Battery Capacity in Watt-Hours Calculator – Wh, kWh & Runtime

When buying a battery, you see something like “100 amp hours” on the box. That’s half the story. It measures how much charge is in the battery, not how much work that charge will do. A 100 amp hour battery operating at 3 volts contain a fraction of the same amount of energy as a 100 amp hour pack running at 48 volts. Watt-hours solve that confusion by bringing the voltage into the equation. You get one number that actualy matters when figuring out your power requirements.

The calculator above takes those abstract ratings and transforms them into concrete runtime estimates to help you stop guessing and start knowing. Charge flow is measured in amp-hours, like fuel volume is measured in gallons. But energy is power times time, so it takes voltage and current to get at that. The true measure of stored electricity is therefore watt-hours.

Why Watt-Hours Matter More Than Amp-Hours

These also appear on your utility bill as kilowatt-hours, and they can be used to compare a phone battery with an electric vehicle pack apples to apples. Why? Otherwise, you might think you can swap two batteries if they have the same amp hours. In fact, you absolutely cannot if one has a higher voltage then the other.

The math in the background is simple, but it works. Amp hours times voltage equals watt hours, which you then divide by 1,000 for kilowatt hours. For smaller cells that are listed in milliamp-hour, the tool will divide by 1,000 first (adjusting them). From there, everything come down to this equation, whether you are checking if your power station can run a fridge overnight or figuring out how big of a solar bank you need.

A real battery bank is typically more than a single cell. A battery bank are constructed of multiple cells wired together in both parallel and series ways. Series connection increase the voltage but retains the capacity: four 3.2 volt cells = 12.8 volts. Parallel strings retain voltage but increase capacity; two 3 amp-hour cells = 6 amp-hours.

The calculator does all that math for you. It multiplies the capacity by the number of parallel cells and the voltage by the number of series cells. It then calculates the resulting energy amount. It eliminates the mental math from designing packs, allowing you to focus on whether it will fit your requirements.

Not all rated energy is available. To protect the life of a battery, it’s not drawn down to zero. Engineers establish a depth-of-discharge (DOD) limit. For example, lead-acid batteries like a light DOD around 50 percent; lithium iron phosphate types can safely and comfortabley deliver 80 to 90 percent to thousands of cycles.

Useable energy in this case means multiplying available watt-hours times your selected DOD percentage. If you have a 720 watt-hour AGM battery with a 50 percent available capacity, that’s only 360 watt-hours, but a 1280 watt-hour bank at 90 percent is 1152. An honest DOD limit protect batteries from dying too soon.

So what does that mean? The practical question on everybody’s mind is answered by the final output, how long will it last? To calculate runtime, take the amount of usable watt-hours divided by your load in watts. That means a 150 watt appliance running on 1152 usable watt-hours will last approximately 7.7 hours.

This number already factors in the limitations of the chemistry and safety margins by using the reduced usable energy instead of the higher nameplate total. Change the load value to see how tweaking it changes the timeline (e.g. This could be a low-wattage LED strip versus a high-wattage heater.

Without having to type anything in, you have quick sanity checks from the reference tables on the page. These show how arrays scale through series-parallel setups, milliamp-hour equivalents for small electronics, and how some common voltage and amp-hour combinations translate into energy. See how a server rack module compares to a normal car battery, or see how different configurations make batteries of similar size have very different capacities. The grids are worked examples, taking the principles and showing how they apply to different applications.

The real world isn’t numbers on a screen. The cells will age over time, the cold reduces their effective capacity, and inverters convert electricity with wasted heat. Consider the calculated runtime the best possible case, not the guaranteed promise. Account for 10 to 20 percent efficiency shavings and you’ll have a buffer that’s going to keep your lights on when the math thinks they should of been out.

That makes it a valuable planning companion for camper van solar, backup power projects, and more, turning vague hope into measured expectation.