- Introduction
- Looking at Lead-Acid Batteries
- Converting Amp Hours to Watt Hours
- The Important Difference With Lead-Acid Batteries
- Lithium Batteries and Usable Capacity
- Conclusion
Introduction
I know I explain this quite often, but I still end up going over it with most new customers. I remember when I first started, and even now, when explaining this to people who don’t deal with batteries or solar at all, it’s easy to forget how to calculate these things. So I figured this week’s article could be an easy revisit to this scenario.
Most traditional lead-acid batteries are rated in amp hours (Ah), while many newer lithium batteries are often rated in watt-hours (Wh), some are still in amp hours, and some show both. Lead-acid batteries, however, have always been rated in amp hours, so we’ll look at the differences.
At first, this can be confusing when you’re trying to compare one battery to another, but once you understand how these numbers relate to each other, the comparison becomes quite simple.
Looking at Lead-Acid Batteries
Most off-grid systems I come across these days, especially when upgrading batteries, still use lead-acid batteries, and these are almost always rated in amp hours (Ah).
For example, you might see a battery labeled:
12 volts 200 amp hours or 12 V 200Ah
Amp hours tell us how much current the battery can deliver over time, but to understand the total stored energy, we also need to consider the battery voltage.
Converting Amp Hours to Watt Hours
To compare batteries properly, it helps to convert everything into watt-hours (Wh), which represents the total energy stored in the battery.
The formula is simple:
Battery Voltage × Amp Hours = Watt Hours
So a 12-volt 200 amp hour battery stores about:
12 × 200 = 2400 watt-hours
This makes it much easier to compare different batteries, especially when some are listed in amp hours and others in watt-hours.
It also makes it easier to understand how much energy your loads would use. For example, a 7-watt LED light bulb uses 7 watts every hour, or 7 watt-hours. If you run it for 6 hours, it would use:
7 × 6 = 42 watt-hours
The Important Difference With Lead-Acid Batteries
With lead-acid batteries, it is important to remember that you usually should not use the full capacity.
In most cases, only about 50% of the battery’s capacity should be used if you want the batteries to last a reasonable amount of time.
So in our example:
2400 watt-hours × 50% = about 1200 usable watt-hours
This is an important detail because many people compare batteries based on their full capacity, but the usable energy is often much less with lead-acid batteries.
Lithium Batteries and Usable Capacity
Lithium batteries are different because they can safely be discharged much deeper than lead-acid batteries.
A 24 volt 100 amp hour lithium battery stores exactly the same as our previous 12 volt 200 anp hour battery
24 x 100 = 2400 watt-hours
In many cases, lithium batteries can be used down close to 0%, which means you can access much more of the stored energy.
However, it is still very important to look at the manufacturer’s specifications and warranty conditions.
Some lithium battery manufacturers specify that the battery should only be discharged to 10% state of charge, which means about 90% of the battery capacity is usable.
If that is the case, it is good practice to set your inverter or system controls so the battery shuts down at around 10% state of charge to stay within the recommended limits.
If the system uses voltage settings instead of percentage, then it is best to follow the manufacturer’s recommended shutdown voltage for that specific battery.
This helps protect the battery and ensures you remain within the conditions required for the warranty.
Conclusion
When comparing batteries, it helps to focus on total energy capacity in watt-hours, not just amp hours.
By converting amp hours into watt-hours and understanding how much of that energy is actually usable, you can make a much more accurate comparison between lead-acid and lithium batteries.
Knowing this, and what you are looking to power, helps when making your load list, giving you a clear idea of the amount of capacity you’ll want moving forward.
Looking to upgrade your batteries? Give us a call or contact us here and we can certainly help you find the solution thats best for you and your system
If, however, you are thinking about adding more capacity, like doubling your current battery bank, you may also need to look at the rest of your system to ensure it can properly charge a much larger capacity. That’s a bigger topic, though, and then this is when I start to ramble on about many other factors, so that one we’ll save for another time.
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