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Seriously Flawed though it has its uses
on 19 August 2010
Despite reading in Watts and Watt-hours and claiming to measure power consumption and energy, this unit actually measures only V-A (Volt-amps). In fact it really only measures amps, which is all it can do without an actual connection to the mains, and then multiplies by the assumed voltage. This leads to HUGE ERRORS because certain appliances draw current out of phase with the voltage, giving rise to vastly inflated readings. The ratio of actual Watts to V-A for an appliance is given as the 'Power Factor', and for most fridges and freezers and other devices with induction motors it is around 0.3, so don't be surprised when your 50 watt fridge reads 160 watts. It gets worse: some electronic devices like induction hobs draw considerable capacitive current when they are switched OFF; probably because they incorporate large capacitors to filter the high-frequency noise they generate and stop it getting back into the mains. My two-hob induction unit reads around 100 Watts on the Owl even when turned off, though it is not actually consuming power, just pumping current in and out of the grid ninety degrees out of phase with the voltage! The true facts were revealed when I connected this and other devices through one of those little plug-in power monitors which do read true power as well as current and power-factor.
The 'base-level' power in many homes is taken by fridge and freezer, plus a few things on standbye, and in my case reads around 350 Watts even though it's actually nearer 100 Watts - very misleading as this is a major contributor to total energy used because it is on all day! At higher readings, when cookers and kettles are used, the readings are fairly accurate, as such devices have a power-factor of one. Interestingly, the added contribution of fridge and freezer is then also correct, because the high resistive loads swamp the out-of-phase current producing an overall power factor very close to one, which is what counts.
Even more confusing is the fact that turning on my induction hob at the wall (but leaving it turned off) REDUCES the reading on the Owl considerably! This is because it draws capacitive (leading phase) current which cancels some of the inductive (lagging phase) current drawn by the fridge and freezer. Add to this the resolution limitation and it's all rather useless, though I actually find it useful to monitor my total load on my solar off-grid system so that I can keep within the 3kW limit of the inverter. Sorry to get a bit technical, but I think this is necessary in order to give some insight into the mysteries reported by other reviewers. If the manufacturers did it right, and didn't make such false claims, there would be no need for all this - they should bring out an alternative clip-on unit which also plugs into a socket, measuring true power by muliplying instantaneous current and voltage, and needing no battery. The monitor itself could remain the same.