What Happens to Solar Power When Batteries Are Full?


When solar batteries are full, what happens is that the excess solar power will either be diverted elsewhere for usage or wasted, depending on whether your system is grid-tied or off-grid.

Grid-tied systems can send extra solar power back to the electric grid, while off-grid systems may need to dump excess solar energy unless there is sufficient load to utilize it.

This article will provide a detailed overview of solar power systems, explain what happens when batteries are full in different setups, and give solutions for effectively using surplus solar energy.

How Do Solar Power Systems Work?

Before getting into what happens when batteries are full, it helps to understand the basic components of a solar power system:

  • Solar Panels – These contain photovoltaic cells that convert sunlight into direct current (DC) electricity.
  • Charge Controller – This regulates the voltage and current from the solar panels to the batteries. It prevents overcharging and damage.
  • Batteries – Deep cycle lead acid or lithium batteries store energy from the solar panels when needed.
  • Inverter – Converts the DC electricity from the batteries into alternating current (AC) home appliances.
  • Electrical Panel – Routes AC power from the inverter throughout the home.

Now, let’s look at grid-tied and off-grid systems in more detail.

Grid-Tied Solar Power Systems

Grid-Tied Solar Power System

Grid-tied solar systems are connected to the utility electrical grid. They use the grid as backup and can send excess solar power back to it.

Here are the components of a basic grid-tied solar power system:

  • Solar panels
  • Charge controller
  • Grid-tied inverter
  • Main electrical panel
  • Net electricity meter
  • Batteries (Optional)

During the day, the solar panels charge the batteries through the charge controller. The grid-tied inverter converts DC from the batteries to AC and feeds it to the home’s electrical system.

The excess is returned to the grid if all the generated solar power isn’t immediately used. The home’s net meter tracks electricity used from the grid and excess electricity sent back.

The net metering process credits the homeowner for excess power produced. At night or on cloudy days, the home pulls electricity from the grid when solar production is low.

Off-Grid Solar Power Systems

Off-Grid Solar Power System

Off-grid solar systems are not connected to the utility grid at all. All power is generated from the solar panels and stored in batteries.

Here are the main components of an off-grid solar system:

  • Solar panels
  • Charge controller
  • Batteries
  • Inverter
  • Generator (optional)

The system works similarly to a grid-tied setup. The solar panels charge the batteries during daylight hours through the charge controller.

The inverter converts DC electricity from the batteries into AC power for the home. No connection to the utility grid exists.

Some off-grid systems have a backup generator to recharge batteries when solar production is insufficient.

What Happens When Solar Batteries Are Full?

Now that you understand the basics of solar power systems let’s get into what happens when the batteries are fully charged.

In a Grid-Tied Solar Power System

With a grid-tied solar power system, any excess solar electricity generated when the batteries are full gets fed back into the grid.

Here’s what happens step-by-step:

  1. Solar panels produce DC electricity during daylight.
  2. The charge controller sends electricity to the batteries until they are fully charged.
  3. Once the batteries are full, the charge controller stops sending current to them.
  4. The grid-tied inverter takes DC power from the solar panels and converts it to AC.
  5. The inverter feeds the AC electricity to the home’s electrical system.
  6. If appliances and devices don’t use all the solar power generated, the excess gets sent back to the utility grid.
  7. The home’s net meter tracks electricity used and excess electricity sent to the grid.

So, the excess solar power returns to the grid when your home isn’t using it immediately. This arrangement with the electric company is called net metering.

The net meter spins backward when you send excess power to the grid, effectively banking electricity for future credit. When solar panels aren’t producing at night, your home pulls power from the grid.

In an Off-Grid Solar Power System

With an off-grid solar power system, no connection to the utility grid exists, so there is no way to send excess electricity back to the grid.

Here is what happens when the batteries are fully charged:

  1. The solar panels produce DC power during daylight hours.
  2. The charge controller sends electricity to the batteries until they are full.
  3. Once the batteries are fully charged, the controller stops sending current to them.
  4. With nowhere to go, the excess power coming from the solar panels is wasted or needs to be dumped.

Since off-grid systems aren’t connected to the grid, excess solar electricity must be used immediately or dumped somewhere. Otherwise, it could overcharge the batteries and risk damage.

Solutions exist for using the excess power, which we’ll get into next. But first, let’s look at how to monitor your solar battery charge level.

How to Tell If Your Solar Batteries Are Fully Charged

Charge controller to monitor solar battery charging

Knowing how to monitor your battery bank’s state of charge is important. Here are some ways to tell if your solar batteries are fully charged:

  • Charge Controller Screen – Most have an LED or LCD screen indicating the battery charge level.
  • Battery Monitor – A separate battery monitoring device can show voltage and charge status.
  • Hydrometer – Measures the liquid density for flooded lead acid batteries to determine the charge state.
  • Multimeter – Use a multimeter to measure DC voltage across the batteries, indicating the charge level.
  • Battery Manufacturer Specs – Refer to the specs for your particular batteries to see the voltage for a 100% state of charge.

For example, a 48V battery bank is typically considered full at around 54V. However, check your specific battery’s specs to be sure.

Knowing when your batteries are full helps prevent overcharging and gives you peace of mind that your solar energy system is functioning properly.

What to Do With Excess Solar Power

If your off-grid batteries are regularly becoming full faster than the solar power is being used, you have some options:

Use the Excess Power

Figure out ways to use the excess renewable energy during sunny times rather than wasting it. Here are some ideas:

  • Run appliances like washing machines and dishwashers
  • Charge power tools, batteries, etc.
  • Use an electric water heater
  • Turn on fans, AC, or electric heaters
  • Pump water for irrigation
  • Run a pool pump

The key is to find uses for the extra electricity rather than letting it go to waste. Get creative and utilize as much of your excess power as possible.

Add More Battery Capacity

Battery bank to store solar electricity

Another solution is to expand your battery bank’s storage capacity. This provides more room to store the excess solar electricity.

With larger or additional batteries, your system won’t reach full charge as quickly. It’s easier to balance your solar panel production and battery capacity.

Remember that batteries are one of the most expensive components of a solar power system. Lithium batteries, in particular, can be quite costly upfront.

However, the investment may be worthwhile if your batteries are regularly full and you’re wasting a lot of potential solar power generation.

Install a Diversion Load

A diversion load gives excess electricity somewhere to go besides the batteries. An electrical resistance converts excess power into heat or mechanical energy.

Once the battery bank is fully charged, a diversion load controller automatically switches on the load. This provides a place to dump the extra electricity from the solar panels.

Common diversion loads include water or air heating elements. The excess power gets converted into heat, which can be useful.

This approach prevents overcharging the batteries while making use of surplus power generation. Diversion loads are common for off-grid solar systems with limited battery capacity.

Add Grid-Tie Capability

The nuclear option is to add a grid-tie setup to your existing off-grid solar power system. This allows connecting to the utility grid to send excess electricity back to it.

Converting to grid-tied involves adding the following:

  • Grid-tied inverter
  • Net electricity meter
  • Bi-directional disconnect
  • Some electrical work

This approach lets you use net metering credits when your solar panels produce excess power. It may be worthwhile if wasting electricity becomes problematic or expanding your battery bank isn’t cost-effective.

Consult with a solar installer to determine if adding grid-tie capability suits your situation.

Frequently Asked Questions

Next, let’s answer some related questions:

Can Solar Panels Overcharge Batteries?

Solar panels cannot directly overcharge batteries when properly wired to a charge controller. The controller regulates voltage and current to prevent overcharging. Without a controller, batteries could be damaged by excess solar input.

Do Solar Panels Work When the Grid Is Down?

It depends on the solar system. Grid-tied systems shut down when the grid loses power as a safety precaution. Off-grid solar continues working during a grid outage but needs batteries to store power.

Grid-tied systems with battery backup (also called hybrid solar systems) can work offline, too.

What Happens if Solar Panels Produce Excess Energy?

Excess solar electricity is sent back to the utility grid in a grid-tied system. Surplus power must be used immediately or wasted with off-grid systems unless special equipment is added to divert excess electricity.

Can a Generator Charge Solar Batteries?

Yes, a gas or diesel generator can charge solar batteries in an off-grid system when solar panels aren’t producing enough power.

Some systems use generators as backup power sources. Just be sure your particular batteries are compatible with being charged from a generator.

How Long Do Solar Batteries Last?

Solar battery lifespan varies based on type, use, and conditions but typically ranges from 5 to 15 years.

Lithium-ion batteries can last 10+ years. Lead-acid batteries age faster and often last only 5 years or less. Proper maintenance extends battery life.

How useful was this post?

Click on a star to rate it!

Average rating 0 / 5. Vote count: 0

No votes so far! Be the first to rate this post.

We are sorry that this post was not useful for you!

Let us improve this post!

Tell us how we can improve this post?