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How Much Can You Back-Up with Solar Energy Battery Storage?

Solar for Homeowners | 4 min read
how much can you back up with solar battery

It’s a snowy Saturday afternoon. You’ve caught up on all your household duties and errands. It’s finally time to sit down on your couch in front of the television for some well-earned R&R.

A few minutes later, your TV screen fades to black. The lights flicker and then turn off. The familiar hum of your heat pump and refrigerator goes silent.


The lights have gone out and the grid has gone down. If you own a solar system or are considering solar, you may think that your solar panels will continue to produce electricity. But unfortunately, as long as you’re hooked up to the electric grid, your system shuts down and you have no electricity. Unless you have a battery-backed solar system.

How much storage does an average battery backup system have, and what devices can you run when the lights go out?

Despite advances in battery technology, backing up your solar system up could still cost you a good deal of money. In order to get the biggest bang for your buck, you’ll want to identify the devices that are the most important to keep running. These essential appliances are known as your home’s critical electricity needs, or your critical loads.


What is a Critical Load?

Your critical electric loads are the equipment and devices that are important to keep running when you lose electricity. These could be things like medical equipment, lights, or appliances.

If you have a solar system that produces enough electricity to power your entire home, why do you have to select specific items to back up? A common misconception is that you’ll be able to store enough energy to power your entire home during a power outage in batteries. Unfortunately, the number of batteries required to do this would cost much more than most homeowners can spend. That’s why many opt for just backing up the essentials.

Once selected, your critical loads will be connected to a sub-panel, which will be connected to your battery bank. When the grid goes down and your solar system is disconnected, your sub-panel will switch on and you’ll be able to draw stored solar energy from your batteries to power just your critical loads.

solar battery when grid goes down

Determining the Critical Loads for Your Home

There are a few different things to consider when planning your battery backup system. First, you will have to prioritize the devices or appliances in your home based on how essential they are during a power outage.

These could be things like keeping on some of the lights in certain rooms, running the fridge so your groceries don’t expire, or providing power to a space heater for a few hours. You then need to find out the peak power requirements of those critical loads. This is the maximum amount of energy these loads will use at one time.

To figure this out, you’ll need to gather the following for all your critical devices:
  • Maximum Running Wattage
  • Starting Wattage

maximum-running-wattage-laptop-chargerThe maximum running wattage is how much electricity is needed to power a device while you’re using it. To get the most accurate value, look on the device itself. Most appliances will have the maximum running wattage on them. If yours does not, check online for the specific model, or get a general idea with this chart.

Certain devices also take extra energy to power up on top of the running wattage. Items that have an inductive motor will require additional power the first few seconds they are running. This includes:

  • Refrigerator
  • Washing Machine
  • Electric Stoves
  • Sump Pump
  • Water Well Pump
  • Window and Central AC
  • Heat Pump

For some devices like a washing machine or stove, you can be sure you only start the device up a certain number of times. However, devices like a refrigerator or air conditioner may start and stop several times during your power outage, thus using more of your batteries’ stored energy.

Like the device’s maximum running wattage, you should be able to find its specific starting wattage on the device itself. This figure should be added to the device’s maximum running wattage to get a full picture of the device’s electrical requirements.

Here are some items that do not typically require additional wattage to start:

  • Light bulbs
  • Toasters
  • Microwaves
  • Coffee Makers
  • Security Systems

The Peak Power Requirements

Before choosing how many batteries you need or finalizing your list of critical loads, you will need to define the peak power requirements of your critical loads. This is the maximum amount of electricity you will use at one time. You can do this by adding up two sets of values: all the maximum running wattages for your critical loads, and all their starting wattages. If a device or appliance does not have a starting wattage (like a light bulb), use their maximum running wattage instead.

Here’s an example:

Critical Load

Maximum Running Wattage

Starting Wattage
(or Maximum Running Wattage
if no Starting Wattage)

5x 60-Watt Light bulbs

300 watts

300 watts (no starting wattage)


780 watts

2,000 watts


100 watts

100 watts (no starting wattage)

Laptop Charger

60 watts

60 watts (no starting wattage)

One Window AC Unit

1,500 watts

2,000 watts


2,740 watts

4,460 watts

In this example, the peak power requirements would be 4,460 watts or 4.5 kW. That is the most electricity your critical loads will require at one time. Compare this number to your battery’s maximum discharge power and your inverter’s backup power rating. If either of these ratings is lower than your peak power requirements, you will need to install more battery storage and/or a larger inverter, or you will need to remove items from your critical loads list. For many 9.8 kWh batteries, the maximum power is 5.0 kW. That means this list of critical loads would be supported by the battery.

Battery and generator Blog_CTA


There is one other important factor in all of this, and that’s time. The amount of time you would like your critical loads to run will determine how many batteries you will need. This is difficult to estimate because the longer the electricity goes out, the more energy you will use. However, batteries are still expensive, and you don’t want to spend your hard-earned money on more than you need.

You can estimate how long your battery will power your critical loads by dividing your battery’s capacity by the total running wattage of your list. For the example above, you will get about 3.5 hours (9.8 kWh / 2,740 watts). That means you’ll be able to power all the items on the above list with one 9.8 kWh battery for roughly 3.5 hours.

If you’re interested in installing batteries for your solar system, click the button below. We can talk about your critical loads list, how long you’d like to power your home, and what kind of battery backup system is right for you.

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