Understanding Inverter Specifications
Inverter designs vary considerably and there is a lot of information presented in the data sheets. Here is some information intended to help you understand key aspects of your inverter specification and capabilites.
Most inverters will generate AC (alternative current) power from the DC (direct current) generated by solar panels. Alternatively, a DC-DC converter may be used to generate DC power, for example, to charge car or caravan batteries. This information is about inverters that generate AC power.
Grid tie inverters are connected to your AC grid supply and synchronise the power they generate with the grid. This allows the inverter to both import from and export power to the grid. A grid tie inverter will shut down if there is no grid supply, unless it is specifically configured to provide an emergency power supply
Standalone inverters operate without being connected to the grid and are used for things like generating AC power for mobile / caravan applications. A standalone inverter cannot be connected to the grid.
This information is mostly about inverters that are grid tied.
The type of grid connection used by your inverter will depend on the grid supply you have available. Single phase uses two conductors (L or Live and N or Neutral), while three phase uses four conductors (L1 or Reference, L2 or Secondary, L3 or Tertiary and N or Neutral). Both types of supply will also have an earth conductor for safety (also known as Protective Earth or PE).
In the UK, the main grid supply is three phase and so three phase supply this is delivered to large commercial and industrial premises. Most domestic supply connections are single phase (houses in an area are distributed across the phases). With a single phase inverter, the grid operator (Distribution Network Operator / DNO) may restrict how much power you can export to a single phase in order to maintain a level of balance across the phases. For example, you may be limited to 6kW. If you wish to export larger amounts of power, you will be required to install a three phase supply. As this can involve digging a new supply and replacing your main incoming fuses, meters and circuit breakers, this can be quite expensive.
In Germany, for example, many more houses are connected to three phase supplies and three phase inverters are commonly used.
Solar only inverters connect to an array of solar panels and export power when the sun hits the panels. When the voltage and power from the panels falls below a certain level, the inverter shuts down.
In addition to connecting to solar panels, a hybrid inverter connects to a battery and willl use the power generated by the solar panels to charge the battery. It can also discharge the battery and generate AC power and use this to reduce your consumption of power from the grid. A hybrid inverter uses a CT clamp or meter (CT1 / Meter 1) to measure the amount of power being drawn from your grid supply and uses the battery to generate power to minimise this (i.e. reduce your grid consumption to close to zero).
A hybrid inverter may be AC or DC coupled:
- With an AC coupled system, DC power from the panels is converted to AC and then used to supply your grid consumption. AC power can also be converted back to DC to charge the batteries. The main benefit of an AC coupled system is the ability to charge the batteries from the grid as well as solar. The main disdvantage is that converting the power from DC to AC and back to DC again is slightly less efficient.
- With a DC coupled system, power from the panels is used directly to charge the batteries. The main advantage of this is increased efficiency, while the main disadvantage is that you cannot charge the batteries from the grid.
Solar inverters are simpler and cheaper than hybrid inverters.
You can combine both types of inverter, using a hybrid inverter for battery storage (your main inverter) and one (or more) solar only inverters to provide additional power generation (a secondary inverter). As the main inverter monitors your grid supply via CT1 / Meter 1, it can detect when excess power is being generated, from the panels connected to the main or secondary inverter, and use this power to charge your batteries. The main inverter can also monitor and report the power being generated by the secondary inverters using a second CT clamp (CT2).
An AC inverter is sized to deliver a certain amount of AC power (killowatts or kW). The bigger the inverter, the more power it can supply. Common sizes of inverter are 3.68kW, 5kW, 6kW, 10.5kW and 12kW. In comparison, boiling a kettle consumes about 3kW, so a 6kW inverter can run 2 kettles at the same time, or 1 kettle and a number of common household appliances.
3.6kW or 3.68kW inverters are specifically designed to export power according with the power limitations imposed by ENA G98. This allows you to install an inverter on a domestic single phase supply in the UK and notify the DNO after the installation has taken place. In order to exceed an export power of 3.68kW, you need to apply to your DNO for a higher export limit in advance of installation of the inverter. In this case, the approval you receive could be G99 or G100 (G99 includes a specified export limit with a direct grid connection, while G100 requires your connection to be routed through a fail safe export power limiter). The delays to obtain G99 / G100 can be considerable (several months), so installers commonly default to G98 installation to speed up the installation process.
If you loose grid power, your inverter will normally shut down. If you want to use a hybrid inverter to provide power to your house during a power cut, you may need to consider the amount of emergency power it can supply and how long this will last with your batteries. More information on EPS is provided here.
Solar panels need to be organised as strings with a set of panels that all point in the same direction, connected in series. Where sets of panels point in different directions (e.g. East and West), they need to be arranged as different strings to operate efficiently. Each string connects to a Maximum Power Point Tracker (or MPPT) that works out the optimal current that can be drawn from the attached panels.
Most common inverters have 2 input strings and 2 MPPT. Larger inverters may have up to 4 strings / 4 MPPT. A less common arrangement is where there are, for example, 3 string inputs connected to 2 MPPT (such as Fox ESS H3 series). In this case, 2 of the inputs feed into one MPPT and both of the inputs need to be connected to solar panels that point in the same direction.
Each string input will have a number of limits that need to be considered when designing a solar array for an inverter in order to get the maximum power from the panels and inverter:
- Maximum DC input voltage: this is the maximum combined input voltage for all the inverter input strings. A common value is between 500v and 600v but larger inverters may go up to 1000v.
- Startup voltage: this is the minimum voltage required before the inverter will take any power from a string. Common values are around 100v. Below this, a solar only inverter will shutdown and a hybrid inverter will not draw power.
- Rated voltage: this is the recommended input voltage for a string. Common values are around 360v. Given the maximum input voltage is 600v, if you have 2 strings and one operates at 360v, the other string is limited to 240v. Compare this to the 'optimal' or 'balanced' panel configuration, where the maximum DC input voltage is divided equally across the input strings.
- Maximum input Current: this is the maximum current that the inverter can handle for each input string. Typical values are between 11 amps and 16 amps.
- Maximum string power: each string input has a power limit, based on the combination of the input voltage and current. Common values are between 3kW and 4.8kW. Once this limit is hit, the power generated by the string will be capped. Unfortunately, manufacturers normally specify the maximum current but not the power limit.
For example, suppose you have 400w solar panels, with a nominal voltage of 30v and 2 input strings / MPPT to the inverter:
- The maximum DC input voltage is 600v, so the total number of panels connected to the inverter must be less than 20.
- The startup voltage is 100v, so a string needs to have 4 or more panels.
- The optimal string input voltage is 300v, so each string should have less than 10 panels connected.
- The maximum current is 13A. If you have 4 panels connected, the nominal string input voltage is 4 x 30 = 120v and the maximum power is 120v x 13A = 1.56kW.
- The maximum string power is 3.5kW. If you have 9 panels connected, the nominal string input voltage is 9 x 30v = 270v and the maximum power is 270v x 13A = 3.5kW, which means the string will be operating at the maximum power
Depending on the design, inverters may have a number of different power limits. These can include:
- The maximum AC output power from solar generation (power generation)
- The maximum AC output power from the battery to the grid (battery discharge)
- The maximum input power to the battery from solar generation (solar charging)
- The maximum AC input power to the battery (battery charge from grid)
- The maximum AC output power exported to the grid
In some cases, these limits may work together. For example, if there is sufficient solar generation, the inverter may output power to your house load, satify your export limit and still use any excess to charge the battery.
Inverters can support a range of work modes. These are explained in more detail here.
The information below summarises the capabilities of a number of common 5 to 6 kW hybrid inverters, based on reviewing the data sheets available in JUne 2023:
Please refer to the latest data sheets for information on any inverters you are looking at.