Going Off the Grid: How Many Batteries Do You Need?

by Terry Holtz


SO YOU WANT TO GO OFF THE GRID? 

Many off-grid homes rely on solar panels for power. If you want to keep the lights on when the sun isn’t shining- without burning diesel- you’ll need batteries. How many? In this blog post, I’ll show you the process used by our Aquion Global Partners to size an Aquion battery system for your off-grid home.

Need some inspiration first? Check out these off-grid residential solar + energy storage examples:

redwood gate ranch hawaii microgrid

off the grid home solar

Step 1: Determine Your Daily Electric Load

List the electrical devices in your home that you want to be powered by your energy storage system. Multiply each device’s maximum power by the number of hours the device runs per day.

Remember that some devices cycle on and off intermittently. For example, the compressor on your refrigerator may run for five minutes every hour, so your refrigerator’s total run duration would be just two hours per day. Sum the energy consumed by all devices to get your house’s total daily electric load.

The best way to accurately determine the energy and power of your home’s devices is to use one or more energy meters and record the load activity of the house during an appropriate period of time. This could be several weeks to several months, depending on the variability of the loads over different seasons.

Here's an example:

Device

Max. power (W)

 

Daily run duration (h)

 

Energy consumed per day (Wh)

Heater

2,000

x

5

=

10,000

Refrigerator

500

x

2

=

1,000

Lights (total)

300

x

6

=

1,800

TV

200

x

3

=

600

Total
3,000

 

 

 

13,400

 

Step 2: Make Sure Your PV Array is Large Enough

When the sun is up, your PV system should be large enough to power your loads and charge the batteries. You can use PV modelling software such as HOMER or PVSYST to determine the average hourly power generation from a PV array in most locations throughout the world. 

Once you know how much energy your PV system generates every hour, you’ll need to compare it to your hourly loads over a 24-hour period.

Aquion has developed a simple tool that can help you do that. You can click here to download!

solar system sizing calculator

Here, the tool estimates that your 4kWp PV array will yield 13.0 kWh of average daily excess energy to charge the batteries, and you only need 8.8 kWh of daily discharge energy from the batteries on average. Everything looks good!

Step 3: Define Your Desired Autonomy

How many days in a row do you want your house to be able to run on batteries alone? Think about how long the cloudy or stormy spells tend to last in your region, or how long it takes you to clear snow or other debris off your solar panels.

Take that number of days and multiply it by the total daily electric load. The result is the number of kilowatt hours your battery bank will need to provide. Our example home would need the following:

  • 1 day of autonomy = 13.4 kWh battery bank
  • 2 days of autonomy = 26.8 kWh battery bank
  • 3 days of autonomy = 40.2 kWh battery bank

Aquion recommends two days of autonomy for most systems, based on our analysis of thousands of load profiles and needs. Our example system would need to store 26.8 kWh of energy.

Step 4: Determine How Many Batteries You Need

Find a single battery’s maximum energy by looking at the latest battery spec sheet.

Here are the latest AHI™ spec sheets:

Step 5: Make Sure You Have Enough Power Supply All the Loads

Power is the amount of energy moving through a system at any given moment. You can reuse the energy table from Step 1 to determine the total power requirement of the house.

Be aware that some loads, such as electric motors, can draw five to six times their rated current when first starting. When sizing your system, it is very important to consider the starting current and power of these devices, not just their rated current and power. Appliances such as HVAC units, refrigerators, and freezers have compressors that are driven by electric motors.

With all the devices running at once, our example house demands 3,000 W of power.

Device

Max. power (W)

 

Daily run duration (h)

 

Energy consumed per day (Wh)

Heater

2,000

x

5

=

10,000

Refrigerator

500

x

2

=

1,000

Lights (total)

300

x

6

=

1,800

TV

200

x

3

=

600

Total
3,000
     
13,400

To calculate the total power capability of the battery bank, find the continuous power rating per battery on the battery spec sheet and multiply the value by the number of batteries. This number should exceed the power required by the installation (here 3,000 W).

Step 6: Contact an Aquion Global Partner for Pricing

Get started with an Aquion Global Partner to size and build your solar system. You can reach out to them here. 

contact aquion sales

Topics: Off Grid, Residential

Written by
Terry Holtz
Terry Holtz is an application engineer at Aquion Energy.
 

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