Where Does Solar Go When Batteries Are Full?

Solar energy is a clean, renewable source of power that is becoming increasingly popular as both the cost and demand for energy continue to grow. Sunlight is harnessed via various technologies and then stored in batteries for later use, especially when it is cloudy or dark. But once these batteries are at full capacity, where does the excess power go?

When batteries are full, and if your system is not tied to the power grid, any energy not being used to power the inverter or charge the batteries doesn’t get generated and is essentially “lost” unless it can be diverted to other uses. If your system is connected to the grid, excess power can be sent into the grid for others.

This article will explore the basics of solar energy and how a solar energy system works. It will consider how batteries charge and what size batteries are appropriate for various uses, as well as alternatives for the use of excess solar energy.

How Does Solar Energy Function?

Solar energy harnesses sunlight and converts it into heat. Sunlight is a combination of radiation, which we feel as heat and ultraviolet light that can be harnessed through various technologies, such as solar cells or steam-driven turbines (source). 

Solar energy systems for the home usually consist of four basic parts. These are the solar panels or cells, the solar charge controllers, the inverter, and the batteries. Together, these function to create power for the home.

Solar Panels

When the sun shines onto solar panels, energy is absorbed via the panels’ photovoltaic (PV) cells. The electrical field in the cells creates electrical charges and causes electricity to flow (source).

Solar Charge Controllers

These manage the charge going into the batteries. When there is sunlight, they ensure that the batteries don’t overcharge and, when there isn’t sunlight, they block the reverse current from going back into the solar panels.

Inverter 

The inverter converts solar panel direct current (DC) into alternating current (AC) that can be used in the home or fed into the electricity grid.

Batteries

The batteries store DC power from the panels for later use. When there is sunlight, the charge controller will charge the batteries and send any extra power to the inverter. 

Once the batteries are fully charged, your charge controller will cut off the charge going to the batteries except for a small amount to keep them charged. You can read more about this in this article, “Can a Solar Panel Overcharge a Battery?”

When there is no sunlight, the system may draw power from the batteries until that power is used up. After that, the home may use grid power until there is sufficient sunlight to recharge the batteries.

Solar systems allow users to store energy generated during the day when demand is lowest and then to use it when demand is highest. This high demand time is when electricity is typically the most expensive, and solar companies are constantly exploring ways to enable better storage of this power.

Grid-Tied or Not

Most homeowners with solar power functionality choose to remain connected to the electricity grid because it offers the convenience of using the utility power grid in need. However, one can choose to be grid-tied, entirely off the grid, or a hybrid of the two. 

Grid-Tied

If your system is grid-tied, then it will operate only when the grid is functioning. This is the least expensive option to install and feeds generated electricity directly into the grid through your meter. 

Net metering will allow you to take the same amount of electricity that you feed into the grid, measured in kWh, for no cost (source). In the US, some utilities offer net metering, while others offer time-of-use rates. 

Off-Grid

Some people choose to be entirely off-grid, and others have no choice if they live in remote locations where they don’t have access to grid electricity. Then they will need to install an entirely independent system that can support itself and store power in batteries.

Off-grid solutions need to be carefully designed to ensure that they can meet the necessary energy demand throughout the year.  

Hybrid

A hybrid system refers to a grid-tied system with battery backup. This is more expensive to install than a simple grid-tied system, but it does mean that stored electricity will be available during outages.

Hybrid systems work well in areas with significant climate variation so that you can choose to use the grid when there isn’t sufficient sunlight.

What Happens to Excess Energy?

If you are connected to the grid and generate excess energy, your utility company will credit you for this, saving you money in the long run, making this a big draw for installing solar energy systems.

Solar systems generally hit peak production during the afternoon, when there is a low demand for energy. Demand is at its highest in the mornings and evenings when people tend to be home.

The excess you can’t use in the afternoon is fed into the grid, and your electricity meter credits you. At least 17 states have instituted net metering, and most have some form of refund program in place (source).

Your system will always prioritize your needs and will only offload power that you aren’t using. By feeding it into the grid, you are paying it forward with clean energy and furthering your positive impact on the environment.

If you are off the grid, any excess power generation will be lost unless you divert it to other activities, such as water heating or refrigeration systems. Off-grid users refer to this as “dump loads” — an automatic system that switches on certain devices when there is extra power available, which is generally on bright and sunny days.

All About Dump Loads

Dump loads, or diversion loads, are very important for users of off-grid solar. A dump load is somewhere to “dump” electricity when it is not needed or when your batteries are full. It’s usually a resistor or heating element because these work better than inductive loads.

Because solar electricity is ultimately quite expensive given the cost of setting up the system, it doesn’t make sense to waste any of it if that can be avoided. You could just allow the system to disconnect via the charge controller when batteries are full, but making use of a dump load makes much more sense. 

Dump loads are not essential for a solar system as they are for wind turbine or water turbine systems because the option exists to simply switch the system off.

Dump loads serve to protect batteries from potentially overcharging and operate similarly to the overflow on a bathtub, where the bathtub is the battery bank and the solar system is the tap. The tap fills the tub until it reaches the overflow and then diverts the excess. 

The dump load will only work when there is “overflow,” and this will therefore be as often as there is extra electricity in the system. 

Best Dump Loads

The best dump loads are resistive loads that contain a heating element such as water heater elements or air heaters. Resistive loads draw current in the same proportion as the applied voltage. By contrast, inductive loads that power motors work less well because they consume power when the current lags the voltage.

Resistive loads are durable and long-lasting. They can accept AC or DC, and they are generally inexpensive. 

Usually, the best use for dump loads is to heat water because excess solar electricity is usually generated in summer when there is lots of sunlight. As a result, there is a low need for space heating, but there is always a demand for hot water, and it’s a good way to use excess electricity.

How Are Dump Loads Diverted?

A dump or diversion load controller will divert electricity from the battery bank once it is fully charged. It will constantly monitor the batteries and will divert electricity back to them as soon as they need charging. 

It’s important that the dump load controller is sized correctly and is big enough to handle the full wattage of your dump load. The diversion load must be able to use more watts than the load can deliver, and, conversely, the load must not be able to draw more amperage than the charge controller can deliver.

There are numerous types of load controllers on the market, and you should get expert advice if you are thinking of installing one alongside your solar power system to ensure that it is sized and installed correctly. 

Reviewing Solar Batteries

There are various types of batteries that can be used to store solar energy. These include traditional lead-acid batteries, lithium-ion batteries, and flow batteries.

Lead-acid batteries are the oldest type available and have been servicing solar energy systems since their inception. They are popular because they are the most inexpensive option, but they also have a relatively short lifespan of three to five years.

Lithium-ion batteries have been gaining popularity because they offer a larger, longer charge and less energy wastage than lead-acid batteries. They’re significantly more expensive, but they last 10–12 years and are, therefore, better investments in the long term.

Flow batteries offer the latest technology whereby the charge is stored in tanks of liquid electrolytes (source). They can store more energy than lithium-ion batteries and have an even longer lifespan of up to 20 years. However, the technology is still new, and the vanadium used in them is very expensive.

When considering which batteries to install in your system, you should assess the following elements.

Capacity 

Capacity is the amount of power the battery can store, measured in kilowatt-hours (kWh). To provide some context, a typical solar battery will have about 10 kWh of capacity, while an average US house will use around 30 kWh per day. 

Power Rating

Power rating refers to the electricity level the battery can provide at one time, measured in kilowatts (kW). For example, a battery with a low power rating and a high capacity could deliver small amounts of electricity for a long time.

Depth of Discharge

Depth of discharge (DoD) indicates how much of the battery’s capacity can be used. A battery with a high DoD will offer more usage of its capacity and will likely be more expensive than those with lower DoD. You should consider batteries with at least 40% DoD.

Manufacturers will specify a maximum DoD for best performance, and you shouldn’t use more than that amount before allowing it to recharge. The lifespan of batteries can be significantly affected by how often they are discharged.

Round-Trip Efficiency

This refers to the difference between the amount of energy used to charge the battery and the amount that is available to use. The higher the round-trip efficiency is, the more economical the battery is. Generally, solar batteries should offer at least 80% round-trip efficiency. 

Warranty and Battery Life

All batteries decline over time and with use, and you should consider average life cycles as well as a warranty to guarantee optimal performance. All batteries have a finite number of times that they can be charged and discharged before they lose functionality. 

Manufacturers will usually express this as how many cycles the battery can perform at a specific DoD. For example, they may say the battery is guaranteed to perform 1000 cycles at 80% DoD. At a lower DoD, this same battery would then have more life cycles.

Your choice of battery will depend on both your budget and your needs, and it is worth considering all the options before investing in batteries for your solar power system. 

Alternatives to Batteries

Given that batteries have a finite amount of storage and are costly on a large scale, there is extensive research being conducted to find better ways to store excess solar power.

One of the solutions is to store it in a kind of molten salt. In liquid form, it’s heated in pipes and used to make steam that powers a generator (source). This project is operating in the Nevada desert and is already managing to produce electricity 10 hours after the sun goes down for 75,000 homes.

Another company is experimenting with storing excess solar as compressed air. This operates on the principles of a scuba tank. When there’s excess electricity on the grid, it fills the giant tank with compressed air, and then this is used to drive a generator when it’s needed.

A third project is experimenting with using temperature to store energy. In this project, a thermal energy storage unit stores energy as ice overnight. This ice then condenses during the day, providing daytime cooling. 

Accessibility of Solar Power

Solar power is growing in prevalence worldwide. In the US, it’s estimated to have grown to 62.5 gigawatts (GW), which is sufficient electricity to power 12 million American homes (source). 

The average cost of purchasing and installing solar panels has also decreased dramatically over the past decade. This has made solar energy more affordable, and, in Hawaii, California, and Minnesota, it is a similar cost to grid power. 

However, there is still much work to be done to make it accessible and affordable across the country. Technological solutions are still needed to bypass the challenges faced in trying to implement large-scale solar energy plans, and utility companies will have to come on board to drive the process.

Solar-Powered Gadgets 

Solar power can also be used in gadgets, especially those used when camping, giving you the comfort of home while in the outdoors. These devices have their own internal solar-powered batteries that can hold a charge for some time and can be constantly recharged when the sun is shining.

The most popular of these include lanterns, showers, flashlights, water purifiers, chargers, coolers, speakers, and radios. These items are affordable and widely available and increase your comfort in the great outdoors.

Another option for camping is to take along portable solar panels and then use this power as you would at home. These come in multiple options, but the most important features are:

  • Lightweight, portable and foldable
  • Weatherproof
  • Able to be locked in place (anti-theft)

While these certainly make camping trips more comfortable, they are only designed to provide usable power, and anything extra, once the batteries are fully charged, will be lost.

Final Thoughts

It’s estimated that the sunlight received by the earth in a couple of hours could supply enough power to satisfy the needs of everyone around the globe for a whole year. Why then is more of this free resource not being harnessed? 

We are limited in our ability to store electricity because batteries have a finite amount of storage available, and much of the excess power generated by solar systems is lost. 

As we take more steps to manage our own electricity production and consumption and also to reduce our carbon footprint, so technology will advance to allow for greater storage opportunities.

Tim C

An outdoor enthusiast for decades and a science and physics nerd since 1980, I have recently become very interested in using renewable energy sources for home primary power and as backup for outages, as well as for camping, boating and tailgating.

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