Inverter

Types of inverters for homes:

In general, an inverter is a device that transforms DC power from batteries or renewable sources into AC power. But many different devices are sold under the name “ inverter “ and they actually have different applications. Experience shows that this list expands each year.

So how do you chose which one is the most appropriate for your home? Well, in our view, it depends on your intended application and your personal preferences. As a first step, our suggestion is to spend 5 min. and read the information below.

Grid-tie (also called “on-grid”) inverters for home:

These are one of the most popular devices (sometimes called on-grid inverters). They convert power from solar panels into AC power and in the same time – they synchronize their output signal with the grid. These systems can be used in homes in order to sale energy to the grid or for your own consumption. The systems do not need batteries.

If the grid fails, the grid-tie inverters immediately stops and prevents any feeding of energy either to the house , or to the grid. Thus, these systems are not recommended when the customer wants to have UPS functionality in his home. Also, only by themselves, these systems cannot be used in remote areas that do not have AC grid present.

Some of the inverters can be combined with smart meters that limit (some to 0) the feed-back of electricity to the grid. Thus, now it is possible to install grid-tie system in your home and not to feed any energy to the grid. The grid-tie inverters are further divided into “micro inverters” (inverters usually with power rating 250-400W), “string inverters” (inverters usually with power rating of 1-25kWp) and “central inverters” (used primary in industrial applications).

How we pick-up brands to work-with:

A system with grid-tie inverter is expected to work 10-25 years at least. Thus, we only choose high-quality on-grid inverters. Also, as experience shows, financial stability of the company is important – if your producer goes bankrupt , it is a bit doubtful whether or not your warranty claim will be covered.

Off-grid inverter

They are used mostly in remote homes, yachts, cars, simple solar systems.

The function of this device is to transform the DC power from batteries into AC power. For example, with such an inverter, one can build a remote solar/wind system (consisting of PV modules, batteries, solar charger and inverter) that can power-up devices like laptop, TV lights, refrigerator, etc, or one can use the devices in his yacht in order to have AC power on it; or one can use it to power appliances on a yacht / boat.

The inverters with full sine wave are capable of powering all types of devices that have lower power rating than the nominal power rating of the inverter.

The inverters with modified sine wave are far cheaper than those with full sine wave. Modified sine wave inverters have their limitations and do not work will appliances.

Also, extremely cheap inverters that are sold in the EU market do not always comply with the EU's safety regulations and pose risks to their users as it can be seen from this link:

https://ec.europa.eu/consumers/consumers_safety/safety_products/rapex/alerts/?event=main.search#searchResults

In the search criteria simply choose Years = 2018, 2017, 2016 or the time period that you want and in the “free text” type “inverter” and then click on “Search”. The inverters on the market that have be found not to comply with the standards will be listed as the image below. Still, just by looking at the products one can find many similar (to the ones that are banned) products on the market currently , that are sold under different brands.

Hybrid inverters ( Inverters with AC chargers )

Another type of devices sold under the name “inverter” are the inverters with AC chargers. The idea is that the AC grid (or sometimes a genset) can be used to power the appliances or to charge the batteries in some cases.

Typically , all inverters with AC chargers can be used as UPS ( uninterrupted power supply ) in combination with batteries. For example, if you want to have reserve power of 1kW in your home for , say, 30min , then you can do that relatively cheaply with one inverter with AC charger (of , say, 1.2 kW ) and one deep cycle battery (of about 100Ah). In this situation the AC grid is always accepted by the inverter. It gets a bit more complicated when you can program the inverter when to accept the AC power.

Some inverters with AC chargers can be programmed to work in ESS ( Energy Storage system) setting ; some can't do that.

Some inverters with AC chargers can accept a generator input ; some can't.

Some inverters with AC chargers can be overloaded (in case AC power is available) , some can't.

Some inverters with AC chargers are equipped with solar MPPT chargers, some are with PWM chargers (usually a bad choice), some do not have any solar chargers.

The most popular system for homes , currently, are the ESS settings.  Use ESS in a self-consumption system, a backup system with solar or a mixture of both: for example using the top 20% of the battery capacity for self-consumption, while keeping the other 80% available as a backup during a utility grid failure.

Optimizing self-consumption:

at times when there is excess PV power, the PV energy is stored in the battery. That stored energy is then used to power the loads at times when there is a shortage of PV power.

The percentage of battery capacity used for optimizing self-consumption is configurable: set it to 100% for (Western Europe / Germany). Or for locations with frequent, or even daily, grid failures, set the threshold to 80%. Only the top 20% will be used to optimize self-consumption, the rest of the capacity is reserved for grid failures. African countries for example.

How to choose an off-grid inverter:

The inverter will be the main power supplier to your consumers, so it has to be able to handle your full load. For small systems, it is good to select the inverter's power rating to be slightly higher than the total installed power

Example: if you have the following consumers:
Coffee Maker 1000W
LED - lamps 3 pieces of 10W each - total 30W
LED TV - 35W
Computer- 300W
Total installed power 1365W

You have two options for selecting the inverter power:
Option 1: An inverter with a higher power than the total installed power. This ensures that even if you turn on all the consumers, the inverter will operate without interruption .

Option 2: inverter with less than total power, but with higher power than the most powerful appliance (in this case the coffee maker). In this case, care must be taken not to exceed the maximum power of the inverter. Otherwise, it will shut down and you will have an electrical interruption.

Power information for each appliance can be obtained from its data plate. Most electrical appliances have a label on the back of which the power and other technical data are written. For some refrigerators - the plate is inside and the door needs to be opened. Data on the power of electrical appliances can be found on the Internet as well as in stores for such equipment.

Starting current
If you have consumers with electric motors or with capacitors, you should also take into account their starting current. In this case, the maximum peak current of the inverter must be higher than the starting current of the consumers.

Options in higher-quality inverters:

Transfer switch - what does it represent and how it works?
It is a device that is used in inverters with AC chargers that are connected to an external power source (public grid, electric generator). You can use the transfer switch to directly and automatically power your consumers from the external power source.
Some inverter manufacturers install a transfer switch with much larger power rating than the inverter. Thus, in the presence of an external power source, it is possible to power-up the consumers exceeding the power of the inverter. However, if the external AS source fails, the total power of the consumers must be immediately reduced below that of the inverter; otherwise the inverter will shut down.

Two outputs of the inverter
There are inverters with two outputs, where second operates only when an external power supply is present. Usually, only power consumers are connected to this output, and when the external source is turned off, they are turned off. This is useful for example in yachts , where so important appliances (like navigation) should always operate without disruption. Loads that should not discharge the battery, like a water heater for example can be connected to this output (second output available on models rated at 3 kVA and more).

Two inputs for external source
Some high-quality inverters have two independent inputs for an external source, such as a public grid and a generator. The inverter finds which source is active and is connected to it. When both sources are active, the inverter selects the source with priority.

Synchronize the output signal with the external source
Some manufacturers of quality inverters synchronize the output signal of the inverter to the external source. Thus, if the inverter is used in a solar or wind system, it uses the energy from the renewable source and, if necessary, can add energy from the AC grid when needed. This is an intelligent solution that eliminates the problem of volatile peaks in consumption.

Power Assist - how it works?
It allows the the inverters to supplement the capacity of the alternative source. Where peak power is so often required only for a limited period, the inverters will make sure that insufficient shore or generator power is immediately compensated for by power from the battery. When the load reduces, the spare power is used to recharge the battery.

It is often used in yachts, campers.

Work in parallel
Quality inverters can work in parallel and thus - multiply output power.
Typically, one inverter is the main (lead). This way an existing and operating system can be expanded to a larger one.

Three-phase operation
Good inverters can work in one-phase system and in a three-phase system. Typically, these are three identical inverters (one at each phase) connected to a small matching device that converts the phases to 120º relative to each other. More than one inverter can be mounted in parallel on the individual phases and a high power type system can be obtained.


Input current limitation

Some inverters can be set to limit the input current from the external source. This is especially useful for inverters in yachts and caravans . Limitation is also necessary when an inverter is connected to a small generator and the generator is in danger of being overloaded by the consumers.

Example: An inverter is mounted on a yacht and at a given port the maximum current that is allowed to draw is 10A.

UPS functionality
Most inverters with built-in chargers can also be used for uninterruptible power supply. When an external network is present, the inverter charges the batteries and remains in standby mode. When the external power supply stops, the inverter starts to operate and supplies the consumers.


Solar inverters

Some inverter manufacturers also add a solar charger controller. There are inverters with an additional built-in AC - switchboard with fuses. In this way, the inverter turns into a multifunctional energy system consisting of a powerful sine wave inverter and a sophisticated battery charger, a high speed AC switch and AC distribution in a lightweight and compact enclosure. There are numerous options here - there are inverters with MPPT chargers and full sine wave with/without transfer switch; with/without power assist, with / without ESS, etc.