Battery storage is one of the fastest-growing parts of the residential and commercial solar market in Australia. For electricians transitioning into solar, it also brings one of the steepest learning curves. Size a battery system incorrectly and a customer ends up with a system that doesn't meet their needs, a battery that cycles inefficiently, or an inverter that isn't compatible with their load profile.

This guide covers the fundamentals of battery storage system sizing: the key questions to ask, the numbers you need, the concepts worth understanding and a practical framework you can apply on the job.

What are you sizing for?

Before you pull any numbers from a customer's electricity bill, you need to understand what the battery is expected to do. There are two primary use cases, and they lead to different sizing outcomes.

Self-consumption optimisation

The most common residential use case. The battery stores excess solar energy generated during the day and makes it available in the evening when the sun is no longer producing. The goal is to reduce grid imports and maximise the value of solar generation.

Backup power

The customer wants the battery to keep certain loads running during a grid outage. This could be the whole house or a defined set of critical loads such as lighting, refrigeration, a medical device or a router. Backup sizing is typically more conservative and often results in a larger battery than a self-consumption-only recommendation.

Many customers will want both. When that's the case, size for backup first. It will usually produce a larger, more appropriate battery than sizing for self-consumption alone.

What you need before you start

Accurate sizing starts with accurate information. Collect the following before making any recommendations.

• Recent electricity bills covering at least three months, to establish daily consumption in kWh and identify any seasonal variation
• Time-of-use tariff details, if applicable. Know when the customer is being charged peak rates and whether they have a controlled load tariff
• The size of the existing or proposed solar PV system in kW
• The customer's load profile if possible. When do they use the most electricity? Are there high-draw appliances such as a pool pump, air conditioner or EV charger?
• A list of critical backup loads if backup is a requirement. Note the wattage of each appliance and estimated hours of daily use
• The site's phase configuration. Single-phase or three-phase determines which inverter and battery combinations are suitable

Key terms explained

The battery industry uses a lot of terms that aren't immediately obvious to someone coming from a residential electrical background. These are the ones that directly affect sizing.

Rated capacity vs usable capacity

Rated capacity is the total storage a battery is advertised to have, expressed in kilowatt-hours (kWh). Usable capacity is how much of that you can actually draw on without causing damage to the battery cells. A 10 kWh battery with 90% depth of discharge has 9 kWh of usable capacity. Always work from the usable figure.

Depth of discharge (DoD)

DoD is the percentage of rated capacity a battery can be discharged to under normal operating conditions. Most modern lithium iron phosphate (LFP) batteries are rated at 80 to 90% DoD. Some manufacturers publish their usable capacity directly, which simplifies the calculation. Check the product datasheet.

C-rate

C-rate describes how quickly a battery can be charged or discharged relative to its capacity. A 1C rate means the battery charges or discharges fully in one hour. A 0.5C rate means it takes two hours. C-rate is relevant when matching a battery to high-draw loads or when the customer wants fast charging from solar during the shoulder of the day.

AC-coupled vs DC-coupled

In an AC-coupled system, the battery connects to the AC side of the switchboard, often through its own hybrid inverter. This makes it easier to retrofit onto an existing solar system with any brand of inverter. In a DC-coupled system, the battery connects on the DC side, directly to the solar inverter. DC-coupling is generally more efficient but requires a compatible hybrid inverter and battery combination from the outset.

For customers with an existing grid-connect solar system, AC-coupled is usually the more practical option. For new installations, DC-coupled is worth considering if the inverter and battery are compatible.

Single-phase vs three-phase

Single-phase properties can use most residential battery systems without issue. Three-phase properties require a battery and inverter combination that supports three-phase output, or three separate single-phase units, one per phase. Sizing a single-phase battery for a three-phase property is a common and costly mistake for installers new to the category.

A practical sizing framework

The following process works for most residential sizing scenarios. For commercial or industrial installations, the same principles apply but the load analysis is typically more detailed.

Sizing for self-consumption

1. Establish average daily consumption. Take the total kWh from the electricity bill and divide by the number of days in the billing period. Note if usage varies significantly between summer and winter.
2. Estimate daytime solar generation. A well-oriented 6.6 kW system in Canberra might generate 5 to 6 kWh in winter and 9 to 10 kWh in summer on a typical day. The difference between total daily consumption and daytime solar generation is the approximate evening demand the battery needs to cover.
3. Size usable capacity to meet evening demand. If the customer uses 20 kWh per day and generates 12 kWh with solar during daylight hours, they'll draw around 8 kWh from the grid in the evening. A battery with 8 to 10 kWh of usable capacity would cover most of that.
4. Convert usable capacity to rated capacity. Divide the usable capacity you need by the battery's DoD. For example: 9 kWh needed at 90% DoD requires a rated capacity of 10 kWh.
5. Check inverter compatibility. Confirm the inverter can handle the battery's charge and discharge rates and that phase configuration matches.
6. Account for system losses. Battery round-trip efficiency for LFP is typically 90 to 95%. Inverter losses add another 3 to 5%. Build a small buffer into your sizing to account for this.

Sizing for backup

1. List the critical loads. For each appliance, note the running wattage (not the startup wattage) and estimated hours of use per day during an outage.
2. Calculate total daily backup energy. Multiply each appliance's wattage by its daily hours, sum the results and convert to kWh. For example: fridge (150W for 24 hours) + lighting (100W for 5 hours) + router (20W for 24 hours) = 3.6 kWh + 0.5 kWh + 0.48 kWh = 4.58 kWh per day.
3. Decide how many days of backup the customer needs. Most residential customers are comfortable with one to two days. Critical care or remote properties may require more.
4. Calculate rated capacity. Multiply daily backup energy by the number of backup days, then divide by DoD. From the example above: 4.58 kWh per day, two days backup, 90% DoD gives a rated capacity of approximately 10.2 kWh.
5. Check peak output. The battery's maximum continuous output in kW determines what loads it can run simultaneously. A battery rated at 5 kW continuous output cannot run a 3 kW air conditioner and a 2.5 kW hot water system at the same time.

Quick sizing reference

The table below provides indicative battery sizes for common residential scenarios based on self-consumption optimisation. These are starting-point figures only and should be validated against site-specific solar design tool output.

Figures assume 90% DoD and are indicative only. Validate with a solar design tool before presenting recommendations to customers.

Common mistakes to avoid

• Sizing from rated capacity instead of usable capacity. A 10 kWh battery is not 10 kWh of available storage. Always confirm the DoD and work from the usable figure.
• Mismatching battery to inverter. Not all batteries work with all inverters. Check compatibility before specifying, particularly when retrofitting storage onto an existing solar system.
• Ignoring phase configuration. Recommending a single-phase battery for a three-phase property will result in a non-functional system or a callback after install.
• Forgetting peak output. A customer expecting to run an air conditioner and a pool pump simultaneously needs a battery and inverter with sufficient peak continuous output capacity.
• Not accounting for battery degradation. Most LFP batteries retain 80% or more of their capacity after 3,000 to 4,000 cycles. For customers asking about long-term performance, build a small buffer into the initial sizing.
• Undersizing for backup goals. Self-consumption sizing often produces a smaller battery than backup sizing. If the customer mentions wanting backup at any point, confirm their expectations before finalising the recommendation.

Using design tools to validate your sizing

Manual calculations are useful for a quick indicative size and for building your understanding of the fundamentals. Before presenting a formal recommendation to a customer, validate the sizing using a solar design tool.

Renewables Warehouse supports OpenSolar and Pylon for system design and quoting. Both tools allow you to input site-specific data including roof orientation, shading, consumption profile and tariff structure, and model the expected performance of a battery system alongside the solar array.

Design tool output gives you a defensible, site-specific recommendation and significantly reduces the risk of an undersized or mismatched system.

Learn more at the Renewables Warehouse Academy

This guide covers the fundamentals. The Renewables Warehouse Academy Battery Storage Systems module takes you further, with hands-on content covering advanced sizing scenarios, grid-interactive systems, battery management systems (BMS), compliance requirements and product-specific training with live hardware. Sessions are held at our Canberra (Mitchell) and Perth (Belmont) warehouses. Class sizes are kept small to allow for practical, hands-on learning.