Confused about kilo-Watt versus kilo-Watt-hours? When even electricians talk about “kilo-Watts” when they actually meant “kilo-Watt-hours” it shows that a little education is in order. Being able to differentiate between power (“kilo-Watt” or kW) and energy (“kilo-Watt-hours” or kWh) will help you make better decisions about your system.
Yesterday I was talking to a very nice couple that was about to build an off-grid home. As with any off-grid system discussion I pointed out that we had to find out first how much energy they were going to use on a typical day in summer and winter, and the rest would follow from that. The result was mighty confusion about why several kilo-Watt of solar panels would not work to run their (planned) 175 Watt heating wire to keep the septic line from freezing in winter. The confusion was about power versus energy.
The difference is really not all that difficult: Let’s take a 100 Watt light bulb. When it is switched on it is using 100 Watt of power. How much energy it uses depends on how long that light bulb is switched on for. After 1 hour it will have used up 1 x 100 = 100 Watt-hour of electrical energy. After 10 hours it will have consumed 10 x 100 = 1,000 Watt-hours = 1 kWh of electricity.
If you are on the grid, your electrical company will bill your for energy used; take a look at your bill and each month you will see kWh’s used that month, and how much you pay depends on that.
In short, power is how much it takes to run a device at a particular instant. Energy is the amount of energy consumed by that device over time.
Here is why it is important to differentiate: Let’s again take that 100 Watt light bulb. Lights tend to be used for several hours in the evening after it gets dark, so say it it switched on for 4 hours before we go to sleep on a particular night. In that time it will have consumed 4 x 100 = 400 Wh = 0.4 kWh. Now, let’s take a 1000 Watt (= 1 kW) microwave, to heat up our dinner that evening. Instinctively we feel that running such a large load is a lot more difficult than running a much smaller light bulb, but is that true? To get the food nice and hot we need 2 minutes in the microwave, that is 0.033 hour. The amount of energy consumed by that microwave is 0.033 x 1000 = 33 Wh or 0.033 kWh. Even though the microwave needs much more power to run, it uses much less energy with typical use, compared to a light bulb.
So how does power and energy come into play for an off-grid system?
Providing electricity off-grid is all about energy needs. The basis of it all, how many solar panels, how large of a battery bank, and how costly the system is going to be, all depend on how much energy that system needs to be able to provide on a typical day. Energy is kWh’s, so it comes down to how many kWh’s per day is needed, in particular in winter when it is generally hardest to produce that energy!
Most off-grid systems use only solar panels (and possibly a backup generator) to provide energy to the system. In our part of Canada, in the middle of winter, one kilo-Watt of solar panels (4 typical panels) will produce just about 1.33 kWh of energy per average winter day (twice that from spring through fall). Of course, some days it is more, and some it is less, this is an average for the month of December around here, with the panels mounted facing south, good sun exposure during the day, at a 60-degree tilt-angle.
Let’s go back to that 175 Watt heating wire, that our customer wanted to use to keep the septic pipes from freezing: To run that day and night in winter would require 175 x 24 = 4,200 Wh = 4.2 kWh per day. So how many solar panels does that require? One kW of solar panels will produce around 1.33 kWh per average day, so this works out to 4.2 / 1.33 = 3.2 kW of solar panels, give or take a little. With a typical panel around 250 Watt these days, that means it takes right around 12 or 13 solar PV modules, just to provide the energy needed to run that heating wire! This does not yet take the energy for the rest of the house into account (which as an aside, goes to show that making heat with electricity is a very bad and expensive idea for an off-grid system).
Each panel may be rated at 250 Watt, that does not mean it can run or provide the energy to run a 250 Watt load. PV module ratings are based on a standard (and very high) sunlight intensity, and a standard temperature. Conditions that are only found a few times a year at our latitude. Their 250 Watt rating is the power they can provide at a given instance, how much energy they produce will depend very much on the length of time that they provide power for, just like the energy used by our earlier light bulb depends on how long it is switched on for.
The size of the battery bank for an off-grid system also depends directly on the energy per average day that is needed. Typical battery bank sizing is to store 3 days worth of energy in the batteries (where after 3 days with nothing coming in they are down to 50% State-Of-Charge). Since much of the cost of an off-grid system is sunk into that battery bank, it is important to keep the energy use, in kWh’s, down to a reasonable number.
So how does power (in Watt or kiloWatt) matter for an off-grid system?
Remember that power is what is consumed at a given instant. The sum of all the appliances and loads that are switched on at any given instant determines power use. For example, if (worst case) that 1000 Watt microwave, 10 light bulbs of 10 Watt, a 350 Watt well-pump, 450 Watt fridge, and 100 Watt television are switched on concurrently, they will be using 1000 + 10 x 10 + 350 + 450 + 100 = 2,000 Watt in power at that instance, and 2,000 Watt is 2 kW. The inverter needs to be able to provide that power at that instance, so you will need at least a 2 kW inverter to run the system. That is how power matters, it determines the inverter size.
Hopefully this makes the difference between kilo-Watt and kilo-Watt-hour clear! And for all those electricians reading this; I expect you to never again say “kilo-Watts” when you should have used “kilo-Watt-hours”. They are very different things!