Search Power ratings reflect the energy-saving “search” or “sleep” mode available in most off-grid inverters. This mode allows the inverter to nearly shut off during times of no-load draw. While the inverter still consumes some power to monitor household loads, the search power consumption is commonly about 75% less than the no-load consumption. Depending on the inverter “wake-up” wattage threshold, there may be some small AC loads that will no longer work if nothing else is turned on. Small, always-on AC loads (security systems, clocks, answering machines, etc.) can keep the inverter awake, consuming energy all of the time. For small loads, one tactic that can work is to shift to a consumer battery (like AA rechargeable) counterpart. Another tactic is to include a small, always-on inverter that is dedicated to those household appliances.
Integrated Battery Charger/Maximum DC Amps—Most of the inverters in the table include battery chargers that work on an AC power source (see the “Integrated Battery Charger” sidebar). The battery charger has a maximum DC current rating that will limit how much from the available charging source can be used. While the generator may be adequately sized, a lower battery charger limit can increase generator run time. One strategy is to install multiple inverters/chargers, which increases battery charger capacity. Ideally, the generator will be sized according to charger capability (see “Engine Generator Basics” in HP131).
Generator Start enables inverters to remotely start and stop a generator. Users can select a low battery voltage value that triggers the inverter to initiate a generator start and run sequence to charge the batteries. Other parameters can also be set to run the generator during times of high power consumption and/or during specific times of the day. While this feature can be handy, there are drawbacks (see “Automatic Generator Start” sidebar).
Dual AC Inputs allow users to use more than one AC power source, such as the grid and a generator, for battery charging. This is useful in grid-tied systems with battery backup, since it allows charging batteries from the grid when it is available and from an engine generator during times of utility outages (and low RE-system output), offering another source of backup power.
Remote Display is useful for keeping tabs on the system from a convenient location (such as the kitchen). These displays usually include user buttons to turn the inverter on and off, and to adjust programmed settings.
Prepackaged with Balance of System Equipment can be a time-saver when it comes to installing a battery-based inverter, since these systems have many components that need to be wired and located in the vicinity of the inverter and battery bank. These additional components are required in battery-based systems because there are multiple power sources (such as a PV array, batteries, generator, and the utility grid), and it is required to have disconnects and overcurrent protection between each system component and each power source. Other components can include charge controllers, meters (and shunts), ground-fault protection devices, inverter bypass assemblies, and communications hubs. Additionally, all of these components need a backplate to be mounted on and neatly fitted and wired together, further increasing the time and hassle savings offered by optional prepackaged power-panel assemblies.
Justine Sanchez is a Home Power technical editor and an instructor for Solar Energy International. She is certified by ISPQ as a PV Affiliated Master Trainer.