Installing Solar Panels: How to Design and Plan for Real-World Conditions

A common first step for most beginners is to look at a few key numbers when you start planning a new solar power project. 

These numbers include projected wattage drain, projected number of sunlight hours, and total panel wattage. 

You can calculate your potential power draw and sunlight hours through SanTan Solar’s calculator here

After you get some idea of the power draw, it can be tempting to just go out and buy enough solar panels to cover your draw. 

If you find that you typically use 2500 watt/hours each day, you may make the mistake of simply dividing that drain by the average number of sunlight hours to buy the appropriate number of panels. 

If you get five hours of sunlight each day, you may think that a single 500-watt panel will be enough for your setup. 

After all, 500 watts for 5 hours does equal 2500 watt/hours, your exact drain! 

If you were to buy one 500 watt panel, you’d almost never generate enough power for your needs. 

This is because a great number of inefficiencies nearly always accompany solar panels.

You must consider these inefficiencies when buying your solar panels, or run the risk of too little power.

We’ll be discussing some of the main inefficiencies that accompany all solar power in this article so you can be more confident in your solar design.

Panel Wattage in Lab Tests vs Real World (STC vs PTC vs Real World)

Most solar panels are rated in two ways, STC and PTC. STC stands for Standard Test Conditions, and are what you think of when you hear of unrealistic laboratory test conditions.

Every variable is perfect for solar panels to produce electricity. 

They’re kept at an efficient temperature, they receive light that comes in at a perfect angle and with a uniform intensity across the entire panel.

Solar panel manufacturers measure their panels using STC. 

Every panel that produces energy within a certain rating is then marked with that rating and sold. 

Needless to say, STC does not represent real-world conditions. 

In order to help installers get a better idea of actual solar panel performance, the PVUSA Test Conditions(PTC) rating was created. 

A PTC rating is measured with a more realistic operating/ambient temperature and wind speed. On average PTC ratings reflect around 90% of the manufacturer’s STC rating. 

This means that a panel rated for 500 watts will typically get a PTC rating of somewhere around 450 watts.

Every solar panel sold in California is required to receive a PTC rating, which means that almost every panel sold in the US has a PTC rating. 

You can see a complete list of PTC ratings at, or download the excel document here:

PTC is vastly more accurate than STC when representing real-world conditions, but it still doesn’t take into account degradation over time, off-angles, dirty panels, or other inefficiencies we’ll be covering later in this article. 

Panel Degradation Over Time

We’ve covered how panels can produce less than their factory rating may imply. 

Another concern to consider when buying panels is that they naturally lose efficiency as they age. 

This fact can be especially important when designing solar setups with long projected lifespans. 

It’s typical for solar panels to degrade by about 1% every year. 

That means that in 10 years you’ll be generating about 90% of what you were when you first installed. 

Let’s look at our example from earlier. 

If we buy a 500-watt panel, we know it’ll likely have a PTC rating of around 450. 

If we then use this panel for another ten years, it’ll drop down to around 400 watts even if we don’t take into account any other inefficiencies! 

This degradation occurs for a variety of reasons. 

Shortly after installation, solar panels are exposed to UV light which can rapidly lower the efficiency by a percent or two before leveling off. 

Another way that degradation occurs involves thermal expansion and cooling. 

As with most materials, solar panels expand slightly when warm and shrink slightly when cold. 

This cycle of shrinking and growing can cause microfractures in the silicon.  

You can see this degradation over time reflected in manufacturers’ performance warranties. 

It’s typical to see a guarantee of 90% production for the first ten years and 80% for the first 25 years.


While you’re likely to notice this degradation over the years, it’s not likely to be the biggest drain on your power production. 

We’ll be going over a few of the most impactful ways that real-world conditions can draw more energy than you’re expecting.

Other Miscellaneous Ways That You Lose Power (PWM MPPT, Voltage Drop, Inefficiencies in Batteries, Etc)

Voltage Drops 

When connecting your solar panel setup to your battery bank, it’s important to consider voltage drop. 

Voltage drop occurs as your electrical current passes through the wires connecting your panels to the rest of your system. 

Wires have a resistance that must be overcome by your current which results in a slight loss of power. 

This voltage drop increases when you have small wires and when your panels and your charge controller are far apart.

You can counter this voltage drop by doing a few things. 

The first is to increase the voltage coming from your solar panels. 

The higher your voltage, the less impact you’ll see from voltage drops. 

One method to increase your voltage is by wiring your solar panels in series to add their voltage together. 

You can read more about the ways to wire panels in our article Parallel Vs Series here

Voltage drop is usually considered acceptable when it’s less than 3-5% of your solar panel’s output.

Solar Charge Controllers PWM vs MPPT

Another key to increasing the efficiency of your solar panels is to buy the right kind of solar charge controller. 

There are two main types of solar charge controllers. 

PWM (Pulse Width Modulation) solar charge controllers limit the voltage coming into your batteries to prevent damage. 

If you’ve got a 12-volt battery bank, a 60-volt current would cause serious damage to your batteries. 

PWM charge controllers prevent this by limiting the amount of voltage. 

All the excess current is lost with PWM charge controllers.

MPPT (Maximum Power Point Tracking) charge controllers are similar in that they limit the amount of current flowing into your batteries to prevent damage. 

However, unlike PWM charge controllers, MPPT charge controllers convert the excess voltage back into the current to charge your batteries faster. 

Depending on the voltage of your solar panels and battery bank, an MPPT charge controller could save you an incredible amount of efficiency. 

MPPT charge controllers are more expensive than PWM charge controllers, but they could pay for themselves depending on your setup.

The last inefficiency we’ll be covering today is inefficiencies inherent in batteries.

Battery Inefficiencies

It’s not uncommon for charging and discharging your battery to be the single largest inefficiency in your solar setup. 

We refer to the energy that batteries lose during the process of charging and discharging as their round trip efficiency. 

There are two main types of batteries: lead deep-cycle batteries and Lithium-ion batteries. 

Lead deep-cycle batteries have been used for decades and are cheap and reliable. 

However, they usually have a round trip efficiency of somewhere in the 80% range.

Lithium-ion batteries have grown in popularity in recent years for their vast and varied advantages over lead deep-cycle batteries. 

Lithium batteries typically have a round-trip rating in the high 90% range. 

However, these batteries are far more expensive than lead deep-cycle batteries, sometimes costing up to ten times as much.


As you can see, the real world has a multitude of inefficiencies that can make planning your solar setup difficult. 

STC ratings can be overly optimistic, panels degrade, and dirt and clouds can make everything even harder to account for. 

On top of that, inefficiencies inherent to batteries and wiring can make the headache of planning even worse!

The main takeaway for your solar setup would be to buy far more power than you may think you’ll need. 

You’ll never complain about having a battery bank that’s always full, or a solar panel setup that recharges them too quickly.

Remember, if it takes 10 solar panels to power an appliance, you can’t power the appliance with 9 solar panels, but you can with 10 or more.

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