Sizing an off-grid solar system is not mysterious. There is a clear calculation behind it that any good installer should be able to explain. Once you understand this, the different quotes will make much more sense.
At PSC Energy, we install hybrid and battery systems across NSW every day and have experience designing off-grid solar systems even though we don’t offer this service anymore. We prefer to give you the formula so you can understand the process yourself and have some questions when approaching an installer.
In this article, you’ll learn about the following:
- Step 1: Work Out Your Daily Energy Use
- Step 2: Size Your Off-Grid Solar Battery Bank – The Three-Day Autonomy Rule
- Step 3: Size Your Solar Array – Oversizing Rules Still Apply to Off-Grid Solar
- Step 4: Size the Generator for Your Off-Grid Solar System
- Step 5: Single Phase vs Three Phase – Where the Numbers Change
- Common Sizing Mistakes to Avoid
- FAQ: Off-Grid Solar and Battery
By the end of this article, you’ll know how to calculate your daily energy use, how to size your off-grid solar batteries for three days of autonomy, how solar oversizing rules apply to off-grid solar power systems, and where single-phase and three-phase setups land differently.
Step 1: Work Out Your Daily Energy Use
Everything else in off-grid solar sizing depends on one number: how much energy your home uses each day, measured in kilowatt-hours (kWh).
If you are moving to a property without an existing power connection, you will not have energy bills to use for this calculation. This is how installers usually figure out your energy needs.
In that case, your installer will help you figure it out. They will usually ask, “Tell me what you’ve got, and we’ll work out roughly what it draws.”
Here’s what that walkthrough covers:
- Appliances and how long you use them: fridge, lighting, TV, washing machine, dishwasher, air conditioning, hot water system, power tools, and EV charger if you have one.
- Hours of use per day: use your honest average, not the worst-case scenario.
- Daily kWh per appliance = watts ÷ 1,000 × hours per day.
- Total daily kWh = sum of all appliances.
Your installer will then add 10 to 20% on top for things you might not have mentioned, like an extra fridge in the shed, standby loads, or extra use on hot days.
Here are a few things that often surprise people: Hot water is often the biggest load.
Electric hot water systems use 3 to 4 kWh per day. Heat pumps are much more efficient and are worth considering for off-grid solar systems.
- Air conditioning can quickly increase your energy use. For example, a 2.5 kW split system running for four hours uses 10 kWh, which can double the daily usage of a modest household.
- Standby loads can add up. Devices left on in standby mode still use power throughout the day.
If you’re interested in learning a bit more about how to save with solar, you might want to check out the following article titled, Self-Consumption: How to Increase Solar Energy Use.
Step 2: Size Your Off-Grid Solar Battery Bank – The Three-Day Autonomy Rule
Once you have your daily kWh figure, the battery sizing formula is straightforward:
- Daily energy use (kWh) × 3 days = baseline battery capacity
- Add a 10–20% buffer for unexpected loads.
- The result is your minimum usable battery capacity.
So if your home uses 20 kWh per day, you need 60 kWh of usable storage as a baseline. With a buffer, this becomes about 66 to 72 kWh.
Three days is the Australian industry standard because it covers most extended bad weather stretches in NSW and gives your generator time to kick in without running constantly. Going below it significantly increases the risk of running flat during a cloudy spell.
One important point: usable capacity is not the same as total capacity. Most lithium batteries discharge to about 20% before needing backup. So, a battery advertised as 10 kWh might only deliver 8 kWh in real use.
Always check the usable figure, not just the nameplate rating.
Without a generator, that buffer needs to grow significantly. Most installers will size for four to six days of autonomy instead of three, which means considerably more off-grid solar battery storage and a much higher upfront cost.
A generator is almost always the more economical fail-safe.
Two more things to factor in:
- Future loads: If you plan to add an EV, a workshop, or a granny flat, consider this now instead of retrofitting later.
- Stackable systems: Batteries like the Sigenergy SigenStor are modular, so you can add capacity over time if your system is designed for it from the beginning.
If you’re interested in learning a bit more about solar batteries and off-grid systems, you might want to check out the following article titled, What’s the Best Battery for Off-Grid Solar?
Step 3: Size Your Solar Array – Oversizing Rules Still Apply to Off-Grid Solar
A common assumption is that going off-grid means leaving the rulebook behind. It doesn’t. The CEC oversizing standards that govern grid-connected systems apply equally to off-grid solar inverters.
Your inverter can only handle a certain amount of solar input compared to its rated output. If you install too many panels, you will not generate more usable power; instead, you will lose the extra. These rules are there to protect the equipment, not because of grid requirements.
Here’s how oversizing works in practice:
- Single phase: up to 200% of the inverter’s rated output. On a Sigenergy system, this is the standard figure.
- Three-phase: the limit drops to 166% of the inverter capacity.
- Why oversize at all? Panels rarely produce their full rated output. Oversizing helps make up for shade, cloud cover, heat, and the fact that peak sun hours are not the average throughout the day.
- The rule applies to inverter size, not the battery bank — these are separate calculations.
One thing that changes off-grid is how much you oversize. When connected to the grid, any extra energy is wasted. Off-grid, you are charging batteries, so most designers choose a higher oversizing ratio to store as much solar energy as possible during good weather.
In short, the oversizing calculations for an off-grid solar power system are the same as for a grid-tied hybrid. If an installer says different rules apply off-grid, ask them to explain their reasoning.
If you’re interested in learning a bit more about oversizing, you might want to check out the following article titled, Oversizing Your Solar Panel System: How to Maximise Your Panels for Bigger Returns.
Step 4: Size the Generator for Your Off-Grid Solar System
The generator is the most flexible in terms of size. Unlike your battery bank and solar array, generator sizing is largely governed by your controller, which decides when the generator kicks in, how hard it runs, and when it shuts off.
Here’s what determines the right size:
- Controller input limit — the generator must not exceed the maximum charge input your controller can accept. Too large an overload, and it won’t charge efficiently.
- Whole-home load capacity — it should run your essential loads while simultaneously recharging the batteries.
- Automatic operation — modern systems activate the generator when batteries drop to 20–30% state of charge and shut it off once recharged. No manual involvement required.
- Run-time goals — if you want batteries recharged in 2 to 3 hours rather than 6, you need enough kVA to deliver that charge rate.
The generator isn’t designed to run your house, it’s designed to recharge your off-grid solar batteries quickly so the battery bank can take over again. For most residential off-grid solar systems in NSW, a quality 8–10 kVA generator hits the sweet spot.
If you’re interested in learning a bit more about off-grid systems, you might want to check out the following article titled, Do You Need a Generator with Off-Grid Solar?
Step 5: Single Phase vs Three Phase – Where the Numbers Change
Many homes in Australia are single-phase. Larger rural properties with pumps, workshops, or heavy machinery will often be three-phase. Knowing which applies to you matters because it affects both your inverter options and your maximum panel capacity.
Here’s where the two setups differ:
- Single phase: up to 200% panel oversizing, simpler design, lower cost, and suitable for most residential off-grid builds.
- Three-phase: up to 166% panel oversizing, often uses multiple inverters in parallel for redundancy, and requires load balancing across phases.
- Three-phase is worth specifying when the property runs large pumps, irrigation systems, commercial refrigeration, or workshop machinery that requires it.
The basic sizing method is the same for both. Your daily kWh calculation, three-day battery target, and generator sizing logic do not change. Phase type affects your inverter specification and oversizing limit, not the main calculations. Confirm which type applies to your property early, as it shapes your inverter options from the start.
If you’re interested in learning a bit more about single-phase and three-phase sites, you might want to check out the following article titled, Single-Phase vs. Three-Phase: How Are They Different?
Common Sizing Mistakes to Avoid
Here are the errors that come up again and again:
- Sizing to nameplate capacity instead of usable capacity: always base your three-day calculation on usable kWh, not just the label.
- Underestimating daily energy use: standby loads, hot water, and air conditioning spikes often catch people by surprise. Make sure to include a buffer.
- Skipping or undersizing the generator: without one, your off-grid solar battery bank needs to be significantly larger. A correctly sized generator is almost always cheaper
- Oversizing the generator past the controller’s input limit: more kVA doesn’t mean faster charging if the controller can’t accept it.
- Ignoring future loads: retrofitting a larger off-grid solar battery or adding more panels later is more expensive and can cause design problems.
- Getting the phase wrong early: choosing a single-phase off-grid solar inverter for a three-phase property means you will need to redesign the system from scratch.
If an installer cannot explain why they have chosen a particular battery size, array capacity, or generator rating, keep asking questions or seek another opinion.
If you’re interested in learning a bit more about shopping for a solar system, you might want to check out the following article titled, 6 Mistakes to Avoid When Buying Solar.
Wrapping Up: No Grid, No Problem
Sizing an off-grid solar system comes down to four numbers in the right order: your daily kWh, your three-day battery bank, your solar array within the oversizing limits, and a generator sized to your controller. Get those right and everything else falls into place.
PSC Energy installs hybrid and battery systems across NSW and has experience designing off-grid solar systems. If you want someone to check your numbers or are ready to turn your calculations into a real system, we are here to help. It’s what we do.
If you’re interested in learning more about the Cheaper Home Batteries Program and it’s recent changes, you might want to check out the following article titled, Changes to the Australian Government’s Cheaper Home Batteries Program Explained.
FAQ: Off-Grid Solar and Battery
How much battery storage do I need for an off-grid solar system?
Start with your daily energy use in kWh and multiply by 3. This gives you the baseline storage for three days of autonomy, which is the Australian industry standard for off-grid solar batteries. Add a 10 to 20% buffer for unexpected loads, and always check that your installer is using usable capacity, not just the nameplate rating.
How many solar panels do I need for off-grid solar?
It depends on your inverter size and whether you have single-phase or three-phase. Single-phase off-grid solar systems allow up to 200% panel oversizing, so a 10 kW inverter supports up to 20 kW of panels. Three-phase systems allow up to 16%.
Do CEC oversizing rules still apply to off-grid solar power systems?
Yes. The same rules that apply to grid-connected hybrid systems apply to off-grid solar inverters. The inverter can only process a certain amount of solar input regardless of grid connection. The rules protect the equipment, not the network.
Do I need a generator with an off-grid solar system?
You are not required to have a generator, but for most properties it is the sensible choice. Without a generator, you need to size for four to six days of autonomy instead of three, which means much more battery storage and a higher upfront cost. A quality 5 to 15 kVA generator is almost always the more economical safety net.
What’s the difference between single-phase and three-phase for off-grid solar?
The main differences are in panel oversizing limits and inverter options. Single-phase allows up to 20%, while three-phase allows 166%. For most residential off-grid solar builds in NSW, single-phase is simpler and more cost-effective. Three-phase is best when the property has equipment that needs it.
Can I add more batteries to my off-grid solar system later?
Yes, provided the system is designed for expansion from the start. Stackable systems like the Sigenergy SigenStor let you add battery modules as your needs grow. Flag it at the design stage so the inverter and controller are specified to accommodate it.
