Battery Bank Sizing Considerations

 In Battery Backup

Adding a Battery Bank to a Solar System

You’ve decided to add a battery bank to your solar system, and that is a wise move since there are several benefits to giving your solar system the ability to power your home or business when the grid is down. These include; 1) electricity will always be available as long as the sun shines, 2) no noisy generators running to keep the lights on, 3) few moving parts, which means no oil changes and minimal maintenance, and 4) it is green energy, so it helps lower your carbon footprint and helps the environment.

There are 2 parts of the battery backup system: the inverter and battery bank. But it’s the batteries that are the most expensive component of the system. A large battery bank quickly makes the cost-effective use of solar a moot point. To help manage costs and keep within a budget, you have to define exactly what loads you want on backup.

What does this mean? When the utility power goes out, you would ideally like your battery bank to power ALL of the loads in your house. You’d like your entire main power panel (the one with all of the breakers) to be energized, so every appliance, light, and clock in your home or building works.

What Should Run off the Batter Bank?

There is no doubt that we can size a battery bank to power your entire home or building. But most locations have energy-hogging HVAC systems and inefficient major appliances that use a ton of electricity. The battery bank needed to provide ongoing backup to power them would likely be huge and costly. (Read more about that in our article about taking your house Off-Grid).

Deciding what loads you want to back up means determining what selected circuits in the main panel you want to be energized and kept running (or able to run) when the utility grid goes down. It’s not necessarily all of the circuits but critical loads you need during a power outage. You most likely will need to trade-off the cost of a battery backup with the loads you want to be powered during an emergency.

Sizing of the Battery Bank

2 considerations when determining the size of the battery backup system are the AMOUNT of power that you need at any one time and the LENGTH of time that power will need to be provided during periods of no or reduced sun. The AMOUNT of power defines the size of the inverter, and the LENGTH of time for power defines the size of the battery bank.

We’ll start with the amount of power needed first. Most houses in the US are built with 200A service. That means the power company (or “POCO”) can provide 200A at a given time to power all the loads in your house. If converted to watts, this is roughly 48 kW at 240V.

To duplicate that output in a power outage environment, you need a 48 kW generator connected to your house’s main electrical panel. That’s one big generator, and we know that most whole-home generators are half that size…probably around 20-24 kW. That’s because you won’t likely ever use 200A at one time.

To do that, you’d have to turn on every electrical appliance in your house at once. That most likely never happen, so the chances that you actually pull 200A from the utility is unlikely. But the POCO provides that if you ever need it…they’re ready to sell you the electricity to do it!

How Much Power is Needed

We really need to look at how much power you’re going to pull at one time. The inverter is the “generator” that takes DC voltage from your battery and inverts to AC power for your loads. To know how much inverter capacity is needed, we need to know what loads will be powered.
Here’s a suggested list of items that a prospect sent us recently to have on backup.

  • Outlets & lights in the living room – 100W
  • Lights in the bedroom – 100W
  • Lights in master bathroom – 100W
  • Outlets & lights in the kitchen – 100W
  • Refrigerator & freezer – 400W each
  • HVAC system, 3-ton – 3,500W
  • Elec Stove – 1,000W
  • Microwave – 1,500W
  • Elec Water heater – 4,500W
  • Outdoor outlet – 100W

We’ve indicated next to each item its wattage, based on the type of appliance or load and using our best guess (based on our experience, Google research, and a list of appliances and their average wattage here.) If all of these loads were running simultaneously, we would need a total inverter capacity of 11,500W.

The Inverter

Maybe a 12 kW inverter (the biggest one available) would be a good size here. However, we also realize that some of these loads have start-up amp draws bigger than the continuous draw listed above. If we thought all of these loads would be on at once, we’d actually need a bigger inverter than 12 kW to cover the additional amp draw when the loads started. There’s a good chance that having the big loads (HVAC and heater) running might overload the inverter and shut the system down.

We’ve got other loads that we’re not taking into account, too. We call these “phantom loads” additional appliances and devices that need electricity throughout the day. Examples include the toaster, coffee maker, cell phone charging, television, and miscellaneous power that you will need during an outage. If these were all running simultaneously as our above list, we’d need even more inverter power.

Sizing the Battery Bank

We know that the likelihood of all of these loads being on at one time is low. We would probably want to look at the big load and see if we could determine the start-up amp draw. Likewise, one or both of those could likely overload the inverter just when they start. In this case, we’d most likely start with a single inverter and be prepared to add on additional capacity if the system requires it.

Now, we need to size the battery bank. The batteries provide electricity to these loads when the sun is not shining at night or on rainy days. The inverter draws DC current from the bank and inverts it to AC for the loads. How much electricity needs to be stored in your battery bank? This is calculated by the sum of wattage loads (W) multiplied by hours in use (hrs). This equals an energy unit call kilowatt-hours (kWh). It’s the same unit of electricity that the power company charges for your electricity use each month.

You’ll need to calculate how much kWh you need to have available for the loads you want to run during an outage.

Listing of Energy Needs

Take the same list of loads you want to backup and estimate, on average, how many hours per day that particular load will run. For example, the refrigerator uses 500W, but only when the compressor is running. We’ll assume that happens 12 hrs during a 24 hr period. Likewise, the microwave may only run 30 mins during that same 24 hr period. Using the same list above, we’ve provided our best estimate of daily usage based on our experience:

  • Outlets & lights in the living room – 8 hrs (0.8 kWh)
  • Lights in the bedroom – 4 hrs (0.4 kWh)
  • Lights in master bathroom – 2 hrs (0.2 kWh)
  • Outlets & lights in the kitchen – 3 hrs (0.3 kWh)
  • Refrigerator & freezer – 12 hrs (9.6 kWh)
  • HVAC system, 3-ton – 8 hrs (28 kWh)
  • Elec Stove – 1 hr (1 kWh)
  • Microwave – 0.5 hrs (0.75 kWh)
  • Elec Water heater – 3 hrs (13.5 kWh)
  • Outdoor outlet – 1 hr (0.1 kWh)

The Calculation

Multiplying the wattage by the average hours each appliance is expected to run each day, you get the energy required for each load…which we’ve added in the parathesis above.
We can’t ignore the phantom loads, either. Those are the additional appliances and devices that need electricity throughout the day. We’ll assume those phantom loads use another 5 kWh/day of energy.

All those variables require 60 kWh of energy storage to run these loads for a single day without sun. You probably want several days of backup in your battery bank to be ready for a couple of rainy days in a row. Referred to as “Days of Autonomy” (DOA), which equates to the number of days a battery bank can support the loads without being recharged by the sun. Assume you want 3 days of autonomy, and you need 60 kWh multiplied by 3 or 180 kWh of battery bank usable energy storage capacity.

Given that 10 kWh of battery storage will cost roughly $5,000, that’s $90,000 just for the battery bank! Then you need the 12 kW inverter (and a second one, most likely) and enough solar modules to provide 180 kWh of energy to the batteries each day. It becomes a costly system.

Managing the Battery Bank

We manage the battery backup costs by educating consumers about the energy usage of the major loads in a house. We start with the “big” loads…these are electrically driven motors or heating elements that use the biggest share of electricity in your house. They include your:

  • Oven
  • Range Top
  • Electric Central Heat
  • HVAC compressor
  • Hot Water Heater
  • Electric Clothes Dryer
  • 220V Pumps (pool, well, etc.)

These appliances pull more than 1 kW, and running them for any length of time can quickly drain a battery bank. For example, looking at our previous list, the HVAC system makes up almost 50% of the daily usage. That is true for every home…the HVAC system will use more energy than all other needs combined. Add in the Water Heater, and those 2 appliances make up about 70% of the daily usage in this example! Just dropping those 2 loads from the backup panel would save 70% of the battery bank size and costs.

However, it might not be feasible or desirable to drop those loads. You may want (or need) your HVAC to run when the utility power is down. We typically design a battery bank for a single Day of autonomy or enough power to supply the needs for one day.

Using a Generator

To make up the difference, we provide a connection for a generator on the system. The generator can augment the system during longer periods of poor sun, or additional energy is needed to run the heavier appliances. The generator can be a large fixed unit or even a smaller, portable one you purchase at a local hardware store. Some of the fixed generators have the ability to be remote started…and we can have the solar battery backup system auto start it when the bank is low on charge or the loads need extra electricity.


Adding a battery bank to your solar system is the only way to ensure you have uninterrupted power forever. Moreover, look at the loads you want to be backed up and make sure your budget can support your plan. Contact us, and we can help you with the system sizing, design, and installation of your solar battery system.

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