If you’re shopping for a home battery, you’ve probably seen terms like “total capacity,” “usable capacity,” and “DoD.” These words can be confusing, especially when you’re trying to work out how much energy you’ll be able to use.
At PSC Energy, we help Aussie homeowners avoid surprises. If your battery doesn’t perform as expected, no one is happy. This doesn’t always mean the battery is bad. It often means nobody explained the fine print. Which is what we’re here to help with today.
In this article, you will learn about the following:
- Power vs Energy: What’s the Difference Between kW and kWh?
- Total Battery Capacity vs Usable Battery Capacity
- What Is Depth of Discharge (DoD)?
- What Is Backup Reserve and Why Does It Matter?
- Battery Degradation Over Time
- FAQ: Home Battery Terms Explained
By the end of this article, you’ll know what these terms mean, how they affect your battery’s real-world performance, and how to make sense of the numbers.
Power vs Energy: What’s the Difference Between kW and kWh?
If you’re comparing battery specs or solar system quotes, you’ll notice two terms that look almost the same: kW and kWh. But they mean very different things.
Let’s break it down.
- kW (kilowatt) = power
- kWh (kilowatt-hour) = energy
Power is a measure of how fast something can work.
Energy is the measure of how much work it can do over time.
Think of it like filling a bathtub:
- The power is how wide the tap is. The wider it is, the faster the water comes out.
- The energy is how much water ends up in the tub.
A home battery’s power rating (in kW) indicates the maximum amount of electricity it can deliver simultaneously.
Its energy rating (in kWh) indicates how long it can deliver the electricity it has stored.
Here’s an example:
- A Powerwall 3 can deliver up to 11.5 kW of power at peak.
- It has 13.5 kWh of usable energy.
That means it can run big loads (like air conditioners and ovens) and still last a few hours, depending on what you’re using.
If your home uses 1.5 kW of power every hour, a 13.5 kWh battery could run it for about 9 hours.
Most Aussie households use between 20 and 30 kWh of electricity per day, but not all at once. That’s why understanding both power and energy is crucial when selecting the correct battery.
Please note that the above figure of 11.5 kW of power is capped in NSW at 10 kWh due to DNSP inverter regulations.
If you’re interested in learning a bit more about power and energy, you might want to check out the following article titled, Power vs. Energy: kW vs. kWh.
Total Battery Capacity vs Usable Battery Capacity
Home batteries store energy in kilowatt-hours (kWh). That’s the same unit used on your electricity bill. However, not all of that stored energy is available for your use.
Total capacity (also called “nominal capacity”) is the full size of the battery.
Usable capacity is how much of that total you can use to power your home.
- Total Capacity: 13.5 kWh
- Usable Capacity: 13.5 kWh
- The Powerwall 3 allows 100% depth of discharge, meaning its usable capacity equals its total capacity.
- Total Capacity: 5.0 kWh
- Usable Capacity: 5.0 kWh
- Enphase states that the IQ Battery 5P offers 100% usable capacity.
Sigenergy SigenStor (8 kWh module)
- Total capacity: 8.0 kWh
- Usable capacity: 8.0 kWh
- That’s 100% usable energy.
PSC only installs the 8kWh SigenStor module. We don’t offer the 5kWh models available in other markets. Each module features its own power conversion system, allowing them to be stacked and scaled easily while maintaining efficiency.
Most modern batteries protect themselves by holding back a small amount of energy at the top or bottom of the charge. This helps extend the battery’s life and reduce wear.
If you’d like to learn a bit more about the cost of SigenStor, we recommend you check out the following article titled, How Much Does the Sigenergy SigenStor Cost?
What Is Depth of Discharge (DoD)?
If a datasheet doesn’t specify the usable capacity, look for a metric called Depth of Discharge (DoD). This indicates the percentage of the battery’s total capacity that you’re allowed to use.
Here’s how it works:
- A 10 kWh battery with a 90% DoD means you can use 9 kWh.
- A 13.5 kWh battery with 100% DoD means you can use all 13.5 kWh.
- A 13.44 kWh battery with 96.7% DoD means you get 13.0 kWh.
Most lithium batteries have a DoD between 90% and 100%. Anything below 90% is a red flag, unless there’s a specific technical reason. Some cheaper batteries hide this number deep in their warranty documents, so it’s always worth asking.
If you’d like to learn more about the price of a Powerwall 3, you might want to check out the following article titled, How Much Does a Tesla Powerwall 3 Cost?
What Is Backup Reserve and Why Does It Matter?
If you want your battery to power your home during blackouts, you need to set a backup reserve. This is a portion of your battery that’s always saved for emergencies.
Let’s say your battery has 13 kWh of usable capacity and you set a 20% reserve. That means:
- 2.6 kWh is always saved for blackouts
- 10.4 kWh is available for daily use
You can usually adjust your backup reserve in your battery’s app. Some people set it higher if they live in an area with frequent outages. Others keep it lower to get more value each day.
Here’s what it might look like for each battery:
- Powerwall 3: 13.5 kWh total, 100% usable, reserve fully adjustable
- Enphase IQ Battery 5P: 5.0 kWh usable, also adjustable
- SigenStor (8 kWh module): 8.0 kWh usable, app-controlled reserve via mySigen
It’s important to understand that a backup reserve reduces your daily energy savings. If you want to keep more for emergencies, you’ll need to size your battery a bit larger.
If you’re interested in learning a bit more about IQ Batteries, you might want to check out the following article titled, How Much Does an Enphase IQ Battery 5P Cost?
Battery Degradation Over Time
Batteries lose a small amount of storage capacity in each year of operation. This is normal. It’s called degradation, and it occurs in every type of battery, including those in your phone and car.
Each solar battery has a performance warranty that indicates how much capacity it will retain after a specified number of years or charging cycles.
Here’s how our top batteries compare:
Tesla Powerwall 3
- 10-year warranty
- At least 70% capacity remaining after 10 years
Enphase IQ Battery 5P
- 15-year warranty or 6,000 cycles
- At least 60% capacity after 15 years
SigenStor
- 10-year warranty
- At least 70% capacity remaining after 10 years
So, if you buy a battery that meets your current energy needs, it may fall short a few years from now. But going too big can also cost you more upfront. That’s where custom system sizing comes in.
If you’re interested in solar batteries, you might want to check out the following article titled, Are Solar Batteries Worth It in NSW? PSC’s Ultimate Guide for 2025.
Wrapping Things Up: Battery Terms Explained
Not all battery capacity is created equal. Always look for usable capacity, understand your backup reserve settings, and consider how degradation will impact you in the long run.
PSC Energy only quotes the real usable capacity in every proposal. We don’t hide specs behind fancy marketing. We want you to know exactly what you’re getting.
If you’re unsure about how much battery storage you need or which battery offers the best long-term value, we’ll help you figure it out. You’ll walk away confident, informed, and never oversold.
If you’re interested in learning a bit more about the price of solar batteries, you might want to check out the following article titled, How Much Are Solar Batteries? A Full Breakdown of Prices, Rebates, and Value in 2025.
FAQ: Home Battery Terms Explained
What is usable capacity in a home battery?
Usable capacity is the amount of energy you can actually draw from your home battery. It’s usually a bit less than the total battery size listed in kWh.
What is the difference between total capacity and usable capacity?
Total capacity is the full size of the battery. Usable capacity is how much of that energy your home can use. Most modern batteries offer 90–100% usable capacity.
What does depth of discharge (DoD) mean?
Depth of discharge (DOD) is the percentage of a battery’s total capacity that can be safely used. A 90% DoD on a 10 kWh battery means 9 kWh is usable.
How does backup reserve affect my battery?
Backup reserve is the portion of your battery saved for blackouts. If you set a 20% reserve, only 80% of the usable capacity is available for daily use.
Do batteries lose capacity over time?
Yes. All batteries degrade. Most solar batteries lose about 20–30% of their capacity over 10 years, depending on usage and battery type.
How do I choose the right size battery for my home?
Start by checking your daily energy use. Then look at usable capacity, backup reserve settings, and expected degradation. A good installer will size it correctly.
Which batteries offer 100% usable capacity?
Tesla Powerwall 3, Enphase IQ Battery 5P, and Sigenergy’s SigenStor all offer 100% usable capacity.
