Minimizing standby losses is an important aspect of optimizing a home renewable electricity system. Every watt-hour you save is a watt-hour your RE system doesn’t need to produce, and money you save in up-front system costs.
The majority of solar-electric (photovoltaic; PV) systems installed are grid-tied, and rely on batteryless inverters to convert the DC electricity generated by your PV array to AC electricity used in your home or sent to the grid. Most batteryless inverters are designed with low standby loss in mind, and will shut down overnight, drawing only a fraction of a watt after the sun goes down. In grid-tied systems with battery backup, standby losses can be more significant compared to batteryless systems, since battery charging and management is required. Correct programming will minimize inverter energy consumption overnight.
Keeping standby losses to a minimum is extremely important in off-grid RE systems. Any electricity demand beyond what the RE system can generate is typically provided by an engine generator, which is noisy, polluting, and expensive to run and maintain. One design approach for large off-grid PV systems (3 KW array and greater) is to simply add additional modules to offset standby energy use by appliances and inverters in idle mode. This approach is most convenient, but requires a larger cash outlay up front. For small off-grid systems (1 KW array or less), unchecked standby losses can quickly become one of the largest loads on the system.
Home-scale, battery-based inverters typically have two modes that affect standby energy usage—idle, and a standby mode commonly referred to as “sleep” or “search” mode. Modern, battery-based inverters can draw as much as 22 W per inverter without loads (at idle). Multiply this by 24 hours in the day and your inverter can quickly become a significant load on the system, consuming more than 500 WH per day. The same inverter, programmed to go to into an energy saving sleep mode if AC loads are not present, will draw about 4 W, or 96 WH per day, consuming only about 18 percent of the energy of idle mode.
Eliminating all phantom loads will allow your inverter to go into sleep mode, saving energy when you’re not actively using AC electricity. Then, if an AC load is turned on, it takes a moment for the inverter to “wake up,” ramp up to full voltage, and power the AC appliance. This momentary inconvenience is well worth it for many small system owners focused on maximal system performance and minimum generator run time.
Comparing PV output and initial costs to energy consumption helps drive home the benefits of minimizing standby energy use. Let’s assume that you have installed an array with 100-watt PV modules at a location with 5 daily peak sun-hours, and appliance and inverter standby losses amount to 1,200 WH per day. For a batteryless, on-grid system, an overall annual system efficiency of about 80 percent is realistic in many locations. To offset the daily standby energy use only, the output of three 100 W modules would be required (1,200 WH/day ÷ 5 sun-hrs./day = 240 W; 240 W ÷ 0.8 (efficiency factor) = 300 W; 300 W ÷ 100 W/PV module = 3 PV modules). Off-grid systems have a conversion efficiency of about 70 percent, so four 100 W modules would be required to offset the standby energy use in that case.
You can expect to pay about $600 for a 100 W module in today’s market. That means your standby energy use would cost you $1,800 in up-front PV module costs in the batteryless grid-tied system described above, and $2,400 in the off-grid system example. Add this to the cost and nuisance factors related to generator use when a few days of cloudy weather appear, and additional installation and balance of system equipment costs, and minimizing standby energy use can amount to big savings.