It is often surprising how much larger an off-grid solar system needs to be than a grid-tie solar system to support the same overall lifestyle. A grid-tie system that completely eliminates the energy usage from the utility company might still be very undersized if the owner wants to go off grid using the same system. There are several reasons for this.

The main reason is simply because a grid-tie system is sized off of the average month (average amount of sun vs average energy usage) and an off-grid system is sized off of the worst month, referred to in the solar world as the “critical month” (highest-usage month compared to the lowest amount of sunshine). While it doesn’t seem like this would make that big of a difference, it does!

There are a few other reasons for this. The Power Conversion Efficiency (PCE) is lower on an off-grid solar system. This is because the inverter technology is a little different and also because most of the energy being used is temporarily stored in a bank of batteries. Depending on battery technology being used, the overall Power Conversion Efficiency of an off grid solar system can range from 65% to 80%, as opposed to about 85% to 90% for grid-tie systems. SolarSet uses Lithium battery technology in all products, so we can achieve approximately a 79% Power Conversion Efficiency in our solar systems.

So, how “critical” is the critical month in an off grid system? If you want reliable power, it’s pretty critical, but here are a few things to consider that can reduce the size of the system:  

• Augment with backup generator – If a combustion engine generator is hooked into the system as a backup, it sometimes is more economical to plan on using the generator for a short time every day during the critical month instead of oversizing the entire system. This should be done with some caution, however. Generators are mechanical devices which take maintenance and eventually wear out, and they also require fuel, which can be costly. Overall, a generator is less reliable than a solar system.
• Limit power usage during the critical month – Remember that in the critical month (usually December or January due to shorter days) a small adjustment in lifestyle can make a large difference in system size requirements. System is often substantially oversized in the summer months, so energy can be consumed without too much concern. In the winter months however, the difference between keeping an outdoor light on through the night and installing a simple motion sensor so it is only on for an hour or two each night might result in a surprising reduction in solar system size requirements. Many such small changes can be made which do not impact the overall lifestyle or quality of life. While for most of the year these changes in usage might not be required, they may be essential during a critical month.

So, can you start with a grid-tie solar system and go off-grid in the future? Of course you can, but it will require adjustments to the system. More panels will likely need to be added, batteries will need to be added, and mostly likely the inverters and electrical equipment will need to be changed out in order to operate with the batteries. In most cases, the installation scope, such as trenching and tying into the breaker panel, can be used as is. Generally speaking, if an owner wants to be grid-tie for a time and then go off-grid in the future, it is more economical to go with a battery backup grid-tie hybrid system from the beginning.