Computing Loads

pvoffgrid · June 2013

Hi, I am using the PV load evaluation spreadsheet from this site in combination with my Kill-A-Watt meters and it is an eye-opener in detailing how much power a particular item uses. As an example, my DeWalt DW9116 NiCD 2A, 120V Battery Charger read on the Kill-A-Watt 1.09A, 70w, 136VA, 59.9Hz, and 0.50pf. The load evaluation sheet computes that if plugged in with a battery charging for 24 hrs per day (recommended by a DeWalt rep) for 7 days a week = 1,680 average daily watt-hours. If I cut that back to 3 days per week, to Friday, Saturday and Sunday when I am most apt to use the DeWalt drill it still computes to 720 average daily watt-hours. So, this helps to pinpoint what items should be unplugged or at a minimum put on a power strip that can be turned off & on to avoid having to always plug something in...easier for a child to push a switch on a power strip that to plug a cord in, plug something in a hard to reach location, etc.

But, I do have a question on the load evaluation spreadsheet: how do you account for hours per day in this example: a/v equipment rack: plugged in 24 hrs per day and only used to watch movies 3 hours per week. It cannot be unplugged or the power strip turned off because it is the central source for the whole house audio and for my family to go downstairs to the theater room to plug the rack in or to to turn a power strip on...well, that is not an option. LOL So, 24 hrs off for 5 days per week, on for movies 3 hours per week, and on for house audio 7 hours per week.... is that a 3 line entry on the load evaluation spreadsheet? How would this be entered?

Thanks.

Cariboocoot · June 2013

Re: Computing Loads

If you have the device (in this case the DeWalt charger) and the Kill-A-Watt meter and a source of AC to run it there is no need to use any spreadsheet to calculate power usage. Plug it in and run it for a week and see what it actually uses. This will be different from a calculation based on its maximum power consumption because it will not use maximum power all the time.

For example you really only need to plug it in at all when it is charging batteries. The rest of the time, unplug it and you don't waste power on a phantom load (running the charger while it is doing nothing).

This applies to the A/V rack as well. If you can not (or family will not) turn off things "for real" they will suck down some power while waiting to be used. Plug it in to the meter and measure the power use, do not calculate it.

pvoffgrid · June 2013

Re: Computing Loads

So how does the Kill-A-Watt work to determining the average daily watt-hours in determining the needed amp-hours of a battery bank? Isn't this what the load evaluation spreadsheet is for, to take the measurements obtain either from a measurement device like a Kill-A-Watt and/or equipment motor labels/electrical specs? Without the load evaluation spreadsheet I would not have known how much power that battery charger used.

Thanks for the help.

Cariboocoot · June 2013

Re: Computing Loads

The K-A-W will run up accumulated Watt hours for anything you run through it. You need only measure it for a day or a week and get some idea of the average power consumption in that time period. Then you divide by the number of days and get an average daily consumption.

Yes, some days you might use more power and some days you might use less. This is why you need the "fudge factors" mentioned in the other thread. The absolute worst thing you can do is try to nail down every number precisely: you will end up with either a system that is expensively oversized due to expectation of maximum power use daily or one that shuts down unexpectedly due to scrimping on capacity. This is why we talk of "balanced systems"; getting the storage capacity and recharging capacity to work with the power usage. All of these things vary from day to day, including the storage capacity. That will change according to the rate the power is used at (faster rate = lower true capacity) and will inevitably go down over time (batteries lose capacity as they age).

So you can never deal with "100%" figures; there has to be a "+/-" involved with everything. This is why actual usage patterns over time of anything plugged in is more valuable than numbers calculated on a spreadsheet.

Using data off equipment labels, btw, is the worst. The Voltage may be right, but the current number will be a maximum (usually; sometimes it's pure fiction) and they will leave out the power factor entirely.

pvoffgrid · June 2013

Re: Computing Loads

That is what my thought was...cannot nail down an exact number else you end up with an incorrect solution. But, the Kill-A-Watt and load evaluation spreadsheet in combination give a good approximation of what is needed, to be used with real-world experience & common sense. And, the label data issues you describe is what I found using the Kill-A-Watt.

Cariboocoot · June 2013

Re: Computing Loads

If you want to you can consider any one device as two different devices: power consumption when on, and power consumption when "off". (Note the quotes because the things don't really go off. We can call it "idle" instead.) Then you have the number of hours on and the number of hours idle.

With something like an A/V rack this can be considerable. It could use 50 Watts idle 22 hours a day and 200 Watts in use 2 hours a day. So you get:
Idle A/V rack = 50 Watts * 22 hours = 1100 Watt hours daily.
On A/V rack = 200 Watts * 2 hours = 400 Watt hours daily.

That is not an unrealistic example and demonstrates just how bad those phantom loads are; they can use up more power daily than the device in use.

BB. · June 2013

Re: Computing Loads

For example, the cordless drill probably takes an hour or so to recharge, then the charger shuts down (or at least drops from 70 watts to less than 5 watts).

So, your charger may use 70 to 140 WH to charge, and use 5 watts * 24 hours per day = 120 WH on standby (pure guess/just for sake of discussion).

This would show that placing the drill charger on a separate switch/switch power bar would save energy. The tool (if charged daily) would use 1/2 the amount of energy if the charger was only turned on when used for charging.

Similar for the A/V equipment... Older equipment (such as my old stereo used about 20+ watts on standby) and Satilite Receivers/Digital TV Receivers-converters/DVRs/etc. can use also use a lot of power when "turned off" (some like DVRs use about the same amount of power when turned on or off--the computer and hard drive run "waiting" to record--frequently even if no recording is scheduled. And for Sattilite Receivers, sometimes they lose "programming/authorization codes when not turned on 24x7. Some folks have to call every couple of months and/or go to website to restore Sat Receiver function).

My own Washer/Drier (natural gas) uses about as much power in "standby" as it does actually washing/drying clothes. I can save almost 50% of their power usage by only switching them on when I am actively using them.

New Energy Star (US) rated appliances are doing much better these days regarding standby loads--But there are some classes of equipment (Sat Receivers/DVRs/Desktop Computers) that still tend to waste a fair amount of power on "standby".

The Kill-a-Watt meter will tell you a lot... for example you measure 0.8 kWH in a 36 hour period, then converted to 1x 24 hour day:

0.8 kWH * 24 hours per day * 1/(36 hour measurement period) = 0.53 kWH per 24 hour period.

It is amazing, but many times the appliances that draw power 12 hours per day (office equipment) or 24 hours per day (DVRs, home computer servers, etc.) can actually use as much or more kWH per full day vs a microwave oven or induction hot plate (for cooking).

-Bill

pvoffgrid · June 2013

Re: Computing Loads

That is what is happening: I am becoming amazed...until you see it it is hard to understand. I knew the phantom loads were there but to see them drawing more than a microwave being used is eye-opening.

I have been viewing the appliances as 2 different devices as you suggested but then got myself confused. As in your example and as I have done, 22 + 2 = 24 hours 7 days a week. But, on Saturday each week my daughter will watch a movie in the theater room so it actually is 24 * 5 for the days it is not in use, and 22 + 2 for that one day, and so one if she watches a movie a different day,etc. And, I just answered my own question....Thanks! I need to look at that way find the days that are common ..in this case, 5 days of no use, then the days it is used. And, unplugging it is an issue given the reasons Bill mentioned about satellite programming codes, etc. Although we dropped DirecTV (treating new customers way, way better than long-term existing customers, IMHO) and went OTA HD, Pandora streaming radio in every room of the house is nice and to be able to grab the remote while upstairs and turn on the a/v equipment that is downstairs in the theater room is nice. If we had to go down to the room to turn the switch on I would hear about..over & over & over.

BB. · June 2013

Re: Computing Loads

There are remotes that can be used on a "whole home" basis--And you could look into a small/dedicated (lower power) system for home audio without having to fire up the A/V center (maybe).

Otherwise, tell them that they can pay $20 per month (or whatever) towards the monthly power bill because of the A/V system use.

-Bill

Computing Loads

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