Thinking about what size solar panel you need to charge a 120Ah battery?
You’re in the right place.
Picking the right solar panel size can save you a lot of headaches, especially when you’re off-grid, camping in the bush, or exploring Australia’s stunning outback.
Nobody wants to wake up to a flat battery after a night under the stars.
A 120Ah battery holds a fair chunk of energy, but getting it properly charged needs the right setup — too small a panel and you’ll be stuck waiting all day, too big and you’re wasting space and money.
The good news?
Finding the right solar panel isn’t hard once you know a few basics, like how many sun hours you get each day and what kind of battery you’re using.
Australia’s strong sunlight gives you a great head start, but weather, seasons, and location all make a big difference too.
Let’s break it down so you can hit the road with confidence, knowing your battery will stay topped up wherever you park your swag.
How Solar Panels Work With 120Ah Batteries
Solar panels and batteries work together like good mates on a camping trip. The solar panel collects sunlight and turns it into electricity.
That electricity flows into your 120Ah battery, where it’s stored for later — running your lights, fridge, or charging your phone.
The trick is picking a solar panel that’s the right size for the battery. Go too small, and you might not fully charge the battery during the day. That can leave you short on power when you need it most.
A 120Ah battery holds a decent amount of energy. If you’re using a solar panel that can’t keep up, your battery can slowly drain over time, even if you’re topping it up each day. This can cause bigger problems, like:
- Undercharging, which shortens battery life
- Limited usable power, especially on cloudy days
- Getting stuck without enough energy for essentials
Choosing the right size panel means you’ll keep your battery healthy and full, without stressing every time a cloud rolls over.
In Australia, where sunlight can be strong but weather changes quickly, making sure your setup is spot-on matters a lot more than you’d think.
Why Battery Voltage Matters (12V vs 24V Systems)
Most 120Ah batteries you’ll find around Australia are 12V. That’s the standard for caravans, 4WDs, and off-grid setups.
When you’re setting up solar, it’s important to match your solar panel voltage with your battery voltage. If they don’t match, you’ll need a good charge controller to handle the difference and protect your gear.
A 12V panel connects neatly to a 12V battery, keeping things simple. If you’re running a 24V system, you’ll need different panels or wire two 12V panels together.
Getting this part right saves you a lot of headaches later and makes your system way more reliable.
Daily Energy Needs of a 120Ah Battery
A 120Ah battery at 12 volts holds about 1,440 watt-hours of energy in total. That sounds like a lot, but you’re usually not draining it all the way down every day.
To keep your battery healthy, you should only use about 50% of its total capacity if it’s a lead-acid type. That means you’re aiming to recharge around 720 watt-hours most days.
If you’re using a lithium battery, you can safely use up to 80–90% of its energy, so your daily recharge target might be closer to 1,200 watt-hours.
The real energy you need to put back depends on how much you’re using overnight — running fridges, lights, water pumps, or charging phones.
If you’re just topping up small daily usage, you won’t need as much solar. If you’re going off-grid for days at a time, you’ll want a bigger panel setup to keep up without stressing your battery.
How Many Watts Needed to Charge a 120Ah Battery
To charge a 120Ah battery properly, you’ll usually need a solar panel that can deliver about 300 watts under standard conditions.
This gives you enough power to replace the energy you use daily, without pushing your system too hard or leaving you short.
There’s a simple way to work it out:
- Start with your battery’s energy storage. A 120Ah battery at 12V holds around 1,440 watt-hours.
- Check how many peak sun hours you get in your area. In Australia, it often ranges between 4 to 6 hours a day.
- Use this formula: Battery capacity ÷ (Sun hours × Panel efficiency)
For example, if you get 5 hours of solid sun and your panel runs close to full efficiency, you’ll need around 288 watts to fully recharge a drained 120Ah battery. But real life isn’t perfect. Clouds roll in, temperatures rise, panels get dirty.
Adding a little extra — choosing a 300W to 400W panel — gives you a good buffer so you’re not cutting it close every day. It also helps if you’re running a fridge, lights, or anything else while still trying to charge the battery.
Australian Sunlight Hours by Region (With Table)
The amount of sunlight you get each day makes a huge difference to the solar panel size you’ll need. Different parts of Australia get different amounts of strong, usable sun. Here’s a quick look:
| City | Average Peak Sun Hours |
| Sydney | 4.5 hours |
| Melbourne | 4.2 hours |
| Brisbane | 5.5 hours |
| Perth | 5.8 hours |
| Adelaide | 5.0 hours |
Peak sun hours mean the number of hours a day when the sunlight is strong enough to fully power your solar panels.
If you’re in Perth, with around 5.8 hours, you’ll need a slightly smaller panel compared to someone living in Melbourne, who gets only about 4.2 hours. Less sun means you’ll need a bigger panel to pull in enough energy during the day.
For example, a 300W panel might work fine in Brisbane or Perth. In Melbourne or Sydney, you might want to step up to a 350W or even 400W panel to stay on the safe side, especially if you’re relying on solar full-time.
Panel Size Adjustments for Weather and Seasons
Weather plays a massive role in how much power your solar panels actually produce. In summer, you’ll usually get plenty of strong sun, longer days, and better charging conditions. A 300W panel can easily keep up with your daily needs when the skies are clear.
Winter is a different story. The sun sits lower in the sky, days are shorter, and cloudy weather can drag on for days. Your panels will make less power even if you’re getting some light.
To handle this, it’s smart to add a 25–30% buffer to your solar panel size. That way, if a few cloudy days roll through, your battery still gets a solid charge without you running out of power.
For example, if you calculated you needed a 300W panel for summer, bump it up to 375W or 400W for year-round reliability. It keeps your system ticking along without any nasty surprises.
Role of MPPT Charge Controllers in Charging Efficiency
A charge controller is what manages the flow of power from your solar panel to your battery. The two main types are PWM and MPPT.
PWM controllers are cheaper, but they’re less efficient, wasting a bit of the power your panels make.
MPPT controllers are much smarter. They constantly adjust to get the most power possible from your panels, especially when sunlight conditions change.
Using an MPPT controller can squeeze up to 30% more energy out of your system compared to a basic PWM.
If you want quicker, more reliable charging, going for an MPPT is a no-brainer.
How to Calculate Solar Panel Size for 120Ah Battery (Simple Step-by-Step)
Working out the right solar panel size for a 120Ah battery is easier than you might think. Just follow these simple steps:
Step 1: Find Total Energy Needed
First, figure out how much energy your battery holds. A 120Ah battery at 12V stores about 1,440 watt-hours (Wh). This is your starting point.
Step 2: Check Your Sun Hours
Next, look up the average peak sun hours for your location. For example, if you’re in Brisbane, you might get around 5.5 hours on a good day.
Step 3: Divide Energy by Sun Hours
Now, take your total energy and divide it by your peak sun hours. 1,440Wh ÷ 5 hours = about 288W. This gives you the minimum panel size you need to fully recharge the battery in one day.
Step 4: Add a 20–30% Safety Margin
Finally, add a bit of extra to cover cloudy days, panel efficiency losses, and system wear.
So you’re looking at a 300W to 400W solar panel for a reliable setup.
Choosing the Best Solar Panel Setup in Australia
Portable Solar Panels vs Fixed Panels
Choosing between portable and fixed solar panels depends on how you’re using your setup. If you’re heading out camping, touring the country in your 4WD, or setting up temporary campsites, portable solar panels are a better fit.
They’re lightweight, easy to pack, and you can move them around during the day to catch the best sun. Some fold up like briefcases, while others roll out like a mat. This flexibility can make a huge difference when you’re chasing that last bit of afternoon light.
For anyone building a tiny home, setting up a permanent caravan, or fitting out an off-grid cabin, fixed panels are the smarter call.
You mount them once, angle them right, and let them do the work. Fixed panels are usually tougher too, built to handle storms, heat, and long-term exposure without needing much attention.
Picking the right setup means less stress on the road and better power when you need it most. You’ll spend less time adjusting and more time enjoying the important stuff, like a cold drink from your fridge or a few lights on after dark.
Monocrystalline vs Polycrystalline Panels
When picking solar panels, the two main types you’ll come across are monocrystalline and polycrystalline. For Australian conditions, monocrystalline panels are usually the better choice.
They handle heat more efficiently, produce more power in the same space, and perform better when the sun isn’t perfect, like early mornings or late afternoons.
Polycrystalline panels can still get the job done, but they’re a bit less efficient and usually a bit bigger for the same output.
If you’re short on space or want the most from every ray of sun, monocrystalline is the way to go, especially in Australia’s tough climate.
Key Takeaway
Choosing the right solar panel size to charge a 120Ah battery is easier once you know what to look for.
A battery that size stores around 1,440 watt-hours, and depending on your daily energy needs, most people should aim for a 300W to 400W solar panel.
Sunlight hours vary across Australia, so adding a little extra to your setup can make a big difference, especially when the weather turns or you’re heading into winter.
It’s always better to go a bit bigger with your panels than to fall short when you really need the power. A little extra upfront can save you plenty of hassle later.
If you’re ready to build a reliable solar setup or need a hand finding the perfect gear, we’d love to help.
Check out our full range of solar solutions at Off Grid WA — we’re locals who know what it takes to make off-grid living smooth, simple, and built to last.
FAQs: Charging a 120Ah Battery with Solar
How long does it take to charge a 120Ah battery with a solar panel?
4-6 hours with optimal sunlight and 300-400W panel.
What size solar panel to charge a 120Ah battery in winter?
Around 400W or more depending on location.
Do I need a solar charge controller for a 120Ah battery?
Absolutely. Preferably an MPPT controller.
Can a 200W solar panel charge a 120Ah battery?
Technically yes, but very slow. Not ideal for full daily recharging.
