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What To Do With Excess Solar Power?

Is your solar system generating more power than what you need but don’t know what to do with it? Find the answers here

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Carlos Huerta
Guide

Solar energy is one of the best renewable energy sources available.  Solar panels work by converting this solar radiation into electricity. The amount of power produced depends on several factors like climate, sunlight exposure, solar panel efficiency, the tilt angle of the panels, the size of the system, and others factors.  

When installing solar systems, you might opt for a solar system smaller than the load, roughly equivalent to it, or much larger. Home and business owners with solar panels producing a higher amount of power than what they consume might not know what to do with. Truth is there are many options, like selling excess solar power to the grid, or storing it for future usage, or even shifting the loads.

In this article, we will explain everything you can do with your excess solar power. Here you will learn about the different methods of using it, storing it, and even what happens if it is not used at all.

How to Use Your Excess Solar Power

When a PV system is producing more power than the load consumes, there are several things you can do with that excess power. Here are the most common solutions:

Inject Excess Solar Power to the Grid

Powering the load with solar energy without having any use for the excess generated solar power is inefficient, which is why one of the best solutions is to be connected to the grid.

When you have a battery-based or grid-tied solar system connected to the grid, you can send excess solar power to the grid. That means when there is a larger production to consumption ratio, this surplus electric power will be accounted for by the Net Metering (NEM) system, granting you solar credits equivalent to the power injected into the grid.

Depending on your utility, you might use those credits in future electricity bills or sell them for a profit. Few utilities have solar credit selling systems, which is why you should ask your specific utility to know what you can do with your solar credits.


Figure 1. Typical Solar Energy Production, Consumption and Export Profile for an Average Home. Source: Unison

Store the Excess Energy to Achieve Solar Self-Consumption

Using a solar energy storage device is one of the best ways to take advantage of excess solar power. When a home generates solar power during the day and stores excess energy to be consumed at night, the home can increase solar self-consumption.

When looking to install a deep cycle battery or a smart home battery system, it is important to properly size the system. There are two main factors to consider when sizing a battery backup system:

Peak Power Demand From the Load

One of the most important aspects to consider when choosing a battery backup system for a home or business is how the peak demand of the load relates to the C-rate of the battery. Battery capacities are designed to be discharged at a certain maximum current to a specific  C-rate, discharging a battery with a larger current will reduce the overall effective capacity and therefore discharge time, as you can see in the figure below.


Figure 2: Typical discharge curves of lead-acid battery as a function of C-rate. Source: Battery University

When choosing a smart home battery, you should consider this factor. To do this, you have to calculate the peak current of your system at a certain voltage by using the following formula:

With this formula, you can calculate an estimated peak current for your home by knowing the peak demand of the system and voltage. With this current, you can size the battery bank accordingly, installing as many batteries in parallel as necessary to deliver that current while limiting the current of each battery to its designed C-rate.

Autonomy of the System

The autonomy of a solar system means how many days the load can be powered with the battery bank without charging them back up. Most systems are designed for 1 or 2 days of autonomy to prepare for worst-case scenarios, such as power outages, especially for off-grid solar systems.

Solar systems connected to the grid do not have to worry too much about autonomy since they can use power from the grid if the batteries are drained. These systems usually have batteries to achieve solar self-consumption or to prepare for power outages.

Increase the Load

If your PV system generates a large amount of excess power and you do not know what to do with it, you can always increase the load. Most homeowners avoid using heating systems and high consumption appliances to avoid increasing energy bills, but if you are generating a large amount of power, you can opt to increase the load of your home. Solar panels can be used to power heavy-duty appliances such as refrigerators and even air-conditioning units.

Installing an EV Charging Station

Another interesting option to avoid losing excess solar power is installing an Electric Vehicle (EV) charging station. Charging an EV vehicle with solar power is the future, is good for the environment, and reduces monthly gas expenses to $0. So if your PV system is generating a lot of excess energy and you have been thinking of getting a new EV, it would be the best time to do it.

Load Shifting

Most businesses have large loads that are turned on during the night when panels are not generating, which is why they consume power from the grid or the batteries. While some of these loads cannot be shifted, some of them can. Shifting the usage period for specific night loads will allow you to use the excess generated power during the day, instead of demanding power from the grid (which would be more expensive) or batteries.

Homes can also benefit from doing this, especially for large loads that can be shifted like washing machines, dryers, dishwashers, and similar loads. Shifting loads might require some effort, but it is convenient to do so to fully use your solar power. This is especially valuable if your utility runs under a feed-in tariff scheme and not a NEM.

Conclusion: What to Do With Your Excess Generated Solar Energy

Many options are available to make the best use of your excess solar power. Some of these are as simple as accumulating solar credits for future electricity bills or installing batteries to achieve solar self-consumption.

If you do not want to inject power into the grid or increase solar self-consumption, then you can also shift your load demand from night to morning time or increase your loads to cover that extra amount. This way you will not let any kWh go to waste while optimizing your solar energy consumption.

Frequently Asked Questions (F.A.Q.)


Which Factors Affect My Solar Production?

The production of solar energy depends on many factors. These are some of the main ones affecting how much energy your panels will produce.
· Location: Depending on your state, you will receive a certain amount of solar radiation per day.
· Temperature: Solar panel efficiency is affected by temperature, decreasing about 0.5% each 1ºC above the 25ºC (depending on the temperature coefficient).
· Shading: Objects projecting shadows over your solar panels, will reduce solar power generation.
· Age of the panels: Solar panels decrease their efficiency by 0.5% each year, which is why age is an important factor to consider.
· Type of solar panels: The type of solar panel that you choose will determine how energy can be generated. Traditional modules such as monocrystalline and polycrystalline will generate electricity using only the front side of the module, but others such as the bifacial solar panels will use the solar diffused radiation to generate electricity using the rear side of the panel.  

What Is The Maximum Excess Energy My Grid-Tied Solar System Can Generate?

States and utilities set a maximum energy offset that limits the amount of annual excess energy that can be generated by solar power. In some cases (like PG&E), the maximum offset can be 100% of the power consumed the previous year, but states like Arizona or utilities like SDGE, allow for homeowners to install PV systems that can generate up to 125% of their annual consumption (meaning up to 25% of excess energy that can be injected to the grid).

This varies depending on your location, therefore you need to check with your utility what is the maximum energy offset that a PV system connected to their grid can provide.


What Happens to Excess Solar Power in off-Grid Systems When Batteries Are Full?


In an off-grid solar system, battery inverters and solar charge controllers have the power to curtail the amount of power that is demanded from solar panels. When the batteries are fully charged and the solar panel is still able to generate more power than what the load or the battery are requiring, the charge controller will curtail the power output to ensure the power flow from the solar panel is equal to the demand, plus the electrical losses.

In other words, whenever the batteries are full and the load is covred, the excess solar energy will be curtailed by the solar charge controller and get lost in the form of heat.

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Author Bio

Electrical Engineer with background in solar PV designs for residential and commercial projects as well as power systems development. Fan of renewable energy topics and projects. Technical writer for papers, articles and research in related topics to sustainability and especially solar power.

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