After reading a posting by Bob Golding a while back about the efficiency, or rather lack of efficiency, of cheap inverters I decided to do a test to prove or disprove this.
I have a large number of 300 Watt Nikkai and Skytronic MSW inverters (identical, just different name) so I set a test rig up and measured the watts into one of these inverters versus the watts out under varying loads ( various wattage incandescent lamps). I made the assumption (rightly or wrongly) that these would behave more or less as a pure resistance and so a direct reading of the AC voltage and current would give a true reading of the watts taken by the load. This cannot be done with loads involving capacitance and/or inductance unless the power factor is known.
The AC voltage and current were measured with a true RMS meter (Fluke 8050A) which arrived today after buying it on Ebay. I had been after one of these for a while so that I could check the inverter output voltages after repairing them. The DC input voltage and current were measured with a normal ammeter and voltmeter.
Load(W).....Watts into Inverter......Watts out of inverter....Efficiency
None .............. 9.4 .............................. 0.0
- ................ 21.5 ............................ 13.8 ................... 64.0 %
- ................ 44.6 ............................ 37.0 ................... 82.8
- ................ 59.5 ............................ 52.4 ................... 88.1
- ................ 79.9 ............................ 71.6 ................... 89.7
- .............. 99.6 ............................ 89.7 ................... 90.0
- ............. 117.6 .......................... 107.2 ................... 91.2
- ............. 130.9 .......................... 122.0 ................... 93.2
- ............. 152.7 .......................... 141.2 ................... 92.5
- ............. 191.4 .......................... 176.0 ................... 92.0
- ............. 230.5 .......................... 209.6 ................... 90.9
- ............. 301.5 .......................... 257.9 ................... 85.7
- ............. 359.7 .......................... 293.0 ................... 81.4
Surprisingly the results showed that the inverter was 90% efficient or more over quite a wide range of loads. Not so bad after all. The efficiency decreases dramatically for very small loads due to the idle current of the inverter being a large percentage of the actual input current. As the loads increase the idle current becomes less and less significant. On these small inverters the fans run continuously and take about 50mA. The peak efficiency seems to be at about half the rated output.
I've got a range of inverters up to 2.5 kW so I will try and repeat the tests for some different sizes. The problem is finding a suitable ammeter to handle a couple of hundred amps!
John
