There is a shortcoming with grid-tied photovoltaic (PV) energy systems of which owners are often unaware until an emergency: Although grid-tied systems can save money and earn their owners incentives while the sun is shining and the utility power grid is up, they must disconnect when the grid is down during an outage, leaving unused solar electricity sitting on the roof. As a safety feature within generic grid-tied systems, when the grid is down, users can neither sell energy back to the utility, nor supply their homes or businesses with power per the UL-1741 safety requirement.
Luckily, there is an upgrade for these types of systems called AC coupling that enables one to store energy for use during power outages and emergencies. AC coupling added to a simple grid-tied system converts it to a grid-interactive or Grid/Hybrid type system, providing grid-tied savings when the grid is up and off-grid independence and security when it is down or compromised.
Figure 1– Single line diagram of an OutBack Radian inverter AC-coupled to a grid-tied inverter
1) What elements are in an AC-coupled system?
Although there are several approaches to AC coupling, the systems have some common elements including:
- A grid-interactive or Grid/Hybrid inverter/charger;
- An energy storage system for back-up power consisting of batteries and an enclosure;
- A coupling device or center to tie the new system into the existing grid-tied one; and
- Additional electrical wiring with a second panel for critical backed-up loads that the new energy system’s back-up power capability will support in the event of an outage.
2) Why is AC-coupling important?
AC coupling is not only valuable to the end users who power their homes or businesses during grid outages; it is valuable for solar installers and distributors looking for new revenue streams. The market scale demonstrates the potential of the technology: More than 300,000 residences in the U.S. have grid-tied solar. Pair this with grid uncertainty due to storms, and AC coupling is a highly attractive retrofit opportunity — nearly every home or business with grid-tied solar is a potential customer for upgrading to grid-interactive with AC coupling.
3) How is equipment choice essential to cost savings, safety and stability?
The biggest difference is not in the coupling device but in the inverter/charger itself (in Outback’s case) is the radian inverter/charger. When deciding on equipment, one should look for six key elements in an inverter/charger to significantly impact the system costs, as well as the durability of the AC-coupled system:
- Dynamic stability: provides clean signal to the inverter, ensuring the inverter stays online and keeps producing power.
- Universal design: makes sure the inverter/charger works with other brands of string inverters.
- Dual AC inputs: allows charging of the energy storage system (batteries) from a backup generator in the event of an extended power outage or when the sun isn’t shining.
- Split-phase design: enables easy integration into standard household wiring, eliminating the need for expensive and inefficient transformers.
- Generator input and protection: protects backup generators from back-feed overload when the PV array is charging.
- UL1741/IEEE1547 ratings: ensures system compliancy by accurately detecting “islanding” conditions and disconnecting from the grid to prevent harm to utility service workers, as required.
4) What does AC-coupling cost?
The median MSRP of a complete AC coupling system with sufficient battery storage is about $16,000 – electronics-only packages run around $8,600. Renewable energy professionals recommend installing backup power to a renewable energy system from the start, as it is less expensive. However, considering the gains in system utility and the low annual cost over the payback period, AC coupling is highly recommended for grid-tied PV systems. Better systems use the best batteries, generally AGM/VRLA types which are essentially maintenance-free and have longer shelf and service lives, which save money over time.