Electronics > Power/Renewable Energy/EV's

LED bulbs don't like MSW inverters?

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I obtained a little Ryobi 150 W inverter with the idea of using it to run a table lamp or two in the event of a power outage. I thought that if an LED bulb uses 7 to 9 W it could run for quite a long time on a 4 Ah battery.

However, I've noticed that while these bulbs do indeed draw the rated power from the mains according to my power measuring device (a Kill A Watt clone), they draw twice as much at the input of the inverter, giving an overall efficiency of 50% or less. Since my measuring device doesn't work properly on the output of the inverter, I cannot tell where all the extra power is going. Is it wasted in the bulb, or is it wasted in the inverter?

(I am measuring the efficiency by running the inverter off a DC power supply and measuring the input current at a fixed 20 V.)

Some LED bulbs don't work at all on the inverter, and I have heard stories of other LED bulbs failing prematurely.

For comparison, incandescent bulbs of a similar 6 - 25 W power rating run the inverter at a reasonable efficiency approaching 90%, but this is not helpful when the total power draw is so much higher.

I have one 14 W CFL that draws 17 W at the inverter input, and although that's a reasonable efficiency it is still 17 W.


* Does anyone know what's the deal with modified sine wave inverters and LED bulbs, and why they don't work efficiently?
* Does anyone have experience of Kill A Watt devices on MSW outputs, and knows of one that works correctly and gives proper readings?

How did you actually measure? Did you just measure Pout/Pin and are surprised that the efficiency of the inverter is lower at lower load? That is to be expected because it has some minimum power draw to operate at all, hence efficiency must approach 0% when load power approaches zero. Or did you measure efficiency of incremental power, (Pout2-Pout1)/(Pin2-Pin1), where Pout2>Pout1, or Pout1=0? Even then, the problem might be that the inverter enters some power saving mode at zero output load.

I attach two graphs. One shows the measured performance with resistive loads (incandescent bulbs), and one with non-resistive loads (various LED and CFL bulbs).

The resistive loads show a consistent pattern with an ultimate inverter efficiency of about 92% and a static power consumption of about 2 W.

The LED and CFL bulbs show a very inconsistent pattern with an operating efficiency often less than 50%. There is quite a contrast between the two load types.

[Edit: I have added the no load point to the first graph]


--- Quote from: Siwastaja on December 04, 2022, 07:36:22 pm ---How did you actually measure? 
--- End quote ---

See my post above. I measured Pout/Pin for various loads, both resistive and otherwise, and looked at the trends. For similar sized resistive and non-resistive loads the efficiencies are markedly different (~80% vs ~50%).

The bulbs that draw more power than expected probably use a capacitive dropper technique.  Your inverter is a modified sine wave (yuck) which may be pushing more power through the dropper (and also likely why your AC power meter doesn't like being on the output of the inverter).


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