What Size Battery Do I Need For My House? (Sizing Guide & Calculator)

For a typical energy-efficient 4-bedroom Australian home with a 6.6kW solar system, a 10kWh to 13.5kWh battery is the standard requirement to cover evening energy usage from approximately 4pm through to 8am the following morning.

Before we go further, let’s clear up one point of confusion. Home batteries are measured in kWh (kilowatt-hours), which represents capacity. If you’ve been researching camping or caravan batteries, you’ll have seen ratings in Ah (amp-hours). This guide covers both measurements and explains how they relate to each other.

You don’t need to run complex calculations to get a ballpark figure. Match your household to one of these common scenarios.

Grab your electricity bill right now and do this:

Find your “Average Daily Usage” figure (it’s usually displayed in kWh). Divide that number by 2. The result is the minimum battery size you need to cover your night-time usage.

If your bill shows 20kWh daily usage, you’re looking at a minimum 10kWh battery. Simple as that.

Want a more accurate figure based on your specific situation? Use our battery sizing calculator to factor in your solar system, usage patterns, and priorities.

This is where most people get stuck. You’ve seen 100Ah batteries advertised for a few hundred dollars and wondered why home batteries cost thousands. Here’s the reality check.

A 100Ah battery at 12V provides just 1.2kWh of usable power. A typical Australian home uses around 20kWh per day. You would need seventeen 100Ah batteries wired together to run a standard house for a single day. Those cheap camping batteries aren’t designed for whole-house backup.

Notice that a 100Ah camping battery won’t even run your air conditioner. The inverter simply can’t handle the load. This is why purpose-built home batteries exist, they’re designed to handle high-draw appliances that 12V systems can’t touch.

A large battery is worthless if your solar panels can’t fill it, particularly during winter when daylight hours are shorter and cloud cover is more common.

Use the 5-hour charge rule to work out whether your solar system and battery are well matched. Divide your battery size in kWh by 5 (representing peak sun hours) to find the minimum solar array output you need.

For a 13.5kWh battery like the Tesla Powerwall, you’d calculate: 13.5 ÷ 5 = 2.7kW of constant solar output required. In practice, accounting for clouds, shading, and household consumption during the day, you realistically need a 6.6kW system to reliably charge a battery of this size.

If you have a 6.6kW solar system, you’ll maximise self-consumption with a 9.6kWh to 13.5kWh battery. This setup captures excess daytime generation without being so large that your panels struggle to fill it.

For 10kW systems, you’re generating significant excess power. Consider 13.5kWh to 20kWh of storage to reduce grid exports and maximise savings to ensure having the battery is worth it. With current feed-in tariffs sitting around 5-7 cents per kWh in most states, storing that power for evening use at 30+ cents per kWh makes financial sense.

Not all battery capacity is actually usable. Understanding Depth of Discharge (DoD) will stop you from destroying an expensive battery within a few years.

You’ve likely heard of the “80/20 rule”, only use 80% of capacity and keep 20% in reserve. For AGM batteries, the reality is even more restrictive. Regularly discharging below 50% dramatically shortens battery life. A 200Ah AGM battery effectively gives you 100Ah of usable power if you want it to last.

Modern lithium home batteries allow 90-100% depth of discharge without significant degradation. This means 100Ah of lithium provides nearly double the usable capacity of 100Ah of AGM. The higher upfront cost of lithium delivers substantially more value over a 10-year lifespan.

If you’re building a custom system, a single 200Ah battery is generally the better choice. It reduces wiring complexity, balances charging loads more effectively, and typically saves space in your installation.

Can you go off-grid with a 10kW solar system? Technically yes, but the battery bank becomes your bottleneck and your biggest expense.

Going genuinely off-grid means sizing for autonomy, not just overnight storage. The standard approach is planning for 3 consecutive days without meaningful solar generation (think heavy cloud cover or rain).

Calculate it this way: multiply your daily usage by 3. If you use 20kWh daily, you need a 60kWh battery bank to maintain power through three overcast days.

At current Australian prices, that battery bank alone will cost between $30,000 and $40,000. Add the solar array, inverters, and installation, and you’re looking at a significant investment. For most suburban homes connected to the grid, hybrid systems make far more financial sense than full off-grid setups.

Battery prices have dropped substantially over the past five years, but this remains a significant purchase.

A Tesla Powerwall 3 currently sits around $12,000 to $14,000 installed, depending on your location and existing electrical setup. Mid-range options typically fall between $8,000 and $12,000 for 10kWh of capacity.

When comparing AGM to lithium, AGM batteries cost less upfront but lithium works out cheaper per cycle over a decade. Given lithium’s superior depth of discharge and longer warranty periods (typically 10 years versus 2-3 years for AGM), most installers now recommend lithium for home battery systems without hesitation.

Pull out your latest electricity bill and check your average daily usage. Divide by 2 for a baseline battery size, then consider whether you want coverage for heavy-use evenings, backup during outages, or maximum self-consumption from your solar investment.

Ready to find the right battery for your home? The team at Skyline Solar can assess your current setup, review your energy usage patterns, and recommend a battery solution matched to your household’s needs and budget. Whether you’re adding storage to an existing solar system or planning a complete solar and battery installation, their accredited installers will handle everything from system design through to commissioning.

Get a free quote from Skyline Solar or call to chat through your options with an expert who understands Australian energy needs.