In the first post of this series, I laid out my general reasoning and plan for incorporating solar power into my new home. This post will go into more detail about what that means in theory, and how it plays out in practice.
There are five general types of systems to consider when having a discussion about electric service with and without Solar power (and/or battery storage):
Electrical Grid Only - This is the configuration of the vast majority of residences and businesses. Power is provided through a connection to the local utility’s electric service, which is metered by equipment present at the customer’s Service Entrance (the round meter you see on the side of your house). Old-school meters had a spinning disc that you could watch go round and round to mark the consumption of electricity, spinning faster at times of higher usage. Most meters are digital now and consumption can be “read” over the grid rather than requiring a physical visit to record how many Kilowatt-hours have been consumed since the last billing period. In this scenario, if the grid is down for any reason, you’re down until they repair the fault.
Electrical Grid Plus Backup (generator and/or batteries) - This is a variation of grid-only, where you have some form of backup power that can be used when the grid is down. I call out this type of system specifically because in this system you only use the backup when the grid is down, and when using your backup power source you must be completely isolated from the power grid through the use of a manual or automatic [transfer switch][0] that severs your grid connection. When using a backup power source you must be isolated to prevent backfeed of power to the grid, where personnel may be working on the lines expecting them to not be energized. Safety First.
Off-Grid Solar - I won’t talk about this system much, other than to say that it’s not interesting for this discussion and generally used when electric service is not available, such as in a mountain cabin or other remote location. Solar panels are used to provide primary power for the site, almost always in conjunction with batteries.
Grid-Tied Solar - Grid-tied solar is the most common form of residential solar installation in the United States. This type of solar installation generates power for use within the home, offsetting power being pulled from the grid. If the solar array is producing less energy than the home is using, power is pulled from the grid to make up the difference. If the solar array is generating more power than is being used, the excess is pushed back to the grid and the meter “spins in reverse,” usually generating credit with the utility company. Many utility companies will bank these credits and carry them over month-to-month, so that you can draw against the credit during low- or no-generation periods (nighttime, winter) so that you can net to zero grid consumption on an annual basis. Some municipalities actual pay excess-generators back if they exceed net-zero, but that seems to be becoming more rare (Washington State’s program stopped accepting new applicants in 2021). The main downside of grid-tied solar is that it requires an active grid connection to function. If the grid goes down, your solar array initiates Rapid Shutdown, stopping potential flow of power back to the grid for the safety reasons mentioned above. This leaves you in an outage situation like everyone else, but with stranded power generation just over your head.
Hybrid/Grid-Forming Solar (with and without batteries) - Hybrid solar is where the real fun begins. In a hybrid solar or grid-forming system, the system is capable of mimicking the function of the utility grid in the event of a grid outage. When the grid is up, it’s a normal grid-tied solar system. But when the grid goes down, the system automatically isolates and can then form a local microgrid to continue providing usable power to your premises. If no batteries are present in the system, you will only have power if enough sunlight is available to support powering the microgrid. With batteries, the microgrid automatically forms and is usable as long as the batteries have sufficient power, night or day. If you have enough battery capacity (meaning, quite a lot), you can run off-grid indefinitely any time of year as the solar array will recharge the batteries during the day. Unless it’s winter and you live close to or beyond one of the Polar Circles. In that case, well, get a generator.
My plan at this stage is to go with a Hybrid Solar + Battery setup. I want to have enough solar installed to get to net-zero grid consumption or slightly beyond. I also would like to have enough battery capacity and power to keep critical loads as well as our heat pump system running even if there is an outage on the shortest day of the year (and therefore, little help from the solar array). This may not end up being financially palatable, but I’m going to see where we end up.
In a Hybrid Solar + Batteries system, attention must be paid to sizing both the solar array as well as the batteries, to ensure that there is enough generation, storage, and output capacity to meet the design specs of the system (e.g., run only a few electrical loads, or run the whole house). Some hybrid systems can even have a generator in the mix to charge the batteries when solar isn’t sufficient.
The batteries have two sizing dimensions that need to be considered: storage capacity (Kilowatt-hours), and power output (Kilowatts). To use a simple water supply analogy, think of these two dimensions as the size of a storage tank (Kilowatt-hours<–>gallons) and the rate at which water can leave the tank (Kilowatts<–>gallons per minute of flow). In order to power electricity-hungry appliances like water heaters, AC units, or ovens, you need 3-5 Kilowatts or more of output power, whereas an LED lightbulb may only need 0.01 Kilowatts. And of course when you use more Kilowatts powering big stuff, you need more Kilowatt-hours of storage capacity in order to not run out too fast (a 4.5kW water heater running for 30 minutes consumes 2.25kWh of battery capacity). This is a major design constraint of Hybrid Solar + Battery setups, since battery capacity and power are drivers of cost as well as physical size. Most Hybrid Solar + Battery setups only provide partial backup for homes, focused on the most critical loads (fridge, microwave, some lights and outlets) and leaving heavy electrical consumers unpowered.
Unmentioned in all of the above is where things may be going as it relates to V2X technologies (Electric Vehicle-to-X, where X is Load, Home, or Grid). EVs have really big batteries, way bigger than most solar battery systems. A decent home system might have 10-20kWh of battery storage, wheras even a small EV has 60kWh or more. Being able to tap into that reserve is very appealing, but the tech isn’t generally there yet.
Now that we have some of the fundamentals in hand, the next step is to evaluate some potential solutions. I have proposals rolling in, and in the next installment I’ll share what they look like and how they compare.
Other posts in this series:
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