Considerations for Off-Grid PV Systems: Page 3 of 4

Inverters

Using AC appliances in an off-grid home requires an inverter to convert the PV array and battery bank’s DC electricity to the AC electricity needed. When choosing an inverter, one should consider options such as metering, programming flexibility, type of waveform, idle power draw, generator backup, surge capability, and overall power needed by the loads.

Sizing an inverter requires adding up all the power needs of the appliances that will be on simultaneously. Some loads with motors, like a refrigerator’s compressor, also require a boost of power to start, called a surge, which is typically two to seven times larger than the appliance’s normal operating power needs—continuous power and surge power need to be considered. Choosing an inverter with a higher power rating may be important if loads increase or there may be future system expansion. Inverters also use some energy in standby mode (waiting for any load to run), so choosing an inverter with a low “idle power draw” is important.

AC appliances require a sine wave that alternates between positive and negative voltages 60 times a second. Inverters take DC current and create this AC sine wave with varying degrees of accuracy. Modified square-wave inverters create a rudimentary waveform that can run most appliances but has trouble with more sophisticated electronics, such as dimmers and computers. A true sine wave will run most equipment, especially motor-based, more efficiently—which translates to more useful energy from the system. Thus, many off-grid homeowners choose true sine-wave inverters, which can run all electronic appliances without a problem.

Most home appliances require 120 V to operate. Other loads, like well pumps or some shop tools, require 240 V. In this case, you’ll need to choose a single inverter that provides 120/240 V, or use two inverters that each produce 120 V but can be connected (or “stacked”) to provide 240 V. Alternatively, a transformer can be used between a 120 V inverter and a 240 V load to step up the voltage when needed.

Off-grid PV systems commonly use a generator for backup. If you’ll be using a generator, you’ll need an inverter with a large battery charger to change the generator’s AC electricity to DC for the batteries, and run any AC loads. Most higher-end inverters offer programming options for interfacing with generators, such as automatic start and stop, pre-set quiet hours, and maximum charge amps. Additionally, metering allows users to see how much charge is going from the generator to the batteries, how much energy is leaving the battery to the loads, and the battery’s state of charge.

Batteries

Battery considerations include the technology type, cost, preferred system voltage, ambient temperature, maintenance requirements, and battery location. Nearly all home battery banks are deep-cycle lead-acid, which can handle being regularly and deeply discharged (up to 80%, but 20% to 50% depth of discharge will significantly increase longevity). For lead-acid, the first decision is whether to use a flooded battery that requires regularly adding distilled water, or a sealed valve-regulated lead-acid (VRLA) battery that does not require watering. Sealed batteries are more expensive and have a shorter life than equally sized flooded batteries, but this trade-off can be worth it in cases where the battery maintenance cannot or will not be done.

Modern off-grid battery banks are typically 24 or 48 V, which allows the use of smaller-gauge wire than 12 V systems, which have higher current for the same power level. Choosing a higher-voltage battery also means wiring more batteries in series to increase the voltage, thereby reducing the number of parallel battery strings required for the same energy available. This, in turn, helps reduce imbalances across the battery bank. If there are 12 V loads that need to be powered, a DC-to-DC converter can be used to supply the right voltage.

It is important to keep flooded batteries out of living spaces and all batteries should be protected from unauthorized access—as they contain caustic chemicals and pose shock and burn risk if not handled properly. Choosing a location with moderate temperatures (77˚F is ideal) is critical for battery longevity. For every 18°F increase in temperature the battery experiences, the number of available cycles drops by half. For example, if a battery is rated at 3,600 cycles at 77˚F (or approximately 10 years, at 1 cycle per day), it would then be expected to last 1,800 cycles, or about five years, if installed in a climate of 95˚F.  At lower temperatures, a battery will gain lifespan, but its available capacity will decrease.