What happens to solar when it is raining or at night?

Solar Power and Inclement Weather

In addition to our Solar 101 page, we also want to create Blog Posts that answer questions we see a lot in our day-to-day interactions. Today I would like to kick off a Solar FAQ blog section beginning with a question that we get a lot: What happens to solar at night?

While this seems like a relatively simple question, it promotes a good discussion on the inter-workings of solar. First, let’s start off with the most direct answer. At night, your solar panels are not going to be producing energy. Now, there are a few rare cases where this statement is not entirely true. For instance, if you are in Prudhoe Bay, Alaska in June/July during the phenomena of the Midnight Sun, you will actually see sunshine at midnight.

But how does this affect MY solar production?

As an industry best practice, solar energy system designers factor in your available solar resource based on your location, weather patterns, and insolation factors (insolation being a term that describes how much sunlight is accumulated in a given area.) This is often referred to as “sun hours”, with the actual measurement being kWh per m² per day. Interesting to note here, is that the solar availability in Virginia, Maryland, and DC is roughly 4.5-5.5 sun hours Photovoltaic_Solar_Resource When you compare the solar potential to Germany, the leader of solar energy, you will see that Germany receives roughly the equivalent of Alaska at about 2.5-3 sun hours.

Check out this image that National Renewable Energy Laboratory (NREL) produced showing how the US and European countries at similar latitudes measure up.

What happens to solar when it is raining?

Sun-hours again incorporate this weather data into their equation.

You see, sunlight reaches a solar panel through three different methods: direct, diffused, and albedo.

Direct, is exactly what it sounds like-sunlight directly reaching the solar panel. For the most part, this is an “ideal” occurrence, as most sunlight is diffused in some way by the atmosphere, clouds, pollutants, etc.

Albedo radiation is actually sunlight that is bouncing off a surface, such as a white roof surface, and reflecting back onto the solar panels. (This idea of reflecting sunlight was a major tenant to the Solyndra company’s technology; but more on that in another article in the future.) To maintain consistency in these production estimates, the majority of the solar industry utilizes resources such as PV Watts, an online solar performance calculator created by the NREL. So, whether it’s raining, night-time, or cloudy, your solar energy system designer has already incorporated these (and other) factors into the overall design and sized the solar array appropriately.

In further response to the question (What happens to solar when it is raining or it is nighttime?), I want to briefly touch on two more subjects. Both Net-Metering and Battery Back-Up require more than just a few paragraphs to do them justice, but for the sake of this blog, I will summarize them in respect to the question.

Can I use my solar when the power goes off?

The majority of residential and commercial solar installations are “grid-tied solar energy systems”-meaning your solar array is connected to the grid and receives supplemental energy from the utility (when the array is not producing). With “grid-tied” solar, you have access to net-metering. Under net metering, you have the ability to spin your power meter backward by producing your own clean, solar energy.

When the sun is out, and you are producing more energy than you are currently using, you build credited energy. At night, when your solar array is not producing, you can first use credited energy before consuming the supplemental utility energy.

For safety, grid-tied solar arrays are required to shut down when there is a power outage. This protects utility workers from an energy “islanding” affect during outages. If your entire neighborhood is without power, but your home is producing energy through solar, the utility worker might unknowingly be electrocuted by the solar energy while trying to repair the utility system. For this reason, a grid-tied solar array will automatically turn itself off during an electrical outage, and back on once the power has been restored.

What about solar with battery backup?

Solar with a battery backup system WILL allow you to store the solar energy you create. Battery backup systems can be used for grid-tied systems, as well as for “stand-alone” (or “off-grid”) solar systems. Stand-alone solar energy systems are used in situations where there is not an available grid to connect to, such as a remote hunting cabin.

While both stand-alone and grid-tied with battery backup systems use battery banks to store energy, they use this stored energy in many different ways. A grid-tied battery backup system will specifically save its stored energy for a power outage and will be sized to last for a certain amount of days (or autonomy).

A stand-alone battery system will constantly charge its batteries from solar, and then discharge them as their load requires. Stand-alone systems are sized to provide energy storage and production to serve the load indefinitely.

Either form of battery-based solar energy systems is much less common than a grid-tied system, due to cost prohibitions. Whether stand-alone or a battery-backup system require additional maintenance and hardware. A device called a charge controller will be needed to prevent batteries from over or undercharging. You also have to maintain and replace the batteries which have a shorter lifespan than the solar modules themselves (about 10 years vs. 25 years).

A solar energy system that has a battery bank will also require a larger, more intelligent inverter. There are also some additional balance-of-system components, such as housing for the battery bank that will also raise the prices of these systems. All in all, you can expect a battery backup system to cost significantly more than a grid-tie solar array (conservative estimates beginning around 25-50% additional cost to the solar array). If you are in the market for a solar energy system with a battery backup or a stand-alone solar energy system, I would highly recommend researching energy efficiency measures such as using a wood stove instead of central heating or utilizing a generator to supply additional power to the battery banks.

The Bottom Line

We are still several years from having battery technology that is cost-effective to be implemented with every solar energy system. For the time being, a grid-tied solar energy system is going to produce a much better return on investment financially, and provide a shorter payback period.

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