VERDICT: I favour the Efergy Elite (2011).
Review: Efergy Elite 2.0 vs Npower Smartpower (+Owl)
(AKA "power monitors" or "energy monitors" or power vampire finders!)
See photos of the two being tested together.
What Energy Monitors Do
These devices give you a fair idea in real-time of how much power your whole house (not just the room you are in!) is sucking from the grid. Various studies indicate that just seeing what is being used when helps economise by 10% to 20%, as the Efergy 'energy saving guide' claims. For example, see much you normally consume, then how much that drops when you turn items off at the wall not in use. Or see the savings from turning lights off when there's enough daylight, for example in pence per hour on the display in front of you.
The style of "clip-on" meter being reviewed here (a sensor simply needs to be clipped onto the main power wire at your meter) is quick and easy to install but does not give perfectly accurate numbers, and also makes it harder to pick out the behaviour and power consumption of multiple appliances.
(Tip: in our relatively-efficient house at less than half typical UK consumption we have little 'vampire'/'standby' consumption so our consumption at night drops to 50W or less (shown as 0.05kW) other than when the fridge runs; let me know if you consistently beat that and still not cut your bills!)
Fitting and Setting Up
Both wireless meters/monitors were easy to set up and install.
Amazingly both seem to work well even with the door shut on our metal meter enclosure with the transmitters inside, and they don't seem to interfere with one another at all.
The monitors have a similar resolution (Elite is 10W, Smartpower is 5W) and seemed to agree closely with one another most of the time, eg one reading 260W and the other 270W.
The sleek black npower monitor display needs to be plugged in, which inhibits keeping an eye on it while walking round the house (eg turning things on and off to see the effect), but has a slightly prettier graphical display than the Efergy. The chunky rubbery Efergy display is small enough to slip into a (big) pocket and has a decent green backlight (so I was able to check at night that we were down to 50W or so) though annoyingly sometimes I could not get the backlight to work when needed because it's set to only work between certain hours.
Solar PV Microgeneration Confusion
I did the initial set-up in the evening where everything behaved as expected, but in the day, when our solar PV grid-tie system started generating/exporting, things went a little wrong; by their basic design these monitors are not capable of handling export from local microgeneration but will instead see it as a load, ie consumption.
As soon as the PV was generating 20W--200W both monitors showed a high load, ~450W for the Efergy and ~495W for the npower, and with the phantom load rising as the export does. (Switching on a 1kW load such as our toaster or travel kettle has the display change from ~450W to ~1150W on the Efergy.)
Not only can the meters not tell which way the power is flowing, but something about the shape of the current flow or whatever is confusing them about its magnitude/size too.
As the amount of generation has gone up, eg to over 600W, the monitor displays are more closely/accurately reflecting the export number (even though it is still in the 'wrong' direction!).
None of this is a critisism of this style of clip-on meter, it is just an inherent limitation of the easy-to-deploy design.
These clip-on meters will have problems with non-resistive loads and with fluctuating voltages too, which they cannot account for correctly.
Both monitors require batteries for the sensor/transmitter, and the Elite requires a further set for its mobile display.
Though the Elite recommends rechargeables throughout, disappointingly the Smartpower insists on disposables thus contributing to waste when it exists in order to reduce waste. The Elite also allows the transmission rate from the sensor to be reduced, presumably to reduce power consumption and prolong batteryy life, also good.
My partner found the meters easy enough to use, mainly the Efergy Elite, and to spot the reading rise when the kettle was on. She's not convinced that she'd use it all the time, but then we've already ruthlessly measured and trimmed and conserved!
Owl Wireless Electricity Monitor CM119
The Owl seems to want entirely alkaline batteries (no rechargables) which is a shame.
The clip-on meter and transmitter are lighter than that for AlertMe, more like the Efergy and Npower/GEO.
The display unit is big and clear (and monochrome) and entirely portable, ie no mains required though there is a socket to plug in a mains adapter. The power/cost is shown in super-large digits, readable across a room, with smaller cumulative and time/date/temperature readings below.
Operating the buttons to set up the unit and change display mode produces quite loud and piercing beeps which could be bad or good (eg good for someone elderly with diminished vision and hearing).
The whole-house consumption shown by the Owl fairly closely tracks that shown by the AlertMe (though both wrong/misleading when our solar PV is generating). The resolution is 1W, and although that is somewhat spurious, given that clip-on meters can easily be a factor or 2 or 3 out on some loads, the layout of the display avoids the need for any mental arithmetic!
The controls (buttons, menus, etc) are reasonably easy to use and indeed to guess without looking at the instruction booklet!
The Owl seems not to have a backlight, which is a slight disadvantage.
Note that unlike the Efergy and Npower/GEO the Owl is potentially able to work on 3-phase system (two more clip-on meter/transmitters would be needed I think). Although the AlertMe system could conceivably accept inputs from three clamps I don't believe that the underlying software would do the right thing currently.
Because of its portability, backlight, and use of rechargeables throughout, I favour the Efergy Elite.