How to Size the All-in-one Inverters
When specifying an inverter (all-in-one inverter/charger) for any off-grid solar power system, you need to consider the voltage for the loads, the maximum power draw, charging capabilities, and the ability for the inverter to supply power when certain loads surge (draw a large amount of power for a very short duration).
Step No. 1: The Voltage for the Loads
Your load analysis helps you know what voltages your client’s loads require. For most residential applications, this voltage is 110/220VAC, which is the voltage required by AC household loads.
Step No. 2: The Maximum Power Draw
Off-grid inverters don’t care about energy consumption. They are simply concerned with delivering voltage and current (power) to loads. How long those loads run doesn’t concern them, which is why you need to take care to determine an inverter’s power output requirements by using the data that you gathered during your load analysis regarding the power draw fo individual loads.
Off-grid inverters are always rated at their continuous power output value, which means you need to make sure that the inverter(s) you select can provide the amount of power required by your client’s loads. You need to estimate which of the loads will be running at the same time and then add up those power values to determine the minimum power rating for the inverter.
Consider future loads that may require power so that the off-grid inverter installed today can handle growth in the next few years. Consider the users; the potential for growth in a system designed for a retired couple is a lot different from that of a system designed for a young family.
Step No. 3: Charging Capabilities of the Off-grid Inverter
When connected to an AC power source (such as a generator or the utility), an off-grid inverter stops turning DC into AC and becomes a battery charger, taking the AC source and recharging the batteries for you. Solar Inverters have a limited charging capability, so you should consider that value in your design. Ideally, an off-grid inverter can charge the batteries at a C/10 rate.
All solar inverters list their maximum charging capabilities in amps so you can directly compare the charger portion of the inverter/charger to the battery bank when you divide the batteries’ capacity (in amp-hours) by 10 (the number of hours required to recharge the battery).
Step No. 4: The Ability for the Inverter to Supply Power When Certain Loads Surge
Any load with a motor (such as refrigerators, washing machines, and well pumps) causes a brief power surge when it starts operating. If the inverter can’t deliver enough power to the loads during that brief surge period, the entire system may crash, and all the loads may go out.
In order to account for inevitable surges, you need to estimate the amount of power the solar inverter will be providing just prior to the surge and then estimate what the power draw will be when the surge happens. By adding these values together, you can verify whether the off-grid inverter can handle regularly occurring surges.
The amount of surge is specific to the appliance and is typically noted on the listing label as the maximum current draw. If you can’t get to the listing label on the load, you can either estimate the surge by multiplying the load’s power draw by three or you can use a clamp meter and record the current draw as the load starts.
11 Top Tips for Going Off-Grid Solar Systems
If a customer is constructing a remote home or isn’t connected to the grid in a current building but wants to make a change from her current system, the first type of solar power system you should suggest is an off-grid (stand-alone, battery-based) system. This option makes the most sense for this type of customer because it offers an electrical supply that isn’t dependent on an engine generator.
Before you make a decision, here is an article about 11 Top off-grid Solar Tips that can help avoid making costly mistakes.