In this series, I’ll cover several important topics surrounding a power or energy budget. What they are, the benefits of having one (especially off grid), and how to create one. We’ll cover calculated power vs measuring it, make some common power measurements to start our online power database, In the last installment we will offer an online calculator to help do your power budget and save energy! In part 1 we started with power budget benefits. Part 2 ran through quick calculations. In this section we’ll go over power measurement.
Earlier in the series I approached the topic of power measurement. The simplest way to get a quick number for the power of an appliance is found on its label. Sometimes the label will list the power in Watts, other times it may provide the AC voltage and current, other times the power supply may indicate the DC output voltage and current.
[editors note: The entire series can now be found on our new Power Budgets page for your convenience!]
The power calculation from the label is simply the volts times the amps. The resulting number is an approximation and does not take in to account a bunch of technical details like AC {en:power factor} and {en:root mean square} power. In any case, all the numbers you use that are derived from the label are a worst case power and can be up to twice the typical power consumption
Now thanks to the mass market and the emphasis on energy efficiency there is low cost device for consumers to measure power. Before this you needed an expensive tool that clamped around the AC power cord. Its called the P4400 “Kill a Watt Monitor” for about twenty bucks and provides a reasonable way to get real power measurements. I’ve had one for awhile and it came in very handy for getting good power numbers on our electric refrigerator that we got last summer.
Using the Kill-a-Watt Monitor
Using the device is quite simple. Just unplug your appliance, plug it into the P4400, and plug the P4400 into the wall. The monitor has an easy to read LCD display and 5 buttons.
The buttons are used as follows:
- Change display to Volts AC
- Chang display to Amps AC
- Toggle display between Watts (Real RMS value) and Volt-Amps
- Toggle display between AC frequency (should be 60Hz US) and {en:Power Factor}.
- Toggle display between accumulated KWH and the Time it has been accumulated for.
All we really need for our measurements most of the time is simply the real Watts display. Having the timer and KWH display is helpful for measuring devices that can change their own power draw like my espresso machine and refrigerator.
Measurement details
When Using the monitor, you want to consider what kind of operation will be typical for you. This is especially true for electronic devices. For example, we recently got a 32″ LCD television. If I display a TV show the power is around 62W, however if I display nothing except the blue screen the power is 90W!
- Check for “Instant On” power draw by plugging in for a minute without turning the device “on”. Here is how we solve the problem.
- Check that the parameters are what you might consider “typical” like brightness on various Displays.
- Check to device/appliance instructions to see if there are any “power saver” or “energy star” modes available. We tested a Samsung LCD TV and the average power went from 60W to 40W.
Other uses
Some uses for the monitor are obvious, and others not so. You may want to consider:
- Use it to monitor the use of thermostatically controlled appliances, like refrigerators or freezers. Monitor when doors are opened too much, or when seals leak out the cold too much.
- Take your P4400 Montor shopping!! What a great way to know your power commitment before you buy an appliance! Sure it may be a little geeky, but it can save you $$ in the long run, and salespeople won’t let this measurement get in the way of a sale.
Anyway I’m busy here in our off grid “lab” collecting data you can use in a real way for your own power budget. This week we’re on backup power and I noticed that the old generator was running a low voltage of ~100Vac. That’s good for Japan’s power but not mine, or for the P4400. It would blink every 30 or 40 seconds with a power measurement, and read the line voltage as 0V!
One other note on the P4400: When you take measurements with a modified sine wave inverter, the monitor never reads 60Hz as you would expect, instead it reads lower 45-55Hz. Its nothing to worry about.
Next in the series:Power Budgets [4] data you can use" rel="bookmark" href="../genverters/power-budgets-4-data-you-can-use/"> Power Budgets [4] data you can use
Do you have a power monitor? Do you know what your appliances really take for power? Is there a particular type of item you’re curious about? Just ask! Leave your comments, observations, and questions below!
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Mine doesn’t have as many bells & whistles as the one you’re using, but works quite well. It measures the wattage or amps & I used it at my house in town to measure all of the appliances I thought I might use while off-grid. I never considered using it “while” off-grid, to check if the results are the same. I’ll have to dig it out from wherever the heck it’s got to now & try this out once I get back to camp.
LCD TV’s are the way to go though. I don’t have one yet but it’s somewhere on the “list of stuff” I could use. Reducing is the “first” key to sucess! When I took my initial readings, I admit, I was very delusional about all of the things I was going to keep “powered up” off a couple of small panels, homebuilt windmill & batteries.
After some experimenting though, I sure learned the reality of whats possible & whats “wishfull thinking”!
Rusty´s last [type] ..I Have My Own Domain name
Yes, I’d rather have the real numbers BEFORE the move and discover that we were too optimistic!