When an inverter is listed as having 90% efficiency what does this actually measure? Is it the efficiency when outputting the maximum amount of power or does it mean 90% efficiency for all loads?
I'm not aware of any standard for their advertising. If they can hit 90% at any random point in their power curve, they advertise it as up to 90% efficient. Even if it's only 68% efficient at some other point in the curve.
Sandia National Laboratories and BEW have worked together to develop a test protocol to measure inverter efficiency as a function of AC output power and DC voltage. This protocol has been adopted by the California Energy Commission (CEC) and any inverter used in a CEC approved PV system must be tested by an independent lab to this protocol
Fundamentally, efficiency is a ratio of the energy (or power) output for a given energy (or power) input.
Feed 100 W DC in and get 90 W AC out = 90% efficiency.
But as @acdoctor says, because efficiency varies and depends on a range of factors, then there are standards for efficiency ratings to at least provide some level of consistency when comparing inverters.
I use the general rule of thumb that all inverters with variable loads are 85% efficient. My own Victron system (Quattro 48/5kW) is only about 65% efficient when you consider idle power draw and my very low "background" loads of ~80W, but it gets notably better when my absorption fridge turns on pulling an additional 450W. Not great, but better.
If targeting for optimal inverter efficiency, regardless of chart/manufacturer, the sweet spot seems to be around 30-33% of rated continuous power.
Inverter efficiency numbers are almost always a reflection of the inverters maximum attainable efficiency.
It is a measure of how much energy the inverter wastes within itself to make the power you require.
Below the efficiency rating is the amount of energy the unit uses just sitting there turned on. And you'd be surprised at how much juice some of these inverters use (waste) just to keep a 100 watt bulb running.
Which path and conditions for efficiency number??? Often purposely left obscure in their spec sheet. It is called 'marketing' specs
All-In-One HF inverters have many different efficiencies depending on path and other conditions. The number quoted in their spec sheet is often the best conditions for efficiency. That would be PV input to AC output when PV Vmp is at its highest allowable MPPT controller input voltage at moderate to lower load power. It gets worse at lower PV input voltage requiring more solar charge controller boost to inverter's high voltage DC bus.
MP Solar does quote battery to AC output at about 93% which is about right for moderate load.
PV input to battery charging is worse.
For LF All-In-One, all present models' solar charge controller converts PV input power directly to battery input of inverter. PV to AC output is very poor efficiency. At high Vmp limit and highest PV power net efficiency is about 75% and likely to choke from overheating unit. There is efficiency of SCC buck converter from PV input to battery voltage then battery to AC output inverter efficiency to take battery voltage to AC output. With the solar charge controller as a buck converter its efficiency is better when PV Vmp is closer to battery voltage.
LF AIO also places high ripple current on battery for PV to AC output conversion. It uses battery as a large filter capacitor. HF AIO for PV to AC output does not put ripple current on battery. Its HV DC capacitors takes most of the PV to AC output ripple current.
LF All-In-One hybrid inverter do have better PV to battery charging efficiency than HF AIO inverters.
