Why Your 5kW Solar System Output Is Lower Than You Expected

Cleaning a dirty sold panel

Dirt is one of 5 losses

Is your new solar system producing the power output it should? 

A very common question I get from people who have just got a new solar system is: “Now the solar panels are on my roof, how do I know what the solar system output should be?”.

The solar system owners are usually a bit concerned because they have bought, for example, a  5kW system but their inverter is telling them that they only got 4kW of peak power  yesterday! Where’s the missing 1kW? Most people expect a 5kW solar system output!

Here’s an email that I got last week from Hans that is a perfect example:

Hi Finn,

Thanks for your advice. Much appreciated.

I had a system installed via you, all very nice and well installed on Thursday 15/3 and very pleasant people to deal with.

5 KW , 20 x  250 W ET panels with a Sunnyboy inverter.

During the test we have measured at peak at 14:20 4.7 kW and we were very pleased.  After that day we had some hotter and  more sunny  days but the capacity of the system always stays below 4 KW at peak. never go above it, strange.

Today a hot clear day 25kWh at peak 4kw.

May I have your opinion about this and can i expect to reach the 4.7 KW again?

What Solar System Output should Hans be getting from his 5kW system?

A fact that isn’t advertised widely in the solar industry is that most solar systems in Australian conditions will usually peak at only 80% of their specified peak power.

This is actually completely normal in almost any industry that sells hardware that has associated specs. Think about the last time you bought a car with a specified fuel consumption from the manufacturer. Did your car actually ever achieve that fuel efficiency on a day-to-day basis? Thought not!

Solar panels systems usually reach only 80% of their specified peak due to “system losses“. So what are these system losses, and how can you work them out for the solar system sitting on your roof?

In Hans’ case he is using ET Solar 250W solar panels. He hasn’t told me the model number, but there are 3 matching panels in my Solar Panel Comparison tool. I’ll assume he is using a ET-P672250 and use those specs in the calcs.

The 5 Losses in Every Solar System

A flowchart showing how the losses in a solar power system add up.

Solar System Losses Add Up!

1. Manufacturer’s power tolerance.  (1%)

All panels have a power tolerance. In the panels that hans is using, this is -1%/+3%. So worst case is that the specifiedpower output is actually 1% lower than spec.

2. Temperature Loss. (10%)

I wrote an entire blog post on solar panel temperature losses. To cut a long story short, solar panels don’t like to be hot. Most solar panels lose about 10% of their rated power on a 25°C day, more if it is hotter. Let’s assume 10% for this estimate.

3. Dirt (5%)

When your solar panels are put on your roof, airborne particulates like dust will settle on the panels’ glass. These particulates block the amount of sunlight reaching the solar cells behind the glass reducing your power. The reduction in power from particulate build up typically lies in the 5%-15% range.  Hans’ panels have just been installed so we’ll assume only a 5% loss.

4. Wiring Losses (voltage drops) (2%)

All the solar panels on your roof are interconnected with wires, then a long pair of DC wires connects the final solar panel to your inverter. All these wires have a small electrical resistance, which means the electricity flowing through them will suffer a voltage drop. This will reduce your power proportionally, typically by around 2%.

5. Inverter Efficiency (4%)

Everything goes through your inverter so the inverter efficiency will directly affect your system output. Hans is using an SMA Sunny Boy inverter. If he has a modern, transformer less model, he can expect an inverter efficiency of about 96%, giving a 4% loss.

Multiply all those together (see the pic on the right)  and you are looking at 20% total losses, giving a real world peak power of 4kW – which is exactly what Hans was seeing.

But what about the 4.7kW he saw on the first day? 

That was probably down to 3 of things. Firstly Hans tells us that the days following this reading were hotter. So there would have been higher temperature losses later on. Also, after the first day, a thin, almost invisible layer of dust probably quickly settled on the panels increasing the dirt losses from a starting point of zero. Thirdly, the panels can initially degrade a percent or two in the first day or so, before they “settle in”, after which they should degrade a lot more slowly (about 0.5% per year). Take all these into account and it would explain why Hans has not seen his 4.7kW peak power since.


About Finn Peacock

I'm a Chartered Electrical Engineer, Solar and Energy Efficiency nut, dad, and founder of SolarQuotes.com.au. My last "real job" was working for the CSIRO in their renewable energy division.


  1. Thanks Finn,
    All clear , i had again 4.5 last week on a cool but sunny day
    I am getting the average daily output in line with forecast/expectations , sunny days 25-28Kwh

  2. Bill Bromley says:

    Dear Sir, my panels produced 94kw according to my last bill and my useage is 224kw in the house. Origin paid me 6c. credit for the 94kw, what I would like to know is how do I get Origin to subtract power generated subtracted from power used ie 224kw-94 kw = 130kw and then be billed accordingly.

    Regards, Bill Bromley.

  3. mario rom says:

    her in the Philippines sun light is good and my work here as electrician i installed several solar panel and the same problem i encounter 270 watts grace solar

  4. Hi, would it be a good idea to add additional panels to counteract the expected losses mentioned in your blog … Say an extra 10-20% worth of panels 6kw of panels with a 5kw inverter ?
    And would this be acceptable to Energex ?

  5. I have noticed that most Inverters are rated at their relevant size e.g. (3kw inverter rated at 3000, 4kw @ 4000,) BUT the 5kw inverters seem to only be rated @ 4600 Why is this ?
    why are the 5kw inverters rated at a reduced Ac power rating ?

  6. Davyd Lewis says:

    I have had a 3 kW solar system in Melbourne since early 2010 and it has been a complete failure. The solar power credits have averaged $30 – $50 per quarter with no noticeable drop in usage from the grid. The installer, Modern Solar, cannot explain the poor performance of the system and completely refuses to do anything to improve it. Is there any reputable tester I can get to assess why the system is not working?

  7. phase.verocity says:

    Well i’m running a baby one,, basically I have 6x250w mono panels hooked up to a 60amp regulator that only goes off grid. I’ve turned my power off because I don’t believe electricity should be worth 24.4 cents a kWh. At the moment its approaching summer and I’m getting around 1200 watts in the best hours of the day or 8-9 kWh’s a day. I know the panels are not at the right angle and that is usually the biggest mistake people make but its near impossible for me to change them because they’re on the roof of a very large building. Resistance also plays a big part in off-grid. The fuller my batteries become the less I can use to charge the 34kwhs of battery power I’m using. Before hooking up to solar I used roughly 12kWh’s a day but with a lot of changes such as instant hot water and a laptop instead of a big computer i’m using around 5-6kWh’s a day. I’ve heard lots of people having problem hooking up to the grid and I don’t trust electricity companies, They’re out for as much as they can get like every other business these days. Would be nice if business picked up some of that “for the people” power thing that was fashionable in the early years.

  8. I had Moderns in and they tried really hard to sell me a 3 kW system on the south west facing side of the roof because that was the only spot they could put 16 panels. Had to sign the contract just to get the salesman out of the house at 7pm. Followed up the next day with a letter to cancel the contract. Eventually went with 1.5 kW facing north west with another vendor – 2 k$ vs 8 k$.

    • That does not surprise me at all about Modern Solar.

    • phase.verocity says:

      Being stand alone, I installed mine mostly myself although with a little help from a solar guy who charged me $150 and made sure they were ok. for the panels and the cable and roof rails etc 6 panels cost me around $1250 plus cable $60 plus rails $250 plus a breaker box $40 = $1750. Thats not installing anything to do with a new electricity panel or grid tie inverter either. So $2k is pretty dam good because the grid tie inverter is likely to cost $2-300. My off grid one that is 5kw cost $490 for a cheap powerjack from china. I know grid tie ones can get very expensive too so you’ve done very well.

  9. I am in the process of finding a solar system and so far the one that strikes me so far is 13 sunpower panels rated at 327w and a sma sunnyboy 4000tl inverter.(11170 fully installed) 6 panels on east and 7 panels on north .If I understand correctly this amount of panels exceeds the CEC guidelines of 2013 for this inverter. Is this correct?

  10. So how many panels can I have hooked up to a 3 kW inverter ,I previously have 12 x250 watt panels now,my other question is I want to go a little bigger and put a 5 KW growatt inverter and another 10 panels where is the best company or best electrician to get these done,I previously had true value solar put this last lot up ,good price but don’t know if good choice any advice grateful thanks…….Eddie.

    • Hi Eddie,

      If you want to upgrade your solar system then the first thing to check is how it will affect any Feed In Tariff you are currently on. Some states will let you upgrade without penalty,in other states you have to be careful how you do the upgrade.

      I explain the things you need to know in great detail here:


      Hope That Helps,



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  2. […] (If you want to learn why panels don’t get to a 100% performance ratio I go in to great detail as to why here) […]

  3. […] A typical solar inverter sold as “a 3kW inverter” can usually have more than 3kW of solar panels safely connected to it, because 3kW of panels never actually produces 3kW of power for the reasons outlined here. […]

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