This article explains that energy and power are different concepts. Confusing energy & power will mislead with solar & batteries. Here’s why.

 Svetlana Podobedova lifts a 160 kg barbell. Pic: original source unknown.

Power is a measure of how quickly energy is used.

Weightlifters like Svetlana Podobedova (above) readily lift huge weights. Doing so demands extreme power. A child can readily lift the same in smaller amounts. This needs little power but the energy used is the same. 

LiFePO4 batteries are weightlifters of the battery world. Lead acids and AGM batteries are the children. LiFePO4 batteries can release a great deal more power. Unless you truly need it, buying them is like hiring a weightlifter to stack. If a lead acid or AGM battery bank exceeds about 350 amps at 12 volts (4200 watts) it can release power like Svetlana anyway.

Some batteries are promoted as producing massive power. They may well do so. But in say, caravan, motorhome and solar use it’s mostly energy, not power, that matters. Here’s why, and how you can save a lot of money by knowing this.

Energy relates to work done

Energy is the ability to perform work, for example, to move an object by applying a force. That work can be electrical, mechanical, chemical, thermal, or nuclear etc. It can be transformed from one form to another, for example – solar to electricity.

Energy can be expressed in watts. It was conceived by James Watt. He sought to define his steam engine’s energy to that of horses. Watt defined the weight a brewery horse could lift in one minute as one horsepower. Its electrical equivalent is 745.7 watts.

How energy and power often confuses

A classic example of how energy and power often confuses is the energy needed to start a car. Many people think it is huge. In reality, it is tiny. It’s only that of a five watt LED for an hour. Were it otherwise it would have been impossible to hand start an engine by manually turning a handle. The vehicle’s alternator replaces that energy within a minute or two of starting. The power needed is about 500 amps for 2-3 seconds. )In the pic below, the energy needed is readily replaced by one gin and tonic!)

Pic: reproduced here by courtesy of the Henry Ford Museum (copyright Henry Ford Museum). 

Why confusing energy and power can matter 

Much of the time, confusing energy and power does not matter. Where it can do, however, is in battery advertising re caravans, motor homes and home solar. Many battery vendors either do not know the difference, or deliberately deceive. They promote their products’ power in areas where power does not matter. For example, the only product in such areas that needs a lot of power is a microwave oven. It typically needs 110 amps (at 12 volts). But any battery bank over about 250 amp hour will do this.

Conversely, a lot of drivers are wary of claims that a small (typically 12-18 amp hour) LiFePO4 battery could start a big 4WD. But doing so requires 500 or so amps – but for so short a time that may be as low as 2 amp hours. To persuade such drivers, the capacity of such units is expressed in milliamp hours – e.g. 12,000-18,000 (that’s 12-18 amp hours). It’s not dishonest – but can mislead. 

Never buy a starter battery for deep-cycle needs (because it is rightly promoted as having more power. Nor a deep cycle battery for engine starting – as it has more energy.

Knowing this difference with big solar systems can save you thousands of dollars!

Our books: Solar That Really Works cover cabins and RVs. Solar Success covers homes and properties. The associated Caravan & Motorhome Electrics covers every aspect of RVs in depth.

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