Sizing the Battery Bank
In the example above, we are looking for at least two days of autonomy, with a total energy consumption of 2.628 kWh—but that doesn’t mean a 2.628 kWh battery bank will be enough. Efficiency losses of about 15% from our inverter and wire resistance bring that two-day need to 3.09 kWh (2.628 kWh ÷ 0.85). And, lead-acid batteries of any formulation—by far the most common and affordable choice for most off-grid applications (see below)—should rarely be discharged below 50%. That means only 50% of the battery bank capacity is actually usable without drastically reducing battery lifespan. A 6.18 kWh battery bank is required (3.09 kWh ÷ 0.5).
Off-grid batteries are rated and sold by amp-hours (Ah) of capacity, but fortunately the conversion to kWh is easy: Just multiply the Ah rating by the voltage of each battery and add them up. For example, a typical L-16 battery provides 350 Ah of storage at 6 V, and 6 times 350 equals 2,100 watt-hours (2.10 kWh). Our required capacity for two days of autonomy was 6.18 kWh, and 6.18 divided by 2.10 equals 2.94 of these batteries. At 6 V each, they must be used in pairs for a 12-volt system; groups of four for a 24 V system; and groups of eight for a 48 V system. Four would be the minimum batteries required here, offering 8.40 kWh of storage which actually increases autonomy time and also reduces average long-term depth of discharge for longer battery bank life.
Sizing the PV Array to the Battery Bank
Here’s where the occupancy patterns of a remote, off-grid cabin can significantly affect the system design and the budget. For a full-time off-grid residence, with such low prices per watt on PV modules, conventional wisdom now dictates designing a PV array to bring the battery bank from 50% SOC to 100% SOC over the course of one sunny “average” fall or spring day—likely less than a day of charging time during the summer months, and likely more than a day during the dead of winter. If the system has more days to charge the batteries to 100% before the next occupied period, the PV array could conceivably be sized smaller, with the option of expanding it in the future if usage patterns change.
Take a typical PV array in Colorado for the example above, sized for the loads, autonomy time, and battery bank for full-time occupation, with 100% of loads powered by PV during spring and fall, an energy surplus during summer and some backup generator run time required in winter. Bringing the four L-16 batteries (a total of 8.4 kWh of energy storage) from 50% to 100% SOC during spring and fall (including a derate of 30% for battery charging system inefficiency and PV output loss from higher-than-STC cell temperatures) would require 6.0 kWh (8.4 x 0.5 ÷ 0.7). With the site’s insolation of 5.5 peak sun-hours per day during that season, about 1,091 W of PV would be required. With four or five days (in an unoccupied situation) to charge the system, it would be no problem to halve that amount of PV—perhaps two modules instead of four, and still have excess charging capacity.
System Equipment Phantom Loads
Inverters are phantom loads—they still use power even if no loads in the home are running. Most off-grid inverters can be set for “search mode,” which drastically reduces their no-load power use. Typical search-mode power drain is only 5 to 10 W. The inverter remains asleep, but will wake up quickly if a load that draws enough power is turned on. The watts required to bring it out of search mode are adjustable. This can be very convenient upon arrival back at the cabin at night for a vacation weekend, with no need for flashlights to find and turn on the inverter again. But too many phantom loads can keep the inverter on and out of search mode, so it’s best to test carefully before buttoning down the system prior to an extended departure.
Set the inverter for search mode, and turn off or unplug all your loads. After a few seconds, the inverter should drop into search mode, usually indicated by a blinking indicator light or a message on the remote. If it doesn’t do this, increase the search mode watts setting on the inverter. Now, turn on the first light you’ll need when you arrive. If it doesn’t work or it flashes, decrease the search mode W setting until the light operates properly.
Keep in mind that if you have equipment such as a Wi-Fi router and security system that must remain running when you are away, search mode won’t be an option, since the inverter will remain on. Inverter power draw when on (but with no loads operating) varies by manufacturer, but 20 to 40 W is typical. And remember that even if your loads use less than that, the inverter’s no-load draw will still be the minimum. You can find the no-load draw on the inverter specifications sheet.