Efficiency in off-grid systems tends to be overlooked as often as safety. The end result is that all too often you are surprised when your system does not perform as well as you calculated. This post offers some clues to help you right-size your system.

These same losses/inefficiencies can exist in grid-connected electrical systems as well. They are just less likely to be noticed because the are represented as an increment in your electric bill rather than a dead battery.

• Batteries are not 100% efficient at storing energy and delivering it later. There are many variables including the type of battery, state of charge, charge and discharge rates relative to the battery capacity, temperature and age. High-end numbers for lead-acid batteries are around 80%. Know your batteries and your load.
• Not all ampere hours are created equal. (An ampere hour is a measure of battery capacity. It just means you multiply the number of amperes you are drawing times the number of hours the battery can supply that much current.) The neglected part of the description is the rate. For example, a 20 hour rate means a load such that it will take 20 hours to discharge the battery. A 1 hour rate means a load such that it will take only one hour to discharge the battery. Expect the slower rates to deliver many more total ampere hours. For example, a battery might deliver 10 amps for 40 hours (400 ampere hours) but if the load was 100 amperes it might only deliver it for two hours (200 ampere hours).
• Wiring, particularly at lower battery voltages can introduce significant inefficiencies.
• Converters (one DC voltage to another) and inverters (DC to AC) introduce losses. Modern ones tend to be fairly efficient (95% is not abnormal) but their efficiency may be significantly lower when lightly loaded. For example, a unit that can deliver 1000 watts maximum may be 95% efficient at full power but only 75% or less efficient if only delivering 100 watts.
• With photovoltaic systems, panels have a peak power point. That is, a point where the product of voltage and current is maximum. That voltage will vary with the amount of sun falling on the panel and panel temperature. A battery also presents a varying voltage load depending on state of charge and charge current. A mismatch can introduce 30% or more losses into the actual power being saved in the batteries. MPPT (Maximum Power Point Tracking) PV controllers are designed to fully load the panels and transfer the maximum amount of power into the batteries.
• Some devices (most anything with a motor but also incandescent and florescent lights) draw more power to start up than their running power.

There are more but that should offer a good start.