Watt-Hours Explained: Wh, Ah and kWh Made Simple

You're comparing two batteries. One says 1000Wh. The other says 100Ah. A third says 1kWh.

They're describing the same kind of thing in three different languages, and one of them is deliberately confusing.

Ten minutes here and no spec sheet in this category will trip you up again.

The only analogy you need

Think about a car.

Watts is your speed. Miles per hour. How hard the thing is working right now.

Watt-hours is your fuel tank. How far you can go before you stop.

A 100W light bulb runs at 100 watts whether it's on for one minute or ten hours. The wattage never changes. What changes is how many watt-hours it has eaten.

Why this matters when you're shopping

Two units, same price. One is 2,000Wh with a 1,000W inverter. The other is 1,000Wh with an 1,800W inverter.

The first has a bigger tank. The second has a bigger engine.

If you need to run a 1,500W appliance, the first one simply cannot do it. It doesn't do it slowly. It refuses, and shuts off.

Capacity tells you how long. Output tells you whether.

Amp-hours, and why they're used to confuse you

Amp-hours (Ah) measure charge, not energy. To get energy you need voltage too.

Amp-hours × volts = watt-hours.

A 100Ah battery at 12V is 1,200Wh. A 100Ah battery at 24V is 2,400Wh. Same "100Ah" on the label, twice the energy.

That's why RV and marine listings quote amp-hours: it makes small batteries sound bigger. Whenever you see Ah, find the voltage before you compare anything.

Kilowatt-hours

One kilowatt-hour = 1,000 watt-hours. It's the unit on your electric bill.

So a 1,000Wh power station holds exactly one kWh. The average US home uses somewhere around 30 kWh a day. verify

That comparison is worth sitting with. Your power station holds about 45 minutes of your normal household consumption.

Which is not a reason to despair — it's a reason to stop trying to run your whole house. You're not replacing the grid. You're keeping four important things alive.

The 15 percent nobody mentions

You don't get the number on the box.

The inverter that turns battery DC into wall-socket AC loses energy as heat. Figure on about 85 percent of the rated capacity actually reaching your appliance. verify

So a 1,000Wh unit gives you roughly 850 usable watt-hours. Cold weather takes more. This is why our runtime figures apply that discount rather than quoting the marketing number.

Look at the microwave and the space heater. Same battery, wildly different outcomes. That's watts doing the work — high-wattage appliances eat a tank fast no matter how big the tank is.

Doing it for your own house

Find the label on the back of the appliance. It gives watts, or it gives amps.

If it gives amps, multiply by 120 for a standard US outlet. A 5A appliance is about 600W.

Then multiply by hours. That's your watt-hours.

Then add 30 percent for inverter losses and the sensible habit of not draining a battery to zero.

That's the whole method. It's the same arithmetic our calculator runs — you can absolutely do it on the back of an envelope.

The one mistake almost everyone makes

Forgetting duty cycle.

Your fridge is labeled 150W. It does not use 150W for 24 hours. The compressor runs about a third of the time, so the real daily figure is closer to 1,200Wh, not 3,600Wh. verify

Anything with a compressor or a thermostat cycles: fridges, freezers, well pumps, sump pumps, air conditioners.

Miss this and you'll size for three times what you need, and spend accordingly. It's the single most expensive misunderstanding in this whole category.