
|Critical backup needs||Days of Autonomy||Battery size (usable capacity)|
|10 kWh per day||1||10 kWh|
|10 kWh per day||1.5||15 kWh|
|10 kWh per day||2||20 kWh|
|10 kWh per day||2.5||25 kWh|
|10 kWh per day||3||30 kWh|
It’s worth noting that a Lawrence Berkeley National Laboratory study found that 10 kWh of battery storage paired with a small solar system can meet critical backup needs for three days in most climate zones and times of year in the US.
What size solar battery do I need?
Choosing a battery size is more of an art than a science because it requires a balancing act between your goals, critical electricity needs, and budget. As a rule of thumb, 10 kWh of battery storage paired with a solar system sized to 100% of the home’s annual electricity consumption can power essential electricity systems for three days.
You can get a sense of how much battery capacity you need by establishing goals, calculating your load size, and multiplying it by your desired days of autonomy.