Off-Grid Inverter Sizing

Sizing an inverter is an important part of off-grid system design. Choose an inverter that’s too small and you won’t be able to run all of your loads, or it might not handle startup surges. One that’s too large will be a waste of some of your purchase money and an inverter that operates under its maximum efficiency—which translates to needing more energy input.

Inverter sizing starts with adding up the power needs of the loads in your off-grid system, as shown for an example home in the table. When sizing the battery and array, also be sure to account for inverter efficiency loss in the calculation. A 10% loss is standard for off-grid inverters, since they often operate at less than maximum efficiency. 

Surge loads are also noted, so that an inverter with enough surge capacity to start them can be selected. For our scenario, an inverter that can handle at least 2,795 W is needed. Ensure that the chosen inverter will surge sufficiently for the loads. It is best to use actual surge values for each appliance, by either measuring or obtaining specifications from the appliance manufacturer. Then add the total run watts to the maximum surge that might occur at any given time. In this example, if all the surging appliances happen to start at the same time the other appliances are on, the surge capacity will need to be 155 + 5,500 = 5,655 W. Another way to roughly guesstimate surge requirements would be to take your maximum simultaneous watts and multiply by three—in this case, that would be about 8,400 W (70 A).

If your budget is small, you can plan for load management so you can specify a smaller inverter. It is usually easy to remember that the inverter will not handle, for example, running both the microwave and the vacuum at the same time, which would reduce the inverter size needed to 1,795 W. However, although restricting the larger loads can be effective, this strategy gets more difficult to implement with each additional person in the household.

Also consider future expansion: Power needs grow in almost all systems, and upsizing to a somewhat larger inverter usually makes sense. Choosing an inverter that can be “stacked” with additional inverters to increase continuous output power will enable easier future expansion. Finally, off-grid inverters typically run most efficiently running at about two-thirds of their rated power. Rather than maxing out its load capacity, choosing a slightly larger inverter could allow it to operate in a more efficient power range. In the long term, this could offer better value—the loads will take less energy out of your system and the inverter will run cooler, subsequently lasting longer.

Possibilities

For our example home’s loads, there are many possibilities that will work, including:

• Using load management: When simultaneous loads total less than 2,000 W, a 2,000 W inverter, like Exeltech’s XP2000, is a relatively inexpensive choice. These inverters are easy to install and have a very accurate sine wave output. However, this choice offers no ability for expansion, greatly restricting system flexibility.
• OutBack’s VFX3524 (a 3,500 W inverter) may be a good choice, since it has a higher power rating and a surge capability of 70 A. 
• If 12 VDC loads are also part of the system, choose an inverter that will work with a 12-volt battery, like Magnum’s MS2812, 2,800 W inverter. Most modern homes do not have DC loads. But if wire runs are short, continuous DC loads, such as an answering machine or fan, can run directly off the battery bank. Running continuous loads on DC allows the inverter to spend more time in its power-saving mode. The Magnum MS2812 has a 30 W no-load draw, but only draws 7 W in sleep/search mode.

Other Considerations

Waveform—Only budget systems have modified square wave inverters. Are there any loads on your list that won’t run on a modified square wave? If so, rule out modified square wave inverters. For instance, in our example, the loads with motors, like the refrigerator, microwave, and vacuum, will run hotter and may not last as long running on a modified square wave inverter. Also, as side effects of the modified square waveform, interference may show up as lines on some TV displays or be heard in audio outputs. Some households choose to use a modified square wave inverter/charger, and separately supply more finicky audio-visual loads with a small pure sine wave inverter.

AC Output Voltage—If there is a generator with 240 VAC output or there are 240 VAC loads, consider inverters that have 120/240 V output to balance the generator output when charging batteries. These inverters can also provide 240 VAC to the loads without having to run the generator. Options here include the Apollo Solar TSW3224 or Xantrex XW4024. Alternatively, you can stack some 120 V inverters for 240 VAC output, or use a single inverter in conjunction with a 120/240 VAC step up/down transformer.

If a generator is used in the system, an AC battery charger will be needed to charge the batteries when RE is not available or the batteries need an equalizing charge. Most off-grid inverters have integrated AC battery chargers. A battery charger that’s too small (compared to the generator’s maximum output) will waste fuel and take a long time to charge the batteries. A charge rate that’s too high will charge the batteries too fast and heat them up, causing harm to the battery bank, so be sure the settings on the AC battery charger are adjusted to the battery manufacturer’s specifications.