Inverters

Inverters: Instead of a UPS, it is possible to buy stand alone inverters that hook up to a car or marine battery, but most people use these for 117 VAC power while camping, or in boats, or cars to power household AC devices. Uninterruptible power supplies (UPS) contain an inverter, small battery & an electronic circuit that switches over to the battery if there is a power failure, as well as charges the battery. Since I have tested some cheap consumer level inverters, I will mention them here. Note that most inverters do not give very high quality sine wave for AC electricity & are instead a square wave AC (which they call "modified sine wave") which can burn out some appliances & cause light bulbs to fail much sooner. They stay square shaped throughout the draining of the battery, although the width & amplitude change.

400 Watt Motomaster brand Eliminator inverter (it's actually 300 Watts of maximum constant power, not 400 Watts) I bought at Canadian Tire ($59.99 CDN). I used the 14 Amp/hour battery & got a total of 161 minutes (2:41 hours) lighting a 35 Watt incandescent bulb before the inverter shut itself off at 10.07 Volts (the inverter was supposed to shut off at 9.5 Volts + or - .5 Volt so it did it's job correctly). The low Voltage buzzer came on after 158 minutes (2:38 hours) when the battery was at 10.69 Volts (the low Voltage buzzer is supposed to come on at 10.5 Volts + or - .5 Volts, so again it did what it was supposed to do). All things consider, for a cheap modified sine wave inverter, this model was far more desirable than the 400 Watt ACDelco inverter (mentioned below) because it's Voltage levels were higher & it's low battery & auto shut down levels were set correctly. However, I was not impressed with the AC Voltage regulation level which ranged from 115.0 - 133.9. That is conducive to a bright bulb, but there would be a wide range in light output (possibly a variance of more than 50% in brightness). The Wattage draw from the AC light was nearly constant throughout the test at 37 - 40 Watts (usually 38 - 39 Watts), but the very wide Voltage range is what concerns me because it's Voltage that determines bulb brightness, not Wattage. High Voltage could also destroy some electronic devices. I measured the DC current (Amperage) & it measured 4.25 Amps at 12.30 Volts with fan on, which is 52.27 Watts drawn from the battery. The Watts that the AC bulb was using at the time was 38, so the difference was approximately 14 Watts being consumed by the inverter. That makes the inverter about 72.5% efficient at a 38 Watt load, meaning that 72.5% of the battery power consumed was being used to power the light bulb & the remaining 27.5% was used to power the inverter (a waste). The manufacturer claims a 90% efficiency factor, although that may be at a 300 Watt load which I did not test. The fan in this inverter only comes on when needed & during this low Wattage test, the fan was not on the whole time. Unlike the test with the 400 Watt ACDelco inverter, the Motomaster 400 Watt inverter did allow the AC Watt meter (WattsUp) to function correctly right to the end. This model is advertised as having a "modified sine wave" which seems to imply that it's not a perfect sine wave pattern but close enough, however there is nothing modified about this sine wave, it is in fact "square wave" which is not easy on some electronic devices.

175 Watt Motomaster brand Eliminator inverter(it's actually 150 Watts of maximum constant power, not 175 Watts) I bought at Canadian Tire ($39.99 CDN). A 14 Amp/hour battery powered the 35 Watt AC light for 111 minutes (1:51 hours) with this inverter. This 175 Watt inverter was 75% efficient with a 35 Watt load. It automatically & prematurely shut down at 11.32 Volts (the inverter was supposed to shut off at 9.5 Volts + or - .5 Volt). This model is advertised as having a "modified sine wave" which seems to imply that it's not a perfect sine wave pattern but close enough, however there is nothing modified about this sine wave, it is in fact "square wave" which is not easy on some electronic devices.

400 Watt ACDelco brand inverter (made by Superex) & bought at Wal-Mart ($49.96 CDN). I used a 20 Amp/hour battery this time & got a total of 223 minutes (3:43 hours) lighting the 35 Watt incandescent bulb before it shut itself off at 11.00 Volts (the inverter was supposed to shut off at 9.5 Volts + or - .5 Volt). The low Voltage buzzer came on after 155 minutes (2:25 hours) when the battery was at 11.6 Volts (the buzzer is supposed to come on at 10.5 Volts + or - .5 Volts). I was not impressed with the AC Voltage regulation level which started low (108.9 Volts), went as high as 114.9 Volts about half way through the test & by the end of the test was back down to 106.4 Volts. This is not conducive to a bright bulb or consistent light output. Wattage levels of bulbs & expected light output are rated at 117 VAC. I couldn't measure the DC current (Amperage) constantly, but it must have been rising through the test because the AC Wattage started off at 31 Watts, midpoint it was 36 Watts, by 90% through the test it was 40 Watts & by the end of the test the Wattage was lower judging by the dimmer bulb. I would want to see considerably better consistency in output of an inverter than that, as this represents a huge change in bulb brightness over the duration of the test. In other words the Voltage regulation of this inverter sucks & it will not give a consistent light or Voltage output. I measured the current on the DC side once & it was 3.68 Amps at about 12.75 Volts, which means that it was drawing almost 47 Watts from the battery, while the AC side was burning the light at 31 Watts. The difference was 16 Watts which means this ACDelco 400 Watt inverter is not as efficient as the Motomaster 175 Watt inverter for this size load. The efficiency factor meant that only 66% of the draw from the battery was going into powering the lamp & the other 34% was wasted to power the inverter. When I disconnected the fan, the inverter drew 1.32 Watts less from the battery. This unit always has the fan running, but some inverters automatically turn the fan on via a thermostat, only when it is needed to cool down the inverter. Interestingly towards the end of my test, the AC Watt meter (WattsUp) did not want to function correctly, so I assume that the quality of sine wave or the low Voltage from the ACDelco inverter was not good enough for the electronics in the Watt meter. If that's the case, then other electronic devices such as CFLs might well perform poorly with a low quality inverter like this one.  This model is advertised as having a "modified sine wave" which seems to imply that it's not a perfect sine wave pattern but close enough, however there is nothing modified about this sine wave, it is in fact "square wave" which is not easy on some electronic devices.

600 Watt Superex Power Inverter Costco sells a 12 Volt DC to 110 Volt AC power inverter that's rated for 600 Watts for $77.99 SKU #292519 (code no. 50-306C) made by Superex Canada Ltd. Unfortunately my experience is that it only put out 101.7 Volts AC (with a light load of a fan) which makes it substandard, since normal appliances expect 117 Volts AC + or - 10%. Just to be on the safe side I tested this power inverter powered by 5 heavy duty marine batteries in parallel (600 Amp/hours total) (equivalent to at least 4 new car batteries) that were fully charged & using thick #8 wire to the inverter. This power inverter simply is too low on AC Voltage & it might cause "brown out" problems on some AC devices so I cannot recommend it. July 2003

PURE SINE WAVE INVERTERS: You will pay more for full pure sine wave inverters, but in my opinion they are worth it. Here are some web sites that offer pure sine wave inverters. http://www.absak.com/catalog/default.php/cPath/29_127_129
http://www.mascot.no/uk/nyeprodukter.asp?mod=9
http://www.invertersrus.com/inverters.html
http://www.voltageconverters.com/pure_sine_power_inverters.html

By Doug Hembruff.
Last updated July 10, 2005