Inverter Efficiency

Re: Inverter Efficiency

SolarSailor wrote: »

Some have stated an inverter would be using about 6-watts of background power, so are adding that to the load being served to determine run time and capacity calculations.

Inverters do have losses just from "running" even if there is no load. Larger inverters tend to have larger "tare" losses. So, if you have small loads, you should use a small inverter... Etc.

If you have a mix of loads, use a small inverter for the loads that are on most of the time (laptop computer, cell charger, a few CFL lamps, etc.) and use a large inverter that is only turned on when the loads are used (such as a well pump).

The problem is using a single efficiency number for an inverter is not really correct... At very low power usages, the efficiency is quite low because of internal switching losses. At high currents, the efficiency also falls because of I^2*R heating losses. Picking a number (like 85% efficiency) gives a "middle ground" number so that people (like me) don't right 5 paragraphs every time somebody asks how big a system do I need to power my cabin.

Inverter also have "tricks" to reduce their power usage... For example, they turn off their output when there is no AC load... And they pulse the output for a second or so, measure the load, and if larger than 8 watts, turn on 100% (the 8 watts is variable between vendors and sometimes programmable).

If you have a fixed/known load for your usage--please feel free to plug in your numbers. One reason I am so "wordy" is that I try and show my work and use simple equations that people can plug in their own numbers and tweak the results to match their installation.

For example, on page 2 of this PDF for the Morning Star 300 watt TSW 120 VAC inverter, you can see their efficiency runs from ~55 watts (near zero power use) to >90% for 50-150 watts, 85% for 300 watts and 70% for 600 watt output... This inverter also has several low power modes where the standby usage is ~0.3-0.6 watts (depending on mode). And ON with no load is ~6 watts.

In general, the "poor efficiency" with small loads is swamped by the losses when operating at rated capacity:

• 6 watt load + 6 watt losses = 50% efficiency is 6 watt loss at 6 watt load
• 300 watt load + 15% losses = 45 watt losses at 300 watt load

What does eat a lot of energy is "stand by losses".... Say I need 300 watts for 15 minutes a day, but leave the inverter on 24 hours per day (water pump for sink/flushing):

• 300W*0.25 hour + 6 watt * 23.75 hours per day =
• 75 Watt*Hours + 142.5 Watt*Hours = 217.5 Watt Hours per day

So, in the above case, the inverter losses by itself is almost 2x the actual power usage by the pump. If the inverter (using the Morningstar 300 watt TSW data sheet) was placed in "standby" where it checks for a load every 5 seconds or so:

• 300W*0.25 hour + 0.55 amps * 12 volt * 23.75 hours per day =
• 75 Watt*Hours + 15.675 Watt*Hours = 90.675 Watt Hours per day

Now, you have much less power use during the 24 hour period.

In addition, it seems, some do and some do not also apply either the 0.77 or 0.52 Derating Factor for DC-AC conversion efficiency of the inverter.

That would be ME!.

When using the PV Watts website to estimate how much power is available from xx KWatts of solar panels, I try and give realistic numbers (perhaps a bit on the conservative side) so that nobody spends $XX,000 on an installation and find that it does not perform to their expectations. (and remember, that solar is weather dependent--and the PV Watts data base is an average of 20 years or so of measurements... So sometimes a site will generate more power, other times less--what average is all about). Backup options (cutting power use, backup gensets, alternative sources like wind or hydro).

So where do the numbers come from? The 77% or 0.77 Derating comes from the US Government (:roll:) via the PV Watts website/solar energy calculator. It is defined for a Grid Tied system with "average" GT Inverter losses, average derating for solar panels (hot panels generate less power), wiring losses, dirt on panels, etc.

You can take a look at the "Derating Help" page here.

I took the 77% derating and added 80% derating for the flooded cell batteries (AGM's are more efficient so you can use 90%). Note that old batteries tend to lose efficiency--so that these numbers probably are more accurate for "well aged batteries" (again being conservative).

And I use 85% derating for the Inverter as it is a solid estimate for an "average" range of loads for a "generic" system.

The math then looks like this:

Derating Factor for Grid Tied = 0.77
Derating Factor for Off Grid, DC Only Output w/ flooded cell batt = 0.77*0.80 = 0.62
Derating Factor for Off Grid, AC Output w/ flooded cell batt = 0.77*0.80*0.85 = 0.52

I tell people that if their system performs within +/- 10% of the predictions--they are doing very well...

For an off grid system where weather and other factors influence power production, +/- 20% of predicted is probably doing good.

And, there is a fundamental difference between a GT and an Off-Grid system in how they store power... My GT system "stores power" (via my Net Metering Based Power Bill) for 1 year (excess in summer can be applied to winter bills--even before the power is generated--i..e, I can run a "negative" amount on my bill in winter and make up the deficit in the summer).

For Off-Grid systems, it is usually only cost efficient to store around 3 days of power... Your 1 month of good power (sunny weather) only charges the bank to 100%. And if you have more than 3 days of clouds/bad weather--you only have 3 says of that sunny month of power stored and available. Hence, the backup generator or other options).

Also, if I want to use more power--I can just pay the utility for it.

For an Off-Grid system, you cannot use more power than you have installed solar panels for (other than more generator run time).

So--another good reason to define your loads accurately, and perhaps even double that amount to allow for future growth (without having to add panels+batteries in the near future). Most people end up using more power than the first planned.

To me, this Derating Factor should account for energy used by the inverter, rather than having to first add it in, then derate it too.

Normally, I try very hard to derate only once. I throw all of the derating factors in the "system losses" of 0.52--then talk about AC loads directly.

I will sometimes break out calculations (you need inverter losses to define wire size and battery capacity)--and it may look look like I double counted--but (most of the time) I have not (do I make mistakes--I will send that question to my wife and kids ).

Seems like double-dipping to me. Actually more than double since it's both added in and then derated as well.

No, I try very hard not to do this... That is why I always post my math--so that people can check my results, understand them, and modify for their specific needs.

I see others too have posted while I typed this up -- I hope that this one helps too.

-Bill