How to Size the Right Solar Panel for Charging a 100Ah Battery
From us here at The Edinburgh Boiler Company
If you’ve got a 100Ah battery and you’re trying to work out what size solar panel you need, you’re not alone. It’s one of those questions that sounds simple on the surface — but once you start looking at watts, amp hours and efficiency losses, it can quickly feel more technical than expected.
The good news is this: almost any solar panel can charge a 100Ah battery.
The real question isn’t can it charge it?
It’s how long will it take — and how reliable will that charging be in Scottish conditions?
Let’s walk through it properly, without overcomplicating it.
Step 1: Understand What a 100Ah Battery Actually Stores
Most 100Ah batteries in small solar setups — whether for backup, leisure, sheds or light off-grid systems — are 12 volts.
So the total energy stored isn’t just “100Ah”. It’s calculated like this:
100Ah × 12V = 1,200 watt-hours (Wh)
That means your battery stores 1.2 kilowatt-hours (kWh) of energy.
That 1,200Wh figure is what we use to size your solar panel correctly.
Step 2: The Basic Charging Formula
In theory, working out charging time is straightforward:
Battery capacity (Wh) ÷ Solar panel power (W) = Charging time (hours)
If everything operated at 100% efficiency, that would be the end of it.
But in the real world, no solar system runs at 100%.
Energy is lost through:
- Charge controller conversion
- Cable resistance
- Heat build-up in the panel
- Sun angle
- Cloud cover
- Seasonal daylight variation
In Scotland especially, designing to a theoretical figure is unrealistic. We normally work with an 80% efficiency estimate to give customers a more dependable expectation.
Step 3: What Charging Looks Like in Practice
Let’s apply that to a 1,200Wh battery.
Using a 250W Panel
Theoretical (100% efficiency):
1,200Wh ÷ 250W = 4.8 hours
Real-world (~80% efficiency):
Effective output ≈ 200W
1,200Wh ÷ 200W = ~6 hours
That means a 250W panel can comfortably charge a 100Ah battery in a strong summer day — but it’s not rapid.
Using a 400W Panel
Theoretical:
1,200Wh ÷ 400W = 3 hours
Real-world (~80%):
Effective output ≈ 320W
1,200Wh ÷ 320W = ~3.8 hours
Now we’re getting into more practical territory for everyday use.
Using a 500W Panel
Theoretical:
1,200Wh ÷ 500W = 2.4 hours
Real-world (~80%):
Effective output ≈ 400W
1,200Wh ÷ 400W = ~3 hours
This starts to feel quick — assuming decent sunlight.
Real-World Charging Time Comparison (~80% Efficiency)
| Panel Size | Approx Real Output | Estimated Charge Time |
| 250W | ~200W | ~6 hours |
| 300W | ~240W | ~5 hours |
| 350W | ~280W | ~4.3 hours |
| 400W | ~320W | ~3.8 hours |
| 450W | ~360W | ~3.3 hours |
| 500W | ~400W | ~3 hours |
| 600W | ~480W | ~2.5 hours |
These figures assume strong, steady sunlight — more likely in late spring and summer than mid-winter in Scotland.
What If You Wanted to Charge It in One Hour?
To charge 1,200Wh in a single hour, you’d theoretically need 1,200W of panel output.
But again, factoring in realistic 80% efficiency:
1,200W ÷ 0.8 ≈ 1,500W of installed panel capacity
So in practical terms, that could mean:
- Six 250W panels
- Four 400W panels
- Three 500W panels
- Two to three 600W panels
And even then, that assumes near-perfect sun conditions.
In Scotland, we don’t design systems around “perfect day” assumptions. We design them to work consistently.
What Really Affects Charging Speed?
Panel wattage is important — but it isn’t the only factor.
Charge controller type makes a big difference. An MPPT controller extracts more usable power from the panel than a PWM controller, particularly in cooler climates like ours.
Battery chemistry also matters. Lithium batteries charge more efficiently and tolerate partial charging better than traditional lead-acid batteries.
Then there’s simple geography. Edinburgh may see 4–5 peak sun hours in summer. In winter, that drops significantly. A system that feels fast in June will behave differently in December.
Even panel tilt and orientation change performance. A south-facing panel angled correctly will outperform a flat-mounted panel of the same size.
Worked Out Your System — Where Do You Buy It?
Once you know roughly what you need, the next question is sourcing it. See where to buy solar panels in Scotland.
So What’s the Right Size?
If you’re aiming for:
Steady, reliable charging over half a day — 300–400W works well.
Faster charging and more flexibility — 400–500W is a strong balance.
Quick recharge in strong sun and future expansion room — 500W+ gives you breathing space.
For most realistic Scottish applications, a 400W panel hits the sweet spot. It’s large enough to charge efficiently without massively oversizing the system.
If you’d like, I can also create a seasonal comparison showing how these charging times shift between summer and winter in Scotland, which is often where expectations need adjusting.
