Solar Inverter vs. Charge Controller

Do you want to install solar panels for your home but not sure if you need a solar inverter or charge controller, or both? You’ll hear both terms used a lot in solar energy discussions, but what’s the difference between them?

Solar inverters convert DC voltage into AC. Charge controllers protect solar batteries from overcharging as they receive current from solar panels.

What are Solar Inverters and How Do They Work?

When sunlight shines on a solar panel, its energy is converted into direct current (DC). The majority of home appliances and electronics require alternating current (120/240V) however.

Solar inverters transform DC into AC so appliances can use it. To be more specific, a solar inverter sends solar DC into a transformer which converts DC into AC.

Inverters don’t literally turn DC into AC. When DC is coursed through a transformer, the inverter moves the current through multiple transistors.

These transistors rapidly turn on and off and the transformer “thinks” it is AC. Since DC is now behaving like AC, it is safe for appliances to use.

Solar inverters are also responsible for syncing solar panels with the grid. This is essential if your home is connected to a power utility company.

Solar Inverter Classification

There are several types of solar inverters. Your solar panel setup will determine what kind to use. Talk to your solar panel installer about what type of solar you want, and they will tell you what inverter to use.

String Inverter

This is one of the most common inverters for solar panels. All the solar panels are connected to one inverter, hence the term ‘string”.

String inverters are affordable and easy to set up. Since all the solar panels are connected to just one inverter though, what happens to one affects the other. If one panel gets shaded, it can affect the rest.

Microinverters

Microinverters are more efficient than string inverters because one is provided for each solar panel. If you have 25 solar panels, there will be 25 microinverters.

A good example of this is the Mixmax microinverter. Because each solar panel has its own microinverter, If one panel gets shaded, the rest will not be affected.

Battery Inverter

A battery inverter transforms battery power into alternating current and runs it through a switchboard.

Choose a battery inverter if you want to separate the battery from the panels and connect it to another inverter. You can also use this inverter if you want to install a battery in your solar array.

Hybrid Inverter

Hybrid inverters are a mixture of a solar and battery inverter. With this you can use it if you are on the grid or off it.

You can charge solar panels or plug it into the grid. It can also pull power from the grid and charge a solar battery.

Central Inverter

These are cabinet-sized inverter with capacities of up to 500kwh or more. They are not for home use and meant for solar farms or commercial / industrial applications.

Features to Look for in a Solar Inverter

Type. It depends on what kind of solar array you want to install.

For a standard grid tied solar system, a solar inverter is the best choice. Most full solar home installations use microinverters, but you can opt for a string inverter if on a budget.

If you want to add a battery, a battery inverter or hybrid inverter is required. At least one battery will be needed if you are off the grid. Probably more if you want to run several appliances when there is no sunlight.

Capacity. Capacity determines how much solar power the inverter can handle. The rule of thumb is inverter and solar capacity should match or be as close as possible

This isn’t always necessary. If your solar array has batteries or you only run a limited number of appliances on solar, a smaller inverter will do.

Efficiency. Some energy is lost when inverters convert DC to AC. A few years ago, an 85% efficiency rating was considered good. Nowadays, do not settle for anything less than 90%.

Warranty. Majority of solar inverters have a 5 year warranty. If you can get an inverter with a 10 year warranty that is even better.

Assuming normal usage, solar inverters pay for themselves after five years, so anything after that is money saved. But of course a longer warranty is preferable.

What are Charge Controllers and How Do They Work?

A charge controller is a device that prevents batteries from overloading and overheating.

Majority of solar batteries require 14-14.5 volts to charge. Solar panels generate 16-20 volts. Without a charge controller, the battery will overcharge.

Is it possible to make solar panels produce just 12 volts? Yes, but if that was the case, solar panels will only produce power under full sunlight and optimum weather.

Solar panels need the higher voltage so they produce power even when there are cloud covers, fog etc.

A 100% charged 12V battery is actually 13.6-14.4 volts when charging and 12.7 volts when resting. This means solar panels have to be at least 12.7 volts.

Types of Charge Controllers

Now that we know what charge controllers are, we can look at the different types available. There are two options to choose from, PWM and MPPT controllers. Their function is the same but how they work is completely different.

Difference between MPPT and PWM Charge Controllers

The most important difference is:

• A PWM charge controller pulls power from the solar panel right above the battery voltage
• With an MPPT charge controller, the power is drawn from solar panels at the maximum power voltage (vmp)

PWM are more affordable but you could end up wasting a lot of power. MPPT charge controllers are more efficient.

This example will show you the efficiency difference. Assume a solar panel has the following specs:

Maximum power voltage (vmp): 18V
Maximum power current (lmp): 5 amps

Assume the battery is:

Battery voltage: 13V

Solar battery voltages range from 14.4V at the absorption level and 10.8V when 100% charged. We are using 13V as an example.

With a PWM charge controller, you get 65W:

13V x 5A = 65W

If you use an MPPT charge controller, you get 90W:

18V x 5A = 90W

Keep in mind that voltage goes down when the temperature goes up so the 90 watts is attainable during ideal conditions only.

But even in less than perfect weather, you will still get at least 17% more current with an MPPT than a PWM charge controller.

While PWM charge controllers have shortcomings, the low cost makes them very popular. A fine example of this is the Renogy controller which offers good performance without breaking the bank.

If you want to get the maximum power from your solar panels, I recommend the Victron SmartSolar It is well worth the investment and pays for itself quickly.

PWM vs. MPPT: Which is the Better Controller?

The example I gave clearly shows that MPPT pulls more current from solar panels than a PWM. So is there any reason to choose a PWM? Under certain situations, yes.

A PWM charge controller is ideal under the following situations.

On a budget. PWM charge controllers are cheaper than MPPT. If you are on a budget this is something to consider. Of course you want to make the most from your solar power investment, so check if the added cost of an MPPT is worth it.

Low powered applications. Low charging devices will not see much benefit from an MPPT. In this case it might be better to use a PWM.

Consistent drawing efficiency. While MPPT controllers extract more current, PWM are generally more consistent. MPPT charge controllers may have lower efficiency when used on low current applications.

An MPPT charge controller is better under the following conditions.

More energy. MPPT charge controllers pull more current from solar panels compared to PWM. The larger your solar array, the more current is wasted by PWM. An MPPT controller can prevent this.

Widespread compatibility. MPPT controllers are compatible with more types of solar arrays than PWM. You can also use it with various solar wires and cables.

Support for large solar arrays. One of the most important benefits of an MPPT charge controller is its support for large solar arrays. This is why you rarely see full house solar arrays use PWM. It is just more efficient to use MPPT.