You Only Get Half the Battery (Here's Why)

By PowerLasts Team

Here is a question that catches almost everyone off guard: if you buy a battery rated at 1000Wh, how much energy do you actually get to use?

If you said 1000Wh, you are not alone. You are also not right.

Depending on the battery chemistry and the system it is connected to, you might only get about half of that. The rest is eaten by physics. Let me explain what is going on.

What “Depth of Discharge” Means

Every rechargeable battery has a limit on how much of its capacity you can safely drain before you start damaging it. This limit is called the depth of discharge, or DoD.

Think of it like a swimming pool with a painted line halfway down. The water below that line exists, but if you drain past it, you crack the foundation. Batteries work the same way. Drain them past their safe DoD and you shorten their lifespan dramatically — sometimes permanently.

So when a manufacturer says a battery holds 1000Wh, that is the total capacity. The usable capacity depends on how deep you can safely go.

Lead-Acid: You Only Get Half

Traditional lead-acid batteries — the kind found in most basic UPS units and car batteries — have a recommended DoD of about 50%. That means a 1000Wh lead-acid battery should only be drained to 500Wh before you recharge it.

This is why car batteries die when you leave the headlights on overnight. The battery was not designed to be drained that deeply. Do it a few times and the battery’s lifespan drops like a rock. A lead-acid battery rated for 500 charge cycles at 50% DoD might only last 100 cycles if you regularly drain it to zero.

Half. You get half. That is the starting point.

Lithium-Ion (NMC): Better, but Not Perfect

The lithium-ion batteries found in most portable power stations (technically called NMC — nickel manganese cobalt) are a big step up. Their safe DoD is around 80 to 85%.

So a 1000Wh lithium-ion battery gives you roughly 800 to 850Wh of usable energy. That is a meaningful improvement over lead-acid, and it is one of the main reasons portable power stations have taken off in recent years.

But it still is not the full 1000Wh printed on the box.

LiFePO4: The Best Chemistry for Backup Power

LiFePO4 (lithium iron phosphate) batteries are the gold standard for backup power right now. Their safe DoD is 90 to 95%, which means you can use almost the entire rated capacity.

A 1000Wh LiFePO4 battery gives you 900 to 950Wh. On top of that, LiFePO4 batteries last significantly longer — often 2,500 to 5,000 charge cycles compared to 500 to 1,000 for NMC lithium-ion.

If you are shopping for a power station and you see “LFP” or “LiFePO4” on the spec sheet, that is a good sign. You are getting more of what you paid for.

The Inverter Takes Its Cut Too

Here is the part that makes things worse. Even after depth of discharge takes its share, you still lose energy when the battery’s DC power gets converted to the AC power your devices use. This conversion happens in the inverter, and it is not perfectly efficient.

Most inverters run at about 85% efficiency. That means for every 100Wh pulled from the battery, only about 85Wh makes it to your device. The other 15Wh becomes heat.

This is not a defect. It is just how power conversion works. But it is another bite out of your usable capacity that most people do not think about.

Stack the Losses and the Real Number Gets Ugly

Let us do the math for each battery chemistry, starting with 1000Wh of rated capacity:

Lead-acid (50% DoD, 85% inverter efficiency): 1000 x 0.50 x 0.85 = 425Wh usable

Lithium-ion NMC (80% DoD, 85% inverter efficiency): 1000 x 0.80 x 0.85 = 680Wh usable

LiFePO4 (90% DoD, 85% inverter efficiency): 1000 x 0.90 x 0.85 = 765Wh usable

Read that lead-acid number again. A battery that says 1000Wh on the label gives you 425Wh of actual usable energy. That is not half — it is less than half.

Even the best chemistry, LiFePO4, only delivers about 77% of the rated number after inverter losses.

Why “Usable Wh” Is the Only Number That Matters

This is why savvy shoppers ignore the big headline number and look for the “usable watt-hours” spec instead. Some manufacturers are upfront about this and list usable capacity separately. Others just print the biggest number they can and hope you do not do the math.

When you are comparing two products, always compare their usable capacity. A 1500Wh lead-acid UPS and a 600Wh LiFePO4 power station might actually deliver similar amounts of real-world energy.

What This Means for You

If you are trying to figure out how long your devices will run during an outage, the rated capacity on the box will give you an answer that is way too optimistic. You need to account for depth of discharge and inverter losses, or you will end up standing in the dark wondering why your “1000Wh” battery died in three hours instead of six.

The good news is that you do not have to do all this math yourself. Our calculator uses usable watt-hours and builds in efficiency losses so you do not get caught short. Plug in your devices, set your desired runtime, and get a recommendation based on real-world numbers — not marketing numbers.

Quick Reference

Here is a cheat sheet you can use when shopping:

Keep these multipliers in your head and you will never be surprised by a battery that runs out “too early” again. The battery is not broken. You just were not getting the whole story.

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