Tag Archive for: solar battery sizing

The Ah vs Wh debate comes up every time you shop for a battery. You see both numbers on every spec sheet. However, most buyers ignore one of them. That is a costly mistake. Ah and Wh measure different things. Confusing them leads to choosing the wrong battery size.

In this guide, Sunlith Energy breaks down both measurements. You will learn the formula that links them. Additionally, you will see real conversion examples. Furthermore, we share a step-by-step method to size your own battery system correctly.

According to the International Energy Agency, battery storage is central to the global clean energy transition. Therefore, understanding how battery capacity is measured matters more than ever. Every buyer deserves to get this right.

What Does Ah Mean? The Charge Side of Ah vs Wh

Ah stands for Amp-hours. It measures electric charge. Specifically, it tells you how many Amps a battery delivers and for how long.

The rule is simple. One Ah means 1 Amp delivered for exactly 1 hour. However, it could also mean 2 Amps for 30 minutes. Alternatively, it could be 10 Amps for 6 minutes. The total charge is always the same — only the rate changes.

When to Use Ah in the Ah vs Wh Decision

• Calculating runtime — how long a battery powers a fixed-current device
• Setting charge rates — C-rate is always expressed relative to Ah
• Designing battery banks — when all batteries share the same voltage
• Comparing batteries of identical voltage side by side

There is one important limitation. Ah is voltage-independent. Therefore, a 100 Ah battery at 12V and a 100 Ah battery at 48V have the same Ah rating. Even so, they store very different amounts of energy. That is the most common battery-buying mistake.

For more on DoD and cycle life, read our guide: Battery Cycle Standards — DoD, SOH, and EOL Explained.

What Does Wh Mean? The Energy Side of Ah vs Wh

Wh stands for Watt-hours. It measures actual energy. Because it accounts for voltage, Wh is the more complete measurement.

Furthermore, battery energy density is expressed in Wh/kg. So understanding Wh also helps you compare weight-to-energy ratios across different chemistries.

When to Use Wh in the Ah vs Wh Decision

• Comparing batteries at different voltages — for example, 12V vs 48V

solar or backup battery system by daily kWh usage

• Calculating how long a battery runs a watt-rated appliance
• Airline carry-on compliance — IATA uses Wh limits, not Ah limits

advantages of BESS across commercial system voltages

The Ah vs Wh Formula — One Equation to Know

Good news: only one formula connects Ah and Wh. Voltage is the bridge between them.

This explains why two batteries with the same Ah can store very different energy. Higher voltage multiplies charge into more usable Wh. As a result, 48V systems deliver far more energy per Ah than 12V setups. That is why 48V has become the standard for modern residential solar.

Ah vs Wh Conversion Examples — Real Numbers

Below are three practical examples. Each one shows how to apply the Ah vs Wh formula step by step.

Ah vs Wh — Which Metric Should You Use?

Both measurements are useful. However, the right choice depends on your question. Use this table as a quick reference:

In summary: use Ah for current and time calculations within a fixed-voltage system. For everything else, use Wh. Comparing batteries across voltages or chemistries? Wh is always the right choice.

Same Ah, Very Different Energy — Why Voltage Changes Everything

Many buyers compare batteries on Ah alone. This is a common and expensive mistake. Voltage changes everything. Below is a clear example:

As the table shows, identical Ah ratings hide very different energy levels. Consequently, always convert to Wh before comparing. For more on how chemistry affects this, see our LiFePO4 vs NMC battery guide.

What Reduces Your Real-World Ah vs Wh Capacity?

Battery labels show the theoretical maximum. In practice, usable capacity is always lower. Several factors reduce what you actually get. Understanding them is essential for accurate sizing.

1. Depth of Discharge (DoD)

Most batteries should not be fully drained. Doing so permanently damages cells. The safe depth of discharge varies by chemistry:

• LiFePO4: 80–90% DoD — consequently, usable Wh = 80–90% of rated Wh
• Lead-acid: only 50% DoD — therefore, you lose half your rated capacity
• NMC: typically 80–85% for a long cycle life

2. Temperature

Cold weather hurts batteries significantly. Below 10°C, deliverable Ah drops by 20–30%. Temperature directly impacts LiFePO4 cycle life — a rise of 10°C above 25°C can halve total cycle life. Heat, on the other hand, temporarily boosts apparent capacity. However, it accelerates permanent degradation at the same time.

3. Discharge Rate (C-Rate)

Drawing current too fast reduces total Wh delivered. For example, a battery discharged at 2C gives fewer Wh than the same battery at 0.5C. Always check the C-rate used during the manufacturer’s Ah test. Because a 0.2C rating looks far better than real-world 1C performance.

4. Battery Aging

Every cycle causes a small, permanent capacity loss. At 500 cycles, most batteries retain about 90%. At 1,000+ cycles, the best LiFePO4 cells still retain 70–80%. Consequently, factor aging into your long-term Wh budget when sizing.

5. System Efficiency Losses

Inverters, charge controllers, wiring, and BMS all consume energy. Modern lithium systems typically achieve 85–95% round-trip efficiency. Therefore, add a 10–15% buffer on top of your calculated Wh need. This protects you from real-world losses.

How to Size Your Battery System Using Ah vs Wh

Now let’s put it all together. Below is a simple four-step sizing method. It is the same approach used in our solar battery sizing guide.

Step 1 — Calculate Your Daily Wh Requirement

List every appliance you want to power. Write down its wattage and daily run hours. Multiply watts by hours for each device. Then add them all together. For example: a 50W fridge runs 24 hours = 1,200 Wh. Four 25W LED lights run 5 hours = 500 Wh. Total: 1,700 Wh per day. Additionally, add 10% for hidden standby loads — bringing the total to about 1,870 Wh.

Step 2 — Apply the Depth of Discharge

Divide your daily Wh by the safe DoD. For LiFePO4 at 80% DoD: 1,870 ÷ 0.80 = 2,338 Wh of rated capacity needed. This step is essential. It ensures you never drain the battery below its safe limit. As a result, both lifespan and warranty are protected.

Step 3 — Add a Safety Margin

Multiply your result by 1.15 to 1.20. This covers system losses, aging, and seasonal variation. In our example: 2,338 × 1.20 = 2,806 Wh minimum rated capacity. Therefore, look for a battery bank rated at or above 2,800 Wh.

Step 4 — Convert Wh Back to Ah

Use Ah = Wh ÷ Voltage. At 48V: 2,806 ÷ 48 ≈ 58 Ah. At 24V: 2,806 ÷ 24 ≈ 117 Ah. At 12V: 2,806 ÷ 12 ≈ 234 Ah. As a result, higher-voltage systems need far fewer Ah. That is why 48V has become the industry standard for residential solar.

Ah vs Wh — Frequently Asked Questions

Q

Conclusion — Ah vs Wh Made Simple

Knowing the Ah vs Wh difference saves you from bad battery decisions. Ah measures charge. Wh measures energy. The formula Wh = Ah × Voltage connects them. Use Ah for runtime and charge rate calculations. For everything else — especially cross-voltage comparisons — use Wh.

Additionally, always apply DoD, temperature effects, C-rate, and aging when estimating real-world usable capacity. The number on the label is a theoretical maximum. Your actual usable capacity will always be lower.

Whether you are planning a home solar install or a commercial BESS project, the Ah vs Wh distinction is the right place to start. Get it right — and every other sizing decision becomes easier.