Understanding the Difference Between Power and Energy

One of the most important parts of planning and managing your retreat is to optimize its power and energy requirements.

The two terms – power and energy – are often used, in normal casual conversation, as meaning the same thing (for example, sometimes you’ll see references to electric power and sometimes to electric energy, although both terms probably are referring to the same thing), and we’ll wager that if we carefully read through the half million words of content already published on this site, we’d find some cases where we too have used one term while meaning the other, so we will try to be scrupulously correct in this article.

Most of the time, there is little harm and no misunderstanding caused when a person is talking, generally, about things to do with power/energy, and when they use the word power or energy in the wrong context.  But, when it comes to ‘doing the sums’ and understanding exactly what your power and energy needs are, you do need to exactly understand the difference between the two concepts, and make sure you are using the correct units (and also making sure that the specifications for the equipment you’re considering are also correct, and/or being able to work through their assumptions to understand exactly what it is you are being sold).

To make things more complicated, both power and energy can be measured in several different types of units.  You are probably already familiar with some of the measurement systems and their names, and some other terms you know of you perhaps didn’t even realize were measuring power or energy.  These would be terms such as horsepower, btu, therm, watt, kilowatthour, joule, erg, calorie, foot pounds, newton meters, and various other terms too.

Power – The Watt

For the purpose of this explanation, we’ll talk about two simple units of measurement – the watt, as a measure of power, and the watt hour as a measure of energy.  First, let’s understand the terms, then we’ll explain what they mean, their differences, and how to convert between them.

In the US system of measurements, the watt is a common measure of power.  You are probably familiar with its use to measure electrical power, and other systems can be used to measure power too – for example, in Europe, the power of a vehicle is usually measured in watts rather than in horsepower.

The abbreviation for the watt is the letter W (an upper case W) – yes, this can be confusing.  If you are using the word, you typically use lower case when writing it, but if using the abbreviation, you should use upper case.

It can also come in smaller units – milliwatts, microwatts, and potentially even smaller numbers.  You will occasionally see things such as small portable appliances have their power requirement described in milliwatts (mW).  A milliwatt is one thousandth of a watt, and is abbreviated with a small letter m and a capital W.  It is very important you do this, because if you write it MW, that means megawatt, which is a totally different number entirely!  You are unlikely to come across measurements in microwatts or smaller, these are quantities that normally only appear in scientific calculations and not in domestic appliances.

Going the other way, to larger quantities than a few watts, you will commonly find kilowatts (kW), megawatts (MW), and sometimes larger quantities such as gigawatts (GW) and even terawatts (TW).  If this sounds sort of familiar, it might be because you see similar suffixes for measuring computer storage, and so you probably already know that kilo means one thousand, mega means one million, giga means a billion, and tera means a trillion.

Note that kilowatts are written as kW, whereas megawatts (and other larger quantities) are written MW, etc. Watts can of course be converted to other units of power.  For example, 1000 watts (ie 1 kW) equals 1.34 horsepower, so your car with 300 hp can also be described as having 224 kW of power.

We’ll stick with watts for this discussion rather than muddy the waters unnecessarily with other terms.  But if you do need to do conversions, you’ll find websites such as this to be helpful.

Power

So, what is a watt?  It is a measure of power, and we’ll give you some examples of what we mean by power.

The first example is to think of a simple electrical heater.  Maybe it is an old-fashioned one with two or three ‘bars’ in it, and you can choose to have one, two or all three of the bars turned on.  Perhaps with one bar turned on, the heater is rated at 500 W, with two it is a 1000 W (or 1 kW) heater, and with three, it is giving you 1.5 kW of power.

Think also of light bulbs.  The more watts the light bulb consumes, the brighter the light, right?  If you think back to the now old-fashioned incandescent bulbs, you would probably be using 60 W or 75 W or maybe even 100 W and sometimes more powerful bulbs to light your rooms.  More watts means more power means more light (with a light bulb) or more heat (with a heater).

Now let’s think of an analogy, which we’ll use to explain the difference between power and energy. Think of a garden hose.  Turn it on a bit, and water will trickle out of the hose, and you can’t squirt it very far.  You would describe that as not very powerful, right?  Turn it on full, and more water will come out, and you can squirt it further.  The flow of water is now more powerful. You can think of electricity and power in general in similar terms to water flow, and whereas we measure water flow in things like gallons per minute, we measure electricity flow and power in general in watts.

Okay, so hopefully now you understand what power is.  Next, we will explain energy.

Energy

Let’s keep thinking about the flow of water through the hose.  The faster it goes, the more power it gives us, right?  And, also, the faster it goes, the more gallons of water it uses.  This is sort of totally obvious.  We can short of think of the total gallons of water used as a measure of the total energy consumed, and the flow as being the rate at which the energy is consumed.

If we had, for example, 100 gallons of water, that could flow through a hose in 10 minutes at a rate of 10 gallons per minute, or it could take 50 minutes at a rate of 2 gallons per minute.

And, there in a nutshell, is the relationship between power and energy.  Energy is like the total store of water, and power is the rate at which the energy is being consumed.

Let’s go back to thinking about our light bulbs and heater, and see how much energy they consume.  We know that the power used by, eg, a light bulb is maybe 100 W which means that is the rate at which electricity is going through the bulb.  If the bulb is on for an hour, then it will have used 100 watt hours of energy.  If it is on for 30 minutes (ie half an hour) it will have used 50 watt hours of energy (or, if you prefer, 3000 watt minutes or 3 kilowatt minutes).

And so on for any other scenario – you are simply multiplying the rate of power usage (as measured in watts) by the time the power is being used.

Watt Hours – Energy

We normally measure energy in watt hours, or kilowatt hours, and so on.  Sure, you could also measure in watt minutes, in watt seconds, or in watt days, but normally you’ll see this expressed in terms of hours.  The abbreviation for a watt hour is Wh or W h (with a space), and of course the abbreviation for other quantities would be, for example, kWh for kilowatt hour and so on.  We generally prefer to omit the space, just to more obviously tie in the h to the W.

If you are starting to get the hang of this, you will realize that a gallon of petrol contains energy (and could be measured in Wh), and the speed at which it is consumed is described as the power of the thing consuming the petrol (and could be measured in watts).  A more powerful thing (eg a car driving faster) will use the energy (the gallon of gas) more quickly than a less powerful thing (a car driving more slowly, perhaps).

Other types of energy measures also exist, of course.  For example, if you consume natural gas, you might see that your gas consumption is measured in Therms or BTUs rather than in watt hours (1 Therm = 29.31 kWh; 1 Btu = 0.293 Wh, and therefore, 1 Therm = 100,000 Btus).

Power is the rate at which we consume energy.  For example, it might take a certain amount of energy to heat your house from 50° to 60° – let’s say it will require 20 kWh of energy to do this.  That means (ignoring heat losses, etc) you could turn on a 1 kW heater and wait 20 hours for it to heat up your house, or you could turn on five 1 kW heaters and wait four hours, or you could turn on your furnace that, for this example, we’ll say uses 10 kW of power, and wait only two hours.

In all cases, you use the same amount of energy and get the same outcome, but you use it at different rates/speeds.

We Pay for Energy, Not Power

Now for the next thing, which hopefully logically flows from the house heating example above.  In most cases with most utilities, we are charged for the energy we use, not for the rate at which we use the energy (there are exceptions to this, particularly for commercial users that sometimes have high power draws, where they get charged for both the energy they use and also the amount of power available to them to draw from).

In the previous example, we will pay for the 20kWh to heat our house, no matter if we use it quickly or slowly.  In case you wondered, you can see on your utility bill the rate you pay for your electricity, and the chances are you’re probably paying 10c – 15c per kWh, so you’d be paying maybe $2 – $3 for the 20kWh.

Both Energy and Power Calculations are Necessary for a Retreat

When you are planning your retreat, you want to of course minimize its total energy requirement.  But you also want to consider its maximum and typical power requirements, too.

The typical US house (if there is such a thing!) consumes an average of 30 kWh of energy a day.  Hopefully, a well designed retreat can get by with much less than that.  Here’s an interesting table showing how energy is typically consumed in an average home.

The good news part of this table is that your greatest energy requirements – for home heating, cooling, and water heating (which between them comprise 60% of your total energy needs) can be greatly reduced by good insulation and home design, and may also be provided, at least in part, through alternate energy sources such as fires for heating and solar for water heating.  You don’t need electrical energy for all your retreat’s energy requirements.

It is important to understand your home/retreat’s total energy needs (and where/how you will source the energy for these requirements).  But you also need to think about the power requirement.  In the most simple sense, think of buying a generator to power your home.  If you consume 30 kWh of energy per day, that sort of seems like you are using 30/24 = 1.25kW of power, and so if you get a 1250 watt generator, you should be in good shape.  Right?

Wrong.  Sure, your house might use in total 30 kWh of energy for a typical 24 hour period, but it does not use this in a steady even flow.  At some times, for example 4am, maybe it is using no power at all.  But at 4pm, maybe it is using energy at a rate that sometimes peaks upwards of 15 kW, because you have some lights on, the stove top on, the vacuum cleaner running, the fridge compressor cycled on, and so on.  Your 1250 watt portable generator isn’t going to be any use to you at all, because any time you turn your stove on, you are needing way more than 1250 watts of power.

You need to understand both the total energy requirements for your retreat, and also the peak power requirements at which the energy will be needed.

Actually, the calculation needs to be fine-tuned even more.  Your retreat will most likely use more energy in the winter months than in the summer months (more heating, more lighting), and so you need to consider not only the typical average daily energy needs, but also the ‘worst case’ peak daily energy needs, and then translate those into the associated power rates needed.

Appliance Power Ratings and Energy Consumption

Most home appliances have a power rating in watts or kilowatts.  Some may also make some sort of vague claim about how much energy they consume a year – perhaps in the form of an Energy Star rating that compares it to other similar products.

The energy an appliance consumes each year depends on its ‘duty cycle’ – how much time each year it is actually turned on and working.  Think, for example, of a fridge or freezer.  Although it is plugged in and switched on 24/7, it actually is only working for perhaps one-third, maybe less, of the time.  Its compressor will turn on, cool the unit down to a certain temperature, then will switch off and wait until the temperature slowly drifts up from the ‘cold enough to stop cooling’ setting to the ‘hot enough to start cooling again’ setting, at which point in will then repeat the cycle.  It is the same for your furnace or your water heater or your oven or stove top element – these things cycle on and off, all the time, probably with you not even noticing.

So it is difficult to translate from a power rating to a total energy consumption, unless you know how many hours a day/week/month/year the device will be operating.  Energy Star ratings can give you some guesstimates, but these numbers, which are typically self-assessed by the manufacturers, are sometimes massively understated, so consider them as indicative best case scenarios rather than as the gospel truth.

The power ratings are useful when working out what your peak power requirements will be.  Simply add together the wattages of everything that you think might be on at the same time.

There’s one more issue to consider, when considering your peak power requirement.  Many appliances draw more power when they first switch on than they then consume while running.  This can be thought of as the extra power to spin their motor up to speed, as compared to the lesser power required to keep it turning once it is at normal speed.  For a couple of seconds, some appliances will draw two or even three times their rated power.

So, potentially, you need to not only plan for a ‘worst case’ scenario with all appliances running simultaneously (or, alternatively, plan your system so this is not possible) but you also need to plan for a scenario where all the appliances start at the same time, too.

We discuss ways to minimize these issues in other parts of this series.

Summary

Watts measure the rate at which something consumes (or creates) power.  There are other ways of measuring power, too, with different names and units, and there are simple conversion tables to convert any unit of power to any other unit of power.

Anything that provides or consumes power can have its power input/output measured in watts – even open fires.

Watt hours measure the total amount of energy something has consumed (or created) over a certain period of time.  There are, again, other measurements of energy in addition to watt hours, and they can of course be converted between the different measuring systems if needed.

It is convenient for us to consider everything in the same units, and we suggest we stick to watts and watt hours.

The most important thing for us as preppers is to understand the total amount of energy we need per day or week or month, and then to understand the rate at which we need the energy provided (the amount of maximum power we need).

Explaining the Power Meter Picture

Finally, in case it remains still unclear to you, an explanation of the ‘power’ meter we showed at the top of this article.  The rotating disk shows the rate of power flow – the faster it turns, the more power is flowing into your house.  The dials are counting up the total energy supplied, and it is the dial reading each month or two which establishes the total energy you have consumed.