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Cent-a-meter “Owl”: watching your energy consumption by Stan Swan 94  Silicon Chip siliconchip.com.au A long with food and water, reliable electricity supplies are rightly considered an essential of modern life, even with the environmental issues regarding power generation. Mains electrical energy may well be a must-have but it’s certainly not free. And today’s switched-on consumers often face energy “bill shock”. Electricity may be hazardous but this has always been assumed to one’s health rather than wealth! In spite of improving appliance efficiency and better-insulated homes, soaring electricity bills often are due to ignorance about just “Watt” in their home is using the family “Joules”. Incidentally, the Joule is the unit of energy while Watt is the unit of power. Watts and Joules are related: Energy = Power x time, so 1 Joule = 1 Watt x 1 second. By contrast, 1-kilowatt-hour “unit” on your electricity bill is equivalent to 1kW for 1 hour = 1000W x 3600s = 3.6 megajoules (MJ). Consumers’ attempts to ease electricity consumption, both sensible (energy-efficient lamps and appliances) and half-baked (wrapping in blankets/sitting around a candle/ cold showers), may be futile if the true culprit is a powerhogging beer fridge in the garage or the pool pump being on for unduly long times. Short of balancing on a chair while trying to read a dusty switchboard meter, it’s not easy to relate high power consumption (arising perhaps from an earlier cold spell) to accounts received weeks later. Behaviour modification usually best occurs when associated with feedback at the time – you don’t stop a dog chasing cars by scolding it weeks later! Fortunately a range of electrical energy monitoring devices is now available. These fall into distinct classes, either simple plug-in units ideal for monitoring single appliances or switchboard-installed whole-of-house models. The Australian-designed Cent-a-meter, reviewed by SILICON CHIP in October 2003, is the best known of the latter type and has had justifiably wide uptake. With a simple clamp sensor installed on the switchboard’s insulated Active wire, it monitors the total household current. This value is transmitted (as a 433MHz data signal) to its portable, LCD-fitted receiver nearby. Features of the original Cent-a-meter included: • Near-instantaneous response to total household load changes • Convenient un-tethered viewing of the display, perhaps even at your bedside to reassure you that things are turned off. • Temperature and humidity displays. • A wide dynamic power range. • A convenient wireless display. However, issues arising have included: • Only apparent power (Volt.Amps) is being monitored, giving deceptive readings for many standby and inductive appliances. • No computer linking (serial or USB) is provided. • Battery life (3 x AA alkalines) on the receiver display is only a few months. (The sender batteries have similar life.) The recent release of an improved Cent-a-meter “Owl”, now apparently sourced from the UK, addresses some of these shortcomings, although its price has increased to siliconchip.com.au The complete Cent-a-meter OWL system: wireless data unit at left, sensor almost hidden at rear and the sometimes confusing display unit at right. around $200. Aside from date and time, its most obvious improved feature is the ability to store readings and thus show accumulated energy usage and running costs over time. Battery life is also said to be improved but my measurements (of several milliamps) indicate that only a few months could still be expected from three alkaline AA cells. For prolonged use a plug-in AC adapter should really be used for the LCD. Wireless coverage of both units (through timber-frame NZ houses) has been found to be a good 30m, with no noticeable susceptibility to interference. Evidence of a more sensitive (three channel) receiver in the new display may usefully boost ranges. What Watts? Due to the very nature of the current transformer clamp pickup, the new Cent-a-meter still does not respond to true billable “Watts” power as does a normal switchboard watthour meter. This means that the Cent-a-meter indicates higher power consumption than will actually be the case when motor driven appliances are being used, although the results will be pretty close for resistive loads (arising from mainly lighting, cooking and heating etc – by far the largest domestic usage), so this may not be too crucial overall. Further deviations may well arise from supply level variations (assumed to be 230V although in Australia it is typically 240V and often higher. Hence, overall it is debatable if A close-up of the inductive pick-up, designed to clamp on to the main Active cable in the rear of the switchboard. October 2008  95 From the publishers of SILICON CHIP PERFORMANCE ELECTRONICS FOR CARS NOT A REPRINT: More than 160 pages of new and exciting projects never published before – all designed to get top performance from your car. FASCINATING ARTICLES: 7 chapters explaining your car – engine management, car electronics systems, etc ADVANCED PROJECTS: You’ll build controllers for turbo boost, nitrous, fuel injection and much more! We explain the why as well as the how to! Available direct from the Publisher ($22.50 inc postage): Silicon Chip Publications, PO Box 139, Collaroy NSW 2097. Ph (02) 9979 5644; Fax (02) 9979 6503; email silchip<at>siliconchip.com.au; or via our website: www.siliconchip.com.au 96  Silicon Chip showing detailed power costs would really be merited. Setup and display Setup of the “Owl” unit should be easy enough for those used to modern appliance interfaces but multiple menus (selected by the three front buttons) make setting some options (currency units etc) rather a chore. Somewhat annoyingly, a strange 10-segment digit has been used for the largest LCD values and although these are readily visible, their “bumpiness” makes for awkward at-a-glance reading. Several users considered the LCD itself rather “too busy”, with the actual running costs etc difficult to spot amongst a sea of digits and display options. Despite politician’s and vested interest groups’ best endeavours, to most families greenhouse gas values are usually of much less interest than the dollars involved! In spite of four energy tariff options provided, a further complication arises due to the fixed line charges often levied by many energy suppliers. Typically, these are 50c$1 daily and for light users (perhaps away on holiday or someone particularly careful with their usage) the monthly bill may be more due to fixed charges than energy actually consumed. Thus even with accurate settings and a resistive load, a typical “ma and pa” Cent-a-meter user may find they receive a bill significantly above what their “Owl” indicates. Of course, ever-present GST will bias this even further. With experience the new “Owl” will provide a useful guide to the level of one’s likely power bill and it can alert consumers to wasteful energy consumption patterns. Note also that the clamp connection for the Cent-ameter should only monitor the Active wired going to the main watt-hour meter. If should not monitor the current drawn by any off-peak hot-water service as the tariff is markedly different, at least it is in most parts of Australia where off-peak tariffs are much lower than the general domestic tariff. Having said that, the fact that the Cent-a-meter monitors “whole-of-house” consumption, spotting the energy hog culprits may be complicated by concurrent activity from other appliances. For such detective work an individual plug-in unit may be better. Given standing non-energy charges and power factor issues, it’s likely that the Cent-a-meter’s indicative cost readings may only be a rough guide to the actual monthly bill. However, if 25% of energy consumption is indeed able to be saved as claimed, then the purchase cost could be covered in just a season. Maybe it could be as simple as “persuading” a household perennial radiator hog to chip in for their fair share of the bill. You can’t beat an investment like that. Of course – thanks to the wireless fed display – late night monitoring of teens watching the plasma TV in the lounge, or messing up the kitchen, may be done from the comfort of one’s bed – or even beside that beer fridge in the garage! SC The Cent-a-Meter Owl is available online from www. centameter.co.nz for $NZ189.95 + $19.95 P&P (~$AU172) or from www.todae.com.au for $AU199.95 inc P&P. siliconchip.com.au