Goodwe’s GW2500-BP DC Converter: The Cheapest Way To Retrofit Batteries?

If you own a solar system and want to add batteries then, in theory, the cheapest way to do this is to simply plug your batteries into your existing solar inverter. The solar inverter can then convert the batteries’ DC power into AC power, just like it does for your solar.

Unfortunately, in practice, regular solar inverters are not designed to be shared by solar and batteries, so this hasn’t been possible and battery upgrades have been jolly expensive as a result.

But Goodwe, a major Chinese inverter manufacturer, has just released a battery DC converter, going by the catchy name of GW2500-BP. It is designed to allow batteries to interface to existing solar inverters.

This DC converter may be the cheapest way to add batteries to an existing solar home. It will also bypass any restrictions that Australian network operators may put in the way of using battery-inverters, such as the Sunny Boy Storage.

By itself it won’t make battery storage pay, but it brings it one step closer to being cost effective.

The Goodwe GW2500-BP Specs

This table shows the technical specifications for Goodwe’s GW2500-BP.

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And the user manual is here:

Download (PDF)

The Three Ways To Install Home Batteries

Currently there are two methods used to install battery storage in Australia:

• The first is to use a multimode inverter.  These are also known as hybrid inverters.  Their drawbacks are they only work with battery systems they are compatible with and they cost considerably more than regular on-grid inverters.
• The second method involves using a special battery-inverter.  This controls the charging and discharging of batteries no matter what kind of solar inverter is used.  Because it is attached, or coupled, to a cable that carries AC current, this method is called AC coupling.  Because it does not need to be compatible with the solar inverter, battery-inverters are a convenient way to add batteries to an existing rooftop solar system.

With the introduction of battery DC converters, such as the Goodwe GW2500-BP, there is now a third way to add batteries.  The GW2500-BP takes DC current from a home’s solar panels at hundreds of volts and converts it into 48 volt DC to charge batteries.  When the stored energy is needed it takes 48 volt DC from the batteries and supplies DC current at the hundreds of volts the solar inverter needs to change it into AC power for the home.

The GW2500-BP works with any solar inverter so there are no compatibility issues.  A communications link from a power meter to the GW2500-BP allows it to know when to charge and discharge the batteries.

How The GW2500-BP DC Converter Is Installed

The GW2500-BP is DC coupled, which means it is connected between the solar panels and the solar inverter.

 

The GW2500-BP Needs A Mini Electricity Meter

In the diagram above, you may have noticed the green dotted line going from a meter to the GW2500-BP. In fact, it is so beautifully done, I’d be surprised if you can take your eyes off it.

The green dotted line represents the communication link between a cheap, mini electricity meter and the GW2500-BP. This lets it know when no grid electricity is being used by the household so excess solar power can be used to charge the batteries. And when grid electricity is being used it knows it can discharge power from the batteries to reduce or eliminate that grid electricity use.

Compatible Inverters

The GW2500-BP is compatible with all string inverters. This is because they don’t need to directly interact with each other to do their jobs.

But string is the thing. This means they won’t work with microinverters, as they supply AC power directly to the home and not the DC power the GW2500-BP requires.

Compatible Batteries Don’t Include The Tesla Powerwall

Goodwe states the GW2500-BP is compatible with lead-acid and lithium-ion batteries. This covers the most common battery chemistries. But it only covers 48 volt battery systems. This means a storage system such as the Tesla Powerwall, which operates at 350V DC would not be suitable.

Intelligent Battery Management

Goodwe says the GW2500-BP has intelligent battery management. That sounds good. However, I suspect the actual intelligence will have to come from the high cholesterol meat inside your head as it won’t be able to work out what it needs to do on its own.  But if you explain you want it to provide battery power during the peak period of your time-of-use tariff, it should be able to reliably do that on its own.

Solar Array Voltage Should Be 480 Volts Or Less

The maximum solar voltage the GW2500-BP will accept is 500 volts and it will only charge the batteries when the voltage it receives is from 150 to 480 volts. This means the GW2500-BP cannot be retrofitted to just any size solar array. Your solar installer may need to reconfigure your solar array to get the total voltage under 480V.

Max Array Size is 6kW, Max Charge/Discharge Is 2.5kW

You can connect up to 6kW of panels to the GW2500-BP but there is a 2.5kW limit on charging and discharging the battery.

If you don’t mind the expense, it may be possible to connect a second GW2500-BP to double the power.  I don’t have any information that says you can’t do this, but that doesn’t mean you can. Technically it could be done, but technical and practical don’t always match up.

Two Separate Strings Of Panels Is A Problem

Many inverters on the market now have 2 Multiple Power Point Trackers or MPPTs. This means they can accept two separate strings of panels and independently attempt to get the maximum amount of power out of each. This is useful because it can allow the separate strings to face different directions, or be different sizes, or use different types of panels, without the performance of one string affecting the other.

The GW2500-BP only has one input and output for DC power. This means if you want your inverter to accept two separate strings of panels, the GW2500-BP can only be attached to one string.

If you only have one string this is not a problem. But with two strings of equal capacity then only half the output of the solar panels would be available to charge the batteries. If the strings are large enough and the battery storage small enough, this may not be a problem on sunny days, as there may always be enough solar energy produced to meet household demand and fully charge the batteries.  But on cloudy days the system might end up exporting some solar electricity that could have instead been used to charge the batteries if they had been had been able to make use of surplus solar electricity from both strings.

So under normal circumstances, where there are two separate strings of solar panels, the GW2500-BP and batteries should always be attached to the string that is expected to produce the most surplus electricity.

The GW2500-BP Loses Solar Energy Converting DC

According to physicists, in this universe there is no such thing as a free lunch. To which I respond, “How come I’m so fat then?”

But when it comes to transforming electricity there are always losses. Or at least, that’s what the last physicist who bought me lunch told me.

The maximum efficiency of the GW2500-BP when charging batteries is 96.0% and when going the other way the maximum efficiency is 96.5%

This is actually a little worse than I expected. Or maybe I am just too optimistic. If any physicists out there want to explain to me that I am being too optimistic, I am available for lunch any time.

Those efficiency percentages are maximums. In practice I would expect them to be maybe half a percentage point lower.

These losses are in addition to the charge and discharge losses of the battery itself. If the efficiency of the GW2500-BP is half a percentage point below its maximum and the round trip efficiency of the battery itself is 95%, together their overall efficiency would come to 88%. This means 1.14 kilowatt-hours of solar DC electricity would have to be put in to get 1 kilowatt-hour of stored DC electricity out.

Vampire Suckage Takes A Toll

In addition to efficiency losses, the GW2500-BP, like most electronic items, also consumes stand by power. According to Goodwe this comes to under 8 watts. If it averages 7 watts, then over a year it would come to 61 kilowatt-hours. If an average of 5 kilowatt-hours of electricity are stored each day then the stand-by power consumed by the GW2500-BP would equal 3.3% of the total energy stored.

It Is Not Suitable For Off-Grid Use

The GW2500-BP is made for on-grid use. While it might be technically possible to get it to work off-grid, that is not what it is designed for. This means if the grid fails you will be left sitting in the dark. If you want the ability to be independent of the grid then a multimode inverter is the way to go.

The GW2500-BP Is A Converter And Not An Inverter

A converter changes DC voltage. An inverter changes DC to AC. The GW2500-BP only changes the voltage of DC, so it is not an inverter. This is important because network operators can limit the total size of inverters connected to a rooftop solar system. As an example, in Sydney, people with single phase power are normally restricted to solar inverters of under 5 kilowatts.

One method of adding batteries to a home involves using a battery-inverter and it is possible for network operators to claim that adding a battery inverter counts as increasing the size of your solar inverter and they can block you from installing one on those grounds.

This might strike you as completely unfair, as adding a battery inverter does not increase the output of a solar system in any way, and households adding battery storage adds to grid stability rather than subtracts from it. But it’s not about being fair. It’s about being a dick to solar owners.

DC Converters May Be Cheapest Way To Add Batteries

It is not currently clear how much the GW2500-BP will cost. Uninstalled it may be around $1,200 but at this point that’s just speculation.

With the Sunny Boy Storage battery-inverter, which has identical power, costing around $2,000 in Australia, it is definitely shaping up to be the cheapest way to add battery storage to an existing system and has the advantage that there is nothing the networks can do about them.

Just how much it would cost installed is not really possible to say. It will depend on the batteries you choose to go with it and whether your solar array needs modification to meet the specs of the Goodwe.

While I imagine there will be competition between the different methods of retrofitting battery storage, battery DC converters such as the GW2500-BP have an excellent chance of being the cheapest method. The fact that they reuse your existing inverter and offer a simple installation bring us one step closer to retrofitted home battery storage paying for itself.