When adding a battery, an important decision has to be made: if I want to use it as a backup power source, should it power all the appliances and outlets (whole home backup scenario) or just the essential ones (partial backup scenario)? This choice will ultimately help you decide on the size of your battery, what components need to be installed and possibly how many modules you should add to your roof.
In this article we will compare the two options (Partial vs Whole Home Backup) by considering two of these aspects: battery size and electrical components needed.
Let’s start by looking at the Whole Home Backup option. This type of system is designed so that when there is a power shut down and the battery is in use (i.e. it is discharging), its energy will reach any of the loads (i.e. appliances) in the home. This result is obtained by connecting directly the battery to the main electrical panel.
Battery size: generally, for this type of solution you would need a battery that can store, at a minimum, the amount of energy that your home uses, on average, in one day. To find out your usage, look at your electricity bill. How many kilowatt hours (kwh) have you used in a month? What is your daily usage? If, for example, you have an average usage of 10kwh/day, that’s about the minimum usable capacity that you would need from your battery.
Components: because your battery will be connected to your existing main electrical panel, in most cases no additional electrical panel is needed. However, a very important safety feature, an automated disconnect, needs to be added. Its purpose is to prevent electricity from getting exported to the grid when it is experiencing a blackout. It can be inside your solar/battery inverter, in the form of a ring around your meter socket, or it can be included in a smart box usually referred to as a ‘gateway’ or ‘system controller’.
In the case of partial backup, you will be able to choose which loads need to be powered during a black out. Typically, these will be medical devices, lights, refrigerators, gate opening motors, internet routers, and (some) power outlets.
Battery size: we can now consider a smaller battery, as the non-essential loads will not be powered by the battery in the event of blackout. To have a better idea of what size battery you need, you can create a Loads Worksheet, i.e. a list of your essential loads and their daily energy usage. It is often advised that the battery capacity should be about 20% more than your loads’ average daily usage.
Components: For partial backup scenarios, the battery will not be connected to the Main Panel. Rather, it will be wired to the Sub-Panel where the critical loads’ circuits originate. Depending on how your home wiring system is designed, a sub panel may already be installed, and your installer will only need to ‘rewire’ it with the critical loads circuit. Alternatively, a new subpanel will need to be installed.
In any case, an automated disconnect will control the subpanel, and disconnect it from the main panel, and hence the electric grid, in the event of a blackout. This disconnect can again be incorporated in your inverter, or be included in the System Controller (or Gateway). Most battery manufacturers now also provide these components. Ask your installer which brand they are most comfortable with!
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