In practice, actual loads may be heavier in the daytime (as when working with power tools) or at night (as when watching movies or running a forced-air furnace).
If loads are applied attentively (or automatically!) one can run electrical loads that do not affect the battery bank's State of Charge at all. Additionally, consuming loads at the right time can also increase the health and longevity of lead batteries.1)
During Absorption the charge current is tapering down as the battery accepts less; the more it tapers down the more is available for loads.4) At the very beginning of Absorption ~0% of “bonus” output is available. At the very end of Absorption 99%+ is available. Mid-way through Absorption5) ~49% of the panels' output is available for loads. See the image at the top of the page for an example of how current demand drops in Absorption.
There will generally be more “surplus” power available when using MPPT charge controllers v. PWM,6)7) though the difference is less dramatic during periods one would run opportunity loads (see below). Siphoning off power can actually help PWM controllers run cooler by reducing the ON-OFF switching activity that generates heat.
Overpaneled systems have even more excess power available.
The best time to run non-essential loads is when there is surplus current over what battery charging needs. This means during Float, or after current has started dropping off in Absorption. For LiFePO4 there is excess power any time after the bank has reached the intended charge state.
You can start the loads manually (ie, start using the power when you have extra). This is error-prone but is free and requires no equipment (except your own memory).
The most precise way to do it automatically is to use a charge controller which turns on the LOAD output only when the batteries are in Float stage. Charge controllers with this feature tend to be expensive.
A free way to automate opportunity loading is to set the normal controller's LOAD or external Low Voltage Disconnect to shut off below Vfloat.
Since the LVD only knows voltage and Vabs > Vfloat this approach will
To run loads heavier than the rating of the LVD or LOAD output use a relay between the LVD and the load.
This setup looks like: [LVD or controller LOAD output] –> relay –> load
A 12v timer could give the system a chance to make some progress in Absorption before starting up opportunity loads.
In this approach the LVD is set just below Vfloat as above, but activation of the loads is delayed by some amount of time. Observation of the system during charging will suggest how long it normally takes takes charging amperage to drop off (ie, when the system has surplus current).
Suggested delay for conservative opportunity loading == the time from [passing Vlvr setpoint during Bulk] to [completed charging].9). This delay might be 2-3hours.
Suggested delay for aggressive opportunity loading == the time from [passing Vlvr setpoint during Bulk] to [far enough into Absorption that enough current is available to power intended loads]. This delay might be 30-60 minutes, depending on the charging rate.
This setup looks like: [LVD or controller LOAD output] –> timer –> relay –> load