Electricity monitors - John C. Kirk
Mar. 29th, 2010
01:09 am - Electricity monitors
I'd like to reduce my electricity bill, so I've just bought two electricity monitors from Home Energy Saving: an OWL (wireless) and a plug-in monitor. That cost about £50, but hopefully I'll save more money than that in the long run. So far, I've been impressed by both gadgets, although you could do a decent "site survey" in a day, so it might make sense to borrow rather than buy.
I heard a phrase a while back: "You can't manage what you don't measure." The purpose of these gadgets is to do that measurement, by quantifying how much electricity you use. Based on that, you can then work out what's important and what's not. For instance, if you leave a light on when you're not in a room, how much does that cost you? Does it make sense to switch to energy saving lightbulbs, or is the difference negligible? Is it ok to leave your TV on standby, or should you turn it off at the wall? This is particularly relevant when it comes to computers: just because you have a 400W power supply, that doesn't mean that it's actually drawing 400W of power from the mains.
It's important to consider the cumulative effect here, since a penny an hour doesn't feel like much. I currently pay £36/month for my electricity. Assuming 30 days per month (for simplicity), that's £1.20/day, or 5p/hour. So, if I spent an extra 2p every hour, that would be an extra £14.40/month, and I'd then be spending £50/month for electricity.
I know some people who don't have to pay an electricity bill (because it's included in their rent), so this will be less relevant to them. However, you can also use these devices to measure the environmental impact. In particular, the OWL has a "GHG" (Green House Gas) mode, which displays how many kg of CO2 you are releasing into the atmosphere each hour, and how many tonnes you've emitted in total (currently 0.001 for me).
The plug-in monitor is very easy to use: you just plug your device into the monitor, then plug the monitor into your wall socket (or 4-way adapter, or whatever). It has several modes, but I've just been looking at the number of watts. Surprisingly, some devices draw power even when they appear to be turned off, so you'd have to unplug them to eliminate that. The main drawback to this monitor is that you can't use it everywhere, e.g. it's no good for ceiling lights (which don't have a plug socket), or for appliances like an oven which are wired straight into the wall. Also, since you have to unplug the relevant device to use the monitor, it's inconvenient for anything with a built-in clock. Since the clocks went forward last night, and I've updated all my gadgets, I was reluctant to go round this afternoon and do them all again.
The OWL comes in two pieces (both battery powered): I attach the sender box to the electricity meter, then I carry the remote monitor around with me. This is pretty easy to set up: the sender box has a sensor that just clips around the outside of the cable (a bit like a pulse oximeter going around your finger), so I'm not tampering with the existing installation. Both units can be wall mounted, but I haven't bothered. You then configure the remote monitor with your tariff, and it will display how much you're currently spending (pence/hour), along with some other info.
This is similar to a speedometer: you won't necessarily use that much electricity in the current hour, but it shows you the current rate of consumption. As you turn things on and off, this figure will increase/decrease. In theory, you could figure this out without the monitor, just by checking the numbers on your electricity meter directly. However, I live above a shop and my electricity meter is in their basement, so I can't keep dashing back and forth. (This also demonstrates that the gadget has a decent range, since the wireless signal easily reaches my flat.) Even if you can check the meter, this saves you the effort of recalculating things. I could see this being particularly useful for families: if you notice that the cost has suddenly jumped up, you can find out what someone else just did to cause it.
The tariff is the only aspect that's a bit fiddly. I get my electricity from British Gas, on their Standard Tariff. On my last bill, it said: "First 99 kWh x 23.022p, next 610 kWh x 10.184p" (not including VAT). Presumably that means that I'm in the Seaboard supply area, so including VAT I pay 24.173 pence/hour for the first 99 units each quarter, then 10.693 pence/hour for all subsequent units. (According to the footnotes on that page, the first tier should apply to the first 125 units. However, since tier 1 is more expensive, I'm not complaining!) The OWL will let you enter up to four tariffs, based on the time of day, but it can't handle this type of shift. Assuming that I'll use at least 100 kW every three months, I've entered the lower rate, i.e. 10.70 pence per hour, but it's slightly misleading.
The amount of electricity I use will vary during the day, but my "baseline" (when I'm not here) is about 4p/hour. I think it's useful to make a list of all the things that I never turn off; my initial list was quite short, then I remembered various other things later on. Here's my list:
* Burglar alarm.
* Oven, microwave (power from wall, clock displayed on front panel).
* Gas boiler (control panel).
* Computer rack (server, switch, UPS, ADSL router).
* Sky+ box, DVD player (standby).
* Wall clock, alarm clock.
If I shut down the server rack, my baseline drops to 1.7p/hour. That's not practical for me, but it does highlight an area where I could make savings. I turned off my central heating on the 17th, which will reduce my gas bill, but it doesn't affect my electricity bill because I need to keep the boiler's control panel turned on (so that I have hot water for taps/shower).
I bought my kitchen appliances from De Dietrich, and I deliberately went for ones with good energy efficiency ratings, although I can't find specs for them anymore. The fridge/freezer costs 0.3p/hour for normal operation; I'm guessing that it will need a bit more power during the summer, when there's a greater temperature difference between the inside and the outside. I don't know how much the oven uses on "standby" (to power the clock and respond to buttons), but it costs 24p/hour when I turn on the fan-assisted oven. When I did a self-clean (putting it up to a very high temperature), it used about 35p/hour. I did a double take when I glanced at the OWL monitor! Sometimes I set the oven to pre-heat, then I hear it beep when I'm in the middle of typing an email so I think "I'll deal with that in a minute", and I forget about it until the next time I walk past. Knowing that it's the most expensive thing in the flat, I'll be more diligent from now on, so these monitors are proving their worth. I haven't assessed the dishwasher or washing machine yet, and I'll be interested to see how much difference the duration/temperature makes.
Here are the costs for my lights:
|Room||Power (W)||Cost (pence per hour)|
|Kitchen||5 x 50 (halogen spotlights)||2.5|
|Bedside lamp||40 (clear)||0.4|
|Hallway||20 + 11||0.4|
The kitchen actually has six lights fitted, but one of the bulbs has gone and I haven't replaced it yet, so presumably that cost would normally be a bit higher. I think it makes sense to have brighter lights in there, so that I can see what I'm doing while I'm preparing food, but I don't need to leave those lights on while the room's empty, and I was quite shocked to see how much that affected the price on the monitor.
"Normal" light bulbs can be clear or pearl, and my rule of thumb is that I'll buy a higher power for pearl bulbs (because the coating obscures some of the light). However, it's quite clear that this power comes at a price. I'm gradually replacing normal bulbs with energy efficient versions, and I can't see any difference between 11W (bedroom) and 60W (lounge) once they're warmed up. I recently bought some energy efficient bulbs for 10p each, so I'm now thinking about chucking away the normal bulbs before they go, although I may keep a normal bulb in the bedside lamp (for reading in bed). The bedroom light obviously takes some power, but it's too low to register on the monitor. I'm not sure what type of bulb is in the shower, but it looks like a flattened version of an energy efficient bulb, and I haven't replaced it since I moved here (in 2004).
Speaking of the shower, that costs 0.5p/hour, so even if I take my time then electricity usage isn't a problem. (The gas bill is a separate issue, depending on how hot the water is.)
Looking at other gadgets around the flat, where I can use the plug-in monitor:
|Name||Power (W)||Cost (pence per hour)||Power (W)||Cost (pence per hour)|
|LCD TV (27")||3||0||123||1|
|Desktop PC||4 (sleeping)||0||60||0.5|
|LCD monitor (19")||1||0||50||0.5|
|CRT monitor (14")||66||1|
There are a few specific points of interest here. Looking at the desktop PC, it sucks up power while the PSU is supplying power to the motherboard, even if the computer itself is turned off. It's a small amount, so I won't normally worry about it, but if I'm going away for a couple of weeks then I might turn it off at the back as well as the front (or turn off the 4-way adapter at the wall). The LCD monitor also draws power while it's turned off (1W), but that's the same as it uses on standby, so there's no real benefit to using the off-switch except for getting rid of the orange light.
The Sky+ box is at the opposite extreme, since it uses almost as much power on standby as it does when it's turned on! There's no actual off-switch, so the only way to turn it off properly is to yank the cable out of the back, which then means that it won't do any timed recording. According to the Sky website: "Experts estimate that Sky boxes account for about 1.5 per cent of home electricity use. [...] Your Sky box uses 30-50% less power when it is in standby mode, compared to being left on." That certainly doesn't match the results I get. In fact, the meter flickers between 23W and 24W when the box is turned on, so I initially thought that it was drawing exactly the same amount of power. At (roughly) 0.5p/hour, that's 12p/day; if that was 1.5%, then I'd be spending £8/day on electricity, or £240/month. That may be true for some people, but it seems quite excessive unless you have electric heating for a huge mansion.
The power for computers (desktop/server) and the monitors does fluctuate a bit, particularly while they're booting up, but these are rough averages. The speakers and headphones hardly use any power, so I can basically ignore them. The desktop has a 500W PSU, and the server has a 400W PSU, so in both cases they're only taking what they need, which is good. In particular, the desktop has an Akasa Power80+ PSU, which claims to be eco-friendly, and I think they're living up to that claim. The LCD monitor uses less energy than the CRT, even though it has a bigger screen, which is nice to know.
I still have a few other gadgets to test, including my shredder and breadmaker; the latter will then help me to compare the overall cost of baking my own bread vs buying it pre-made. I'll also see how much electricity it takes to charge my mobile phone, and whether it costs extra to leave the phone plugged in once it's fully charged.