How to Size a Battery Bank for a Solar Power System

When it comes to building a solar power system, few components are as critical as the battery bank. Whether you live full-time off-grid, travel in an RV, or just want reliable backup power, sizing your battery bank ensures you always have the energy you need.

In this guide, we’ll walk you through how to size a battery bank step-by-step—so your system runs smoothly and efficiently.

If you're new to solar, you may also want to read our guide to building an off-grid-solar-power-system for context before diving in.

What Is a Battery Bank and Why Does Sizing Matter?

A battery bank is a group of batteries wired together to store electrical energy generated from your solar panels. That stored energy powers your devices and appliances when the sun isn’t shining—such as overnight or during cloudy weather.

Proper sizing is crucial. Undersize it, and you risk running out of power when you need it most. Oversize it, and you waste money and space on unnecessary capacity. A correctly-sized battery bank balances reliability, performance, and cost.

Step 1 – Calculate Your Daily Power Consumption

Start by calculating your total daily energy usage.

List every device you plan to use, along with how many watts it draws and how long you use it per day. Dropping this data into a spreadsheet can be really helpful, but even a simple list written out in columns on a piece of paper will do the job.

If you are not sure of the wattage requirements for a device, then you can use an energy meter, such as a plug-in watt meter, to get precise measurements of individual appliances.

Or, just do it the old-fashioned way and Google it.

We recommend keeping a daily usage log for a few days to see how your energy needs vary. You can also use whole-home monitoring systems to track total consumption, especially if you're sizing a system for your house or cabin.

Here are some sample power requirements for a variety of standard devices and appliances.

Okay! You now have your baseline energy requirement. Let's move on to the next step.

Step 2 – Determine Your Required Battery Capacity

With your daily consumption in watt-hours (Wh), use the following formula to calculate the total battery capacity, which is measured in amp hours (Ah), you will need:

Battery Bank Capacity (Ah) = (Total Daily Load in Wh × Days Without Recharge) ÷ (System Voltage × Depth of Discharge)

For example:

• Daily load: 1,600 Wh

• Days without recharge: 2

• System voltage: 24V

• Depth of discharge (DOD): 0.8 (80% is typical for lithium batteries)

(1,600 × 2) ÷ (24 × 0.8) = 166.67 Ah

That means you need at least 166.67 amp-hours of usable capacity at 24 volts.

Step 3 – Choose the Right System Voltage (12V, 24V, or 48V)

Your system voltage affects everything from wiring size to inverter selection:

• 12V: Ideal for small-scale RV or boat systems

• 24V: Most common for mid-size setups and DIY solar kits

• 48V: Preferred for whole-home or commercial systems

Higher voltages reduce current, allowing you to use thinner wires and improving efficiency. Match your battery bank voltage to your inverter and charge controller specifications.

Step 4 – Decide How Many Days of Autonomy You Need

"Autonomy" means how many days your battery bank can power your system without solar input (e.g., during cloudy weather).

• 1 day: Cost-effective, minimal backup

• 2 days: Balanced, standard recommendation

• 3+ days: Best for critical applications or limited sun exposure

Think about your environment and how much downtime you can tolerate. Those in sunny regions may get by with one day; those in forested or cloudy areas might need more.

Step 5 – Select the Right Battery Type

Your battery choice impacts your system's performance, longevity, and maintenance needs. Here are the three main types:

• Lithium (LiFePO4): Long lifespan, fast charging, deep discharge, high efficiency. Ideal for most use cases.

• AGM (Absorbed Glass Mat): Maintenance-free and spill-proof but fewer charge cycles than lithium.

• Flooded Lead-Acid: Least expensive upfront, but requires maintenance and has a limited usable capacity.

We strongly recommend lithium batteries for solar users who want the best long-term value.

Example Battery Bank Sizing Scenario

Let’s look at an example:

• Daily load: 1,600 Wh

• Days of autonomy: 2

• System voltage: 24V

• Battery type: Lithium (80% usable capacity)

Required capacity = (1,600 × 2) ÷ (24 × 0.8) = 166.67 Ah

To meet this need, you could:

• Use two 12V 100Ah lithium batteries wired in series

• Or use one 24V 200Ah battery for simpler setup

Pro Tips for Battery Bank Setup

• Recharge window: Make sure your solar array can refill your battery bank in 4–6 hours of sunlight. Consider pairing it with a solar charge controller for optimal performance.

• Battery monitoring: Use a battery monitor to track state of charge and avoid over-discharge.

• Thermal protection: Insulate or ventilate your battery space to prevent damage from temperature extremes. You can find temperature-controlled enclosures and accessories to help regulate battery temperature.

• Leave a buffer: Add 10–20% above your minimum required capacity to improve performance and reduce battery stress.

Final Thoughts: Make Sure Your Battery Bank Works for You

Sizing a battery bank correctly is one of the smartest things you can do for your solar setup. With a properly sized system, you won’t have to worry about running out of power when it matters most.

Still Need Help Sizing A Battery Bank?

Reach out to our friendly portable power experts and we'll be happy to help!

Toll-free & text: 888-976-5443
Email: support@outboundpower.com