If you’ve got a 100Ah battery and you’re wondering what size solar panel you need to keep it charged—especially while you’re camping, caravanning, or running an off-grid setup somewhere remote—you’re not alone.
This is one of the most common questions Aussies ask when sorting out their solar gear.
Makes sense too.
With so much sun across most of Australia, solar’s a no-brainer. But with so many panel sizes out there, it can get confusing quick.
Too small and your battery won’t charge properly. Too big and you’ve overspent. Plus, sunlight hours change a fair bit depending on whether you’re in Darwin, Melbourne, or out in the bush.
So how do you figure out the right solar panel size for a 100Ah battery that actually works in Aussie conditions?
Let’s break it down.
Best Solar Panel Size for a 100Ah Battery in Australia
The best solar panel size for a 100Ah battery in Australia often lands around 300W, especially when you’re using a 12V system.
A 100Ah battery at 12 volts gives you about 1,200 watt-hours (Wh) of energy to replace.
To do that in one day, your panel needs to produce that much during the hours when the sun’s really shining.
Most places in Australia get 4 to 6 peak sun hours daily, depending on where you are and the time of year.
In Perth or Brisbane, summer brings up to 6 hours of solid sun.
Sydney averages closer to 5.
If you’re down in Tasmania or heading out in winter, expect closer to 3–4 hours.
That makes a big difference to how much energy you can realistically generate each day.
Why a 300W Panel Isn’t Always Enough
Even if your panel is rated at 300W, it won’t hit that number every single hour.
You’ll lose around 20–25% to real-world factors like:
- Heat – panels get less efficient when they’re hot
- Wiring resistance
- Shading from nearby trees, antennas, or gear
So even on a decent day, you might only get around 225–240W per hour from a 300W panel.
That’s why going up to 350W or 400W makes sense if you’re in a cloudier area or travelling in winter.
Also helpful if your panel is fixed flat on a van or roof and can’t be tilted toward the sun.
Here’s a quick guide:
- Brisbane or Perth (summer) – 300W is usually enough
- Hobart or Blue Mountains (winter) – go for 350W–400W
- Off-grid setups – add a second panel or use a bigger solar controller
Planning for the worst-case sun conditions means you won’t be stuck with a drained battery when you need power the most.
And don’t forget—panel quality matters.
A cheap 300W panel might lose efficiency fast in the heat, while a better one keeps delivering solid output.
So the same wattage on paper can give you very different results in real life.
Solar Panel Size Calculator (for 12V & 24V Systems)
Choosing the right solar panel size depends on your system voltage and how quickly you want to charge your battery.
The basic formula is simple: Watt-hours (Wh) = Amp-hours (Ah) × Voltage (V)
So for a 100Ah battery:
- On a 12V system: 100Ah × 12V = 1,200Wh
- On a 24V system: 100Ah × 24V = 2,400Wh
Once you know your energy needs, divide that by the number of peak sun hours in your area (usually 4–6 in most of Australia).
Then factor in about 20–25% system losses (heat, wiring, shading).
Here’s a quick guide:
| System Voltage | Battery Capacity | Recommended Panel Size | Average Charge Time |
| 12V | 100Ah | 300W | ~4–5 hours |
| 24V | 100Ah | 600W | ~4–5 hours |
If you’re using a MPPT charge controller, you’ll get better efficiency—especially with higher voltages or slightly oversized panels.
Just make sure the controller can handle the input from your solar panels, especially on a 24V setup, where the voltage and current are higher.
It keeps your battery safe, and your charging smooth.
Factors That Affect Charging Efficiency in Australia
There’s more to solar than just panel size. A few key things can make a big difference in how fast—and how well—your battery charges.
1. Sunlight Hours: Darwin vs Melbourne
Darwin gets a lot more sunshine than Melbourne year-round.
In Darwin, you’ll often see 6 to 7 peak sun hours most days. Down south in Melbourne, especially during winter, you might only get 3 to 4 hours.
This difference alone can double or halve your charging time.
2. Heat Isn’t Always Helpful
Australia gets hot, but that doesn’t always work in your favour.
When solar panels heat up too much, they actually lose efficiency.
Even in full sun, a panel might produce 10–15% less power on a scorching day than on a cooler one with the same light.
It’s one of those counterintuitive things—hotter days don’t mean faster charging.
3. Dust, Tilt Angle, and Shade
Your panel’s position matters.
- Dust buildup can block sunlight
- Flat-mounted panels don’t catch as much light in the morning or afternoon
- Shade from trees, antennas, or even your own gear can cut output sharply
Try to keep your panels clean and angle them towards the sun when possible—especially if you’re staying put for a few days.
4. Battery Type: AGM vs Lithium
Your battery also affects charging speed.
- AGM batteries are reliable but take longer to charge and don’t like being deeply discharged
- Lithium batteries (like LiFePO4) charge faster, can handle deeper discharges, and don’t lose as much energy to heat
If you want quicker top-ups and better performance in hot conditions, lithium is the better pick—especially for solar.
How Long Will It Take to Charge a 100Ah Battery?
Charging time depends on a few key things—panel size, how much the battery has been drained, and what type of battery you’re using.
Here’s a basic rule of thumb:
- A fully discharged 100Ah battery at 12V needs around 1,200Wh of power
- More efficient panels and batteries = faster charge
- Less sun or a smaller panel = longer wait
Let’s break it down with some examples:
- 300W panel + Lithium battery (LiFePO4) → Around 4–5 hours
- 200W panel + AGM battery → Around 6–8 hours, sometimes longer
Battery chemistry matters here. Lithium batteries charge faster, accept higher current, and don’t lose as much power as heat. AGM or Gel batteries are slower and can’t be pushed as hard.
Another big factor is the depth of discharge. If your battery is only half empty, you won’t need as much time to top it back up.
Don’t forget the seasons. In summer, you’ll get more sun and faster charging across most of Australia. In winter—especially down south—it might take an hour or two longer with the same setup.
Best Time of Year for Solar Charging in Australia
Solar charging works best when you’ve got plenty of sun—and that means spring and summer are your best mates.
During those seasons, most of Australia gets longer days and more peak sun hours, which means faster, more consistent charging.
Winter, especially in southern parts of the country, slows things right down.
Cloud cover, shorter days, and lower sun angles all chip away at your panel’s performance.
Where you are makes a huge difference too.
Darwin stays fairly sunny all year, while Hobart can get pretty gloomy in the colder months.
Here’s a quick look at average daily peak sun hours across a few cities:
| City | Summer (hrs) | Winter (hrs) |
| Darwin | 6.5 | 5.5 |
| Brisbane | 6.0 | 4.5 |
| Sydney | 5.5 | 3.8 |
| Melbourne | 5.0 | 3.5 |
| Hobart | 4.8 | 3.0 |
If you’re planning solar around the seasons, size your panels with winter in mind so you’re not left short when the sun’s hiding.
Off-Grid vs On-Grid Solar Panel Use
Most of the time, a 100Ah battery is used in off-grid setups—like caravans, boats, camper trailers, or remote shacks.
They’re perfect for keeping things running when you’re nowhere near mains power.
You’ll see these setups all over regional and outback Australia, where staying self-sufficient is essential.
Off-grid systems rely on solar to recharge daily, making panel size and sunlight hours super important.
On-grid use is less common with just one 100Ah battery, but it’s possible. In homes with hybrid solar systems, battery banks store excess energy from the grid or panels for later use.
That setup works well for blackout protection or cutting down on power bills—especially with smart charge controllers and energy monitors.
Key Takeaway
Getting the right solar panel size for your 100Ah battery doesn’t have to be complicated.
If you’re running a 12V system, a 300W panel is usually the sweet spot for most Aussie conditions.
You’ll get solid charging performance in places like Brisbane, Perth, and Sydney, and if you’re headed somewhere a bit cloudier or cooler—like Hobart or the Blue Mountains—going up to 350W or 400W gives you more wiggle room.
The sun’s different across the country, so what works up north might fall short down south.
Battery type matters too—lithium charges quicker, AGMs take their time, and weather, shade, and angle all play a part in how well things charge.
If you’re running off-grid, on the road, or just like the idea of reliable power without the drama, getting your setup right from the start saves a heap of hassle.
And hey—if you’re putting together your own system and want gear that actually works in real Aussie conditions, check us out at 👉 Off Grid WA.
We’re here to help you stay powered up, wherever your adventure takes you.
Frequently Asked Questions
What size solar panel do I need for a 12V 100Ah battery?
A 300W panel is ideal for Australia.
Can a 100W solar panel charge a 100Ah battery?
Technically yes, but it would take a full day or longer, depending on sun hours.
How many solar panels do I need to charge a 100Ah battery?
One 300W panel, or two 150W panels, assuming full sun.
