Designing off-grid wind and solar installations differs from grid tied installations in a number of important ways such as:
- The solar array must be in a certain restricted voltage range
- The solar and/or wind generator must be kept very close to the battery bank
- Expanding the system in the future can involve complex wiring
SMA has addressed many of these issues in their Sunny Island product range by offering a unique system configuration where all power generation sources can be connected directly to the AC bus as illustrated below:
The system certainly is innovative and offers a number of advantages over traditional DC coupled systems, but there are potential downsides too.
In this article, I’ll explore the advantages and disadvantages of AC side coupling using the Sunny Island and Sunny Boy with more traditional DC side coupling using the Outback MPPT charge controller.
Flexibility and simplicity in wiring
Using the 230V AC bus as the central component of the off-grid system means that wire size can be reduced. Photovoltaic panels and wind turbines can be located up to 1 km from the Sunny Island and the batteries. And the system can very easily be expanded by simply connecting more Sunny Boys or Windy Boys to the AC bus. Since almost everything is garden variety 230V AC the system could easily be installed by any electrician familiar with domestic wiring – no need for complicated DC wiring.
With a DC connected system, even with an MPPT charger, you could still only use a maximum voltage of about 120V between panels and charger, which means that wires would have to be thicker and/or shorter.
The Sunny Island changes the grid frequency to manage the available and required power. This means that you could connect frequency dependent relays to the Sunny Island mini-grid to turn optional loads on or off (This feature is built into the Sunny Island through a relay anyway, but you could do it externally based solely on grid frequency). You could also attach synchronous generators that would start or stop based on the grid frequency. Flexibility is the Sunny Island’s middle name.
Cost
SMA’s strategic decision to promote AC side connections was more likely made in their board room, not in their engineering department. SMA are well known for the grid tied Sunny Boy and Windy Boy inverters and the Sunny Island was a relatively new addition to their product line, so it makes a lot of business sense for them to promote their existing products instead of branching out into charge controllers.
Inverters cost substantially more than charge controllers of the same capacity. Taking the popular Outback FM80 charge controller which could charge a 48V battery from a 3.8kW PV array, it retails for about 700 Euros. An SMA Sunny Boy inverter 3800 inverter retails for about 1700 Euros, more than double the cost! That’s an additional 1000 Euros that could be spent on more PV.
SMA do produce a 40A charge controller for smaller systems, but this retails for about 900 Euros – again more expensive than the Outback which provides double the charging capacity.
Now there are some cost savings to be made by going the AC route with the Sunny Boys which I’ll touch on in the next section…
Additionals
On a DC connected system, in addition to the charge controller you’d also typically need a combiner box to parallel the PV strings and a DC breaker to disconnect the PV panels. But when using a Sunny Boy these ancillaries are sometimes not required. The Sunny Boy includes a built in ESS switch for disconnecting the PV from the inverter so there’s no need for a dedicated DC breacker. Most models of Sunny Boy also include connections for up to 3 parallel strings. The more powerful models may even include more than 1 MPPT tracker so that you can connect strings with differing orientations or a string that is partly shaded. Although these ancillary items could be saved, they might still not warrant the additional costs of the Sunny Boy.
Efficiency
This is potentially a big issue for AC side coupling. There are two important factors at work when considering AC side connection of power sources:
- Battery charging is less efficient because of double conversion from DC PV to the AC bus and then from the AC bus through the Sunny Island back to DC charging of the batteries.
- Direct consumption of PV power on the AC bus is more efficient because of the high efficiencies of the Sunny Boys (up to 97%, but typically 95%).
This means that the system will be more or less efficient based on how much power is consumed directly off the AC bus and how much is consumed from the batteries. SMA’s training material rather unhelpfully compares the best case AC side coupling with the worst case DC side coupling. I used these figures to instead compare best case AC side coupling with Sunny Boys compared to best case DC side coupling with the Outback FM80. The efficiency values I used were as follows:
| Sunny Boy |
95% |
| Sunny Island Charging |
92% |
| Sunny Island Inverting |
95% |
| Battery charging |
85% |
| Outback FM80 charging |
98% |
Using these values, we can then compare the efficiencies of the 2 systems based on how much power is consumed directly off the AC bus:
Therefore if you’re using 60% of the power directly from the AC bus then the AC side coupling starts becoming more efficient than DC side. But the efficiency of an off-grid PV system is most important in winter, when there’s the least amount of sun. In fact, if you’ve designed an off-grid home then the size of the PV array will largely be determined by the low sun hours in winter and the depth of your wallet. Winter is when efficiency matters and this is precisely the time when you would typically be charing and using more power from the batteries. Averaging 60% usage directly from the AC bus might be applicable to certain systems, such as businesses or farms that don’t have a constant energy requirement throughout the year, but that have peak requirements in the summer months. For off-grid homes that have a more or less constant energy requirement throughout the year and much less sun in winter, DC side charging remains the most efficient.
You can find the full spreadsheet used to calculate the efficiencies here.
Flexibility in PV panels
An additional complication with traditional DC side coupling of PV panels is that you are often limited in choice for PV panels because they have to conform to smaller voltage tolerances. MPPT charge controllers have increased this range with their ability to accept up to 140V from the PV panels and convert that to charge a battery bank of 24 or 48V, but still there are limitations in how you configure the strings and which PV panels can be used.
Since AC side coupling involves using standard Sunny Boy inverters that are usually used for grid tied applications, it means you have complete freedom in choosing PV panels and can work with much higher string voltages, typically up to 500V per string. This could also contribute to cost savings because it might mean that you could use thin-film panels for your off-grid home that would otherwise not fit within the voltage ranges for charge controllers. Higher voltages also mean thinner and cheaper cabling in the PV array.
Conclusions
The choice between AC or DC side coupling essentially boils down to a question of cost versus flexibility.
AC side coupling with Sunny Boys is more expensive than DC, both in terms of capital outlay as well as lost efficiency in winter months. But it offers the advantage of simpler and cheaper wiring and a wider choice in PV panels and string configurations which might offset these costs. The full mini-grid management features allows for added flexibility in expanding off-grid systems and in connecting power suppliers some distance away from the batteries. This added flexibility will likely be less important for off-grid holiday homes or partially occupied homes but increasingly important for larger systems such as farms or other commercial environments or for rural electrification of villages.
AC side coupling is an innovative feature and certainly allows for a freedom in off-grid system design that was not before possible. But freedom comes at a price, and whether it will be worthwhile for your next system really depends on the application and the factors listed above.
