How Much Power Does Your (eg) 250 watt Solar Panel Actually Produce?

So there you are, thinking about buying some solar panels.  You’ve noticed they come in a range of semi-standard shapes and sizes, and maybe you’ve even noticed that the slightly unusual dimensions (such as 39.4″) actually make sense if converted to metrics (ie 39.4″ = 1 meter).

You also notice all sorts of acronyms floating around in the specifications and warranties, and you sort of wonder if, when comparing the power output of Brand X’s 250 watt panel with that of Brand Y’s 255 watt panel, if there truly is 5 watts of difference, and, for that matter, whether either panel will ever produce close to the claimed 250 watt output, and what that actually means in terms of total kWhrs a day.

Perhaps even worse are advertisements with no acronyms or qualifiers at all, just a list of unexplained specifications.  Who is making those claims, and how credible are they?

These are all good questions.  We’ll try to answer them for you.

The good news is that there are some official standards that can apply to how solar panel power outputs are measured.  The not so good news is that while these official standards might provide a level playing field for how to measure one panel’s power output alongside another panel, the results obtained by the standards do not necessarily match the real world experience you’ll get (a bit like how the official mpg figures for new cars are seldom the same as you get yourself in real-world driving).  But first, let’s understand exactly what solar panels give you, and why it so quickly becomes difficult to establish their true power output.

All solar panels provide their power in DC volts and amps.  The actual power they provide (which is measured in watts) is calculated by multiplying their output voltage by the amps of current that flows at that voltage – this might seem like a simple calculation, but it isn’t – the voltage level varies based on the amps that are flowing, and both also vary based on the intensity of the sunlight falling on the panel.  So even a simple seeming power measurement isn’t quite as simple as it should be.

It gets worse.  When you start connecting a series of panels together, the real world practical power you might get is not necessarily the simple sum of the power outputs of each individual panel.

However, simple or not, a DC watt specification is the most direct measure of their power output.  Occasionally you may see panels with an AC wattage rating – these would be panels with individual ‘micro-inverters’ that convert the DC output of the panel immediately to AC, right at the panel.

At least until recently, it has been most common to connect together the DC output from multiple panels, then feed that combined power to a single central inverter that then converts it to AC.  But there are convincing studies to suggest that micro-inverters are a very good thing, and while they might slightly add to the cost of a solar array installation, they might also result in you getting appreciably more power out of the system in real life, as compared to the implied power outputs quoted by the specifications.

For now, simply be aware that all inverters involve a slight and inevitable power loss (typically an inverter is anywhere from 95% to 98% efficient) and so if you are seeing an AC watt rating, this has already had the inverter power loss removed.  For example, a 250W DC panel, after passing through a 96% efficient inverter, would end up giving you 240W of AC power.

In other words, AC watts are generally ‘better’ than DC watts, when comparing numbers.

Now for some official ‘standards’ for solar cell power measurements, and note that usually power measurements are made by the manufacturer, rather than by an independent third-party, so there is a certain amount of trust required when accepting these numbers, no matter what the standard may be that they are claimed to have been measured by.

Many cells are rated based on a STC rating.  STC stands for ‘Standard Test Conditions’.  These are an ambient temperature of 25°C/77°F, sunlight of a 1000 W/sq m intensity falling directly on the panel, an air mass of 1.5, and zero wind speed.

Another rating is the NOCT rating.  This is the Normal Operating Cell Temperature rating, and it will always give a lower rating.  NOCT ratings assume 800 W/sq m of solar power falling on the cell, a 20°C/68°F ambient temperature, and a wind of 1 m/sec (2.24 mph) blowing on the backside of the solar panel for cooling.

Even this is optimistic.  The way most solar panels are laid out prevents any underneath cooling, and so their temperatures can rise appreciably, and as they get hotter, they become less efficient (once the air temperature gets up into the high 80s, you’re probably going to start losing 1% of power for every two degrees F of temperature rise).

But wait – there’s more.  Would you be surprised to learn that California does its own thing?  It uses a different standard, the PTC standard.  Unlike the STC rating, the PTC rating is not a measured rating, but a theoretical rating.  That might seem like a backwards step, but it is based on adjusted realworld data, and unlike the self-assessed STC rating, the PTC rating, at least as expressed by California’s CEC (California Energy Commission) requires independent lab results rather than accepting manufacturer claims.

PTC stands for Photovoltaics for Utility Systems Applications Test Conditions, in case you wondered.

Here is an interesting table of PTC ratings for solar panels.  If you go down the list, you’ll see that sometimes panels with a manufacturer stated lower power capacity than another panel actually test as giving more power, and you’ll see appreciable differences between panels all offering apparently the same output.

So maybe you can decide that your 250 W panel actually produces 225 watts to start with.

But then, you need to start adjusting further down.  Perhaps the panel was slightly under specs when it came from the manufacturer.  It will probably lose almost 1% of its output each year that it is operational – do you want to plan your system based on its best case output when brand new, or its mid-life output when it is 5, 10, even 20 years old?

The allow for inverter losses, additional losses through its wiring, and some shading/uneven lighting losses (both from clouds as well as from things like trees around your site).  Add a bit more for other miscellaneous electrical losses, and some for dirt on the panels, and all of a sudden, that 250W panel is starting to promise you more like 150 watts of real power.

One of the subtle but potential huge power losses is from shading.  Now you sort of understand that if the sunlight halves in ‘strength’, then so too does the power output of your panels halve.  But did you know that a partial bit of shading, on only perhaps 10% of your panel, can reduce its output by 50%?  That’s an amazing but observably true issue.  There’s a good discussion about that issue here.

There’s another related factor to keep in the back of your mind as well.  Not all the power your panels will create is necessarily generically usable power.  For example, let’s say you have 2kWh of power produced in a day – that seems like a meaningful chunk of power.  But that doesn’t mean you can run a 1500 watt appliance for over an hour, because perhaps the power is trickling in at only 300W, over a seven hour period.

You’ll never be able to run your 1500 watt appliance from the 2kW of power you got that day, unless you feel the power into a battery bank over the day and then take it all out at the 1500 watt rate – oh yes, and if you do that, you’ll then have to factor in the additional inefficiencies of converting from the AC power to DC power for the battery charger, then the loss in charging up the battery, then the loss in discharging the battery, and the loss in converting the battery DC power back to AC power for your appliance.

Summary

So, your 250 watt panel will probably never ever give you 250 watts of power, under any conditions.  We’d suggest that you use the Californian PTC test results to convert your panels’ claimed power outputs into more realistic output levels, and then reduce those by at least 10% to convert from panel power output in DC to actual AC power available in your home.  In other words, expect less than 200 watts – in best case conditions – from your 250 watt panels, and in worst case conditions (but still with nice sunny weather) you could be dropping down closer to 150 watts as your theoretical maximum.

The bottom line for us as preppers, and remembering we are planning for a future where solar panels aren’t just a fashionably nice ‘green’ supplement to our normal power from the utility company, but rather are our only power source, is this :  Massively over-build your solar array, because no matter how big it is/becomes, it will disappoint and leave you wanting more when you actually start living off the power.

This is a further part of our series on solar energy.  Please also visit our sections on energy in general and solar energy in particular for more related articles.