2. ESS system design
2.1. PV
2.1.1. MPPT solar charger and/or grid-tie inverter
ESS can work with either an MPPT Solar Charger, a grid-tie inverter, or a mix of both.
Generally speaking, the MPPT Solar Charger will be more effective than a grid-tie inverter in a small system. This is because an MPPT Solar Charger is up to 99% efficient, whereas the PV energy coming from a grid-tie inverter is first converted from DC to AC, then back from AC to DC, causing losses up to 20 or 30%. This will be even more noticeable when the energy consumption occurs mainly in the mornings and the evenings.
When most of the energy consumption occurs during the day - say in an office with air-conditioning - a grid-tie inverter will be more efficient. After (very efficient) conversion to AC, the air-conditioning unit uses the PV energy directly.
In the case of 'no Feed-in', consider using an MPPT Solar Charger or a Fronius PV Inverter, and then use the Zero Feed-in function. This will lead to a much more stable system.
2.1.2. Feed-in or no feed-in
The rules around Feed-in differ all around the world. In various countries:
Energy can be sold back to the grid- or to reduce the electricity bill by running in reverse.
Feed-in is allowed, but not rewarded: All energy being fed back is lost in the sense that the utility provider will not pay for you it. It is, however, an ecologically-sound power-contribution
Feed-in is absolutely not tolerated - even for a few seconds: there are certain prepaid meters in South-Africa that will disconnect from the grid when they detect Feed-in.
Feeding-in results in inflated bills because the electricity meter can only count in one direction - up. Every kWh fed-back to the grid is erroneously counted as energy used, and will be charged for.
Feed-in
Feed-in of PV power via an MPPT Solar Charger can be enabled or disabled in the Energy Storage Systems menu on the CCGX. Note that when disabled, the PV power will still be available to power AC loads.
Feed-in of PV connected to grid-tie inverters occurs automatically. There are no settings or special design considerations to be considered whether connected on the input and/or output of the inverter/charger.
No feed-in
Feed-in of PV power via an MPPT Solar Charger can be enabled or disabled in the Energy Storage Systems menu on the CCGX.
For grid-tie inverters, the only option is to use a Fronius grid-tie inverter and use the Fronius Zero Feed-in function. See chapter 2.1.3.
Using other brands of grid-tie inverters in a No-feed-in system is not recommended. With ESS it is not possible to prevent feed-in where other brands are installed. And using the Hub-2 Assistant as an alternative method leads to a less-than-perfect installation. There can be problems with flickering lights - or even a whole-system shut-down, through overload, when a large load is switched on or off.
2.1.3. Fronius zero feed-in
For Fronius grid-tie inverters ESS has a special feature: Zero feed-in.
With the Zero feed-in option enabled, the ESS system will continuously monitor and actively control the output power of the Fronius grid-tie inverter. See chapter 4.3.11 for detailed requirements and settings.
2.1.4. MPPT solar chargers
All Victron MPPT solar chargers can be used: both the models with a VE.Direct port as well as the models with a VE.Can port.
2.1.5. Grid-tie inverter in parallel or on AC-out
There are two options when connecting the grid-tie inverter:
in parallel with the Multi or Quattro.
on the AC out.
When connected on the AC out, the factor 1.0 rule must be adhered to. There are no exceptions to this. Also use the factor 1.0 rule in countries where the utility grid rarely fails; and also when connecting a Fronius grid-tie inverter on the AC out, and employing 'Zero feed-in'.
2.2. Battery bank capacity
In a grid-parallel system, the size of the battery bank has these effects:
Small batteries will be more cost effective: but all available storage capacity is used every day
Small batteries will be charged and discharged with high currents. This will cause lead batteries, in particular, to have a shorter life.
Larger batteries, combined with a relatively large PV installation, can store excess power on sunny days. Power might then be available during several consecutive days of poor weather.
Larger batteries provide longer autonomy during a power outage. When the installation is required to operate as an Uninterrupted power supply a large battery capacity provides secure power provision for longer periods.
In a backup system, the battery size is calculated by the required autonomy during a mains failure.
See AC-Coupling minimum battery capacity for minimum battery sizes of systems with a grid-tie PV Inverter connected on the AC output of the Multi(s) or Quattro(s).
2.3. Inverter/charger size
The required size of the inverter/charger depends on the type of installation.
In a grid-parallel installation, the size of the inverter/charger can be (much?) smaller than the highest expected nominal and peak loads. For example, to cover the base load of a two-person household, an 800VA inverter/charger may be sufficient. For a family, a 3000VA inverter/charger can run most appliances - as long as not more than one of them is running at the same time. This means that the system can reduce grid power consumption from late spring to early autumn - perhaps to zero - with sufficient storage.
In a backup installation, the inverter/charger needs to be sized according to the expected loads.
2.4. Anti-islanding
ESS always requires anti-islanding. This also applies to a system without feed-in.
For several countries the built-in anti-islanding in our products can be used, for example, the MultiGrid in Germany, and the MultiPlus in the United Kingdom. See certificates on our website for details.
In case there is no certified product available for the country of installation, install external anti-islanding.
More details here: VEConfigure: grid codes & loss of mains detection.