How to Size a Solar Battery for Your Home’s Energy Needs

You’ve got the solar panels. The sun shines, your meter spins backwards, and for a few hours a day, you’re living the dream of free, clean energy. But then 5 PM hits. The sun dips, the panels switch off, and your home starts pulling expensive, fossil-fuelled power from the grid again. It feels like the job is only half done.

Finding the right solar battery size is the key to finishing that job—to capturing all that free daytime energy so you can use it long after sunset. But the process can feel buried in technical jargon, confusing formulas, and conflicting advice.

This guide changes that. We're going to walk you through a simple, three-step process to find the perfect battery size for your home. Forget the complexity. By the end of this article, you'll have a reliable number that gives you peace of mind and complete control over your power.

"The perfect battery isn't the biggest one; it's the one that's perfectly matched to how you live."

First, Choose Your Mission: Savings, Security, or Freedom?

Before we touch a single number, the most important question is: what do you want your battery to do? Your primary goal is the single biggest factor in determining the right solar battery size. Most homeowners fall into one of three camps.

The Bill Buster: Use Solar Power After Sunset

This is the most common mission. With Feed-in Tariffs at an all-time low and peak electricity prices soaring in the evening, storing your own solar is the smartest financial move.

Your battery charges for free during the day. Then, as soon as the sun sets and the expensive "peak" tariff window opens (usually 3 PM - 9 PM), your home automatically switches to run on that stored solar energy. For this goal, the perfect battery size is one that covers your energy use from sunset until you go to bed.

The Blackout Shield: Keep the Lights On, No Matter What

If your top priority is keeping your home powered during a grid outage, you're looking for "autonomy." This means having enough stored energy to run your essential appliances—lights, the fridge, internet, medical devices—when the grid fails.

Sizing for autonomy often means choosing a slightly larger battery than you would for just bill savings. It’s about creating a deep reserve of power, ensuring your family has ultimate peace of mind when the unexpected happens.

The Freedom Package: Maximum Grid Independence

For those wanting to get as close to off-grid as possible, the goal is maximum self-sufficiency. This involves a battery large enough to cover your entire overnight and morning energy needs, virtually eliminating your reliance on the grid.

This approach is also ideal for joining a Virtual Power Plant (VPP), where you can sell your stored energy back to the grid for a profit.  This mission requires a larger battery, often paired with more solar panels to ensure it gets fully charged every day.

The 3-Step Recipe for Your Perfect Battery Size

Ready to find your number? Grab your latest power bill and a calculator. This simple formula is the foundation for a professional-grade sizing calculation.

Step 1: Find Your Daily Energy Use (kWh) on Your Bill

Look on your electricity bill for a section labelled "Average Daily Consumption" or similar. It will show a number followed by "kWh". This is the average amount of electricity your household uses in a 24-hour period, and it's our starting point.

If you can, check both a summer and winter bill. We always want to size for your season of highest use to ensure you're covered year-round.

Step 2: Calculate Your Nightly Need (The 70% Rule)

Your total daily kWh is the big picture, but your battery only needs to cover the period when your solar panels aren't working. For most homes, about 70% of energy is used in the evening and overnight.

[Your Daily kWh] x 0.70 = Your Ideal Battery Capacity

For example, if your bill says you use 20 kWh per day:
20 kWh x 0.70 = 14 kWh

This "sunset-to-sunrise" number of 14 kWh is the baseline for your perfect battery size.

Step 3: Add a Future-Proof Buffer (The 20% Rule)

Sizing a battery only for your past usage is like driving while looking in the rearview mirror. Your energy needs will likely grow. The biggest game-changers are:

• Electric Vehicles (EVs): Can easily double your daily energy consumption.
• Heat Pumps & Air Conditioners: Switching from gas heating to electric will significantly increase winter usage.
• All-Electric Appliances: Induction cooktops and electric hot water systems.

A smart rule of thumb is to add a 20% buffer to your nightly need.

[Your Ideal Battery Capacity] x 1.20 = Your Future-Proofed Size

Using our example:
14 kWh x 1.20 = 16.8 kWh

This means a battery system around 13.5 kWh to 19 kWh is the perfect, future-proofed target for a home using 20 kWh per day.