Battery Bank Sizing Consideration Ensuring That You Have Uninterrupted Power Forever #lithium #solar

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.

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.

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.

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.

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)


Adding a battery bank to your solar system is the only way to ensure you have uninterrupted power forever.