GLSP5: Residential Off-Grid

GLSP5.2 Electrical Analysis

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In short, going off-grid is expensive but paying your electric bill every month for your entire life is also expensive, such that going off-grid isn’t necessarily uneconomic depending on your current location, especially when you’re facing just really bad consumer solar policy. Often a customer requests to go off grid and has a digital meter which could provide 15 or 30 minute interval data yet the customers does not always have access to the same data that the power provider has. A good starting point is to request this “interval data” from the power company.

You may already have access to this data in your online electrical account – a benefit of those digital meters. But sometimes you must request this data directly from the power company yourself. Often the starting point is your month by month electric bill so let’s explore how to convert month-by-month electric consumption data into an hourly interval data estimate.

There is a very big misconception between service panel size, such as a 200 amp service panel, and the actual power draw of a typical home. Electric code requires you to have so many power outlets for every few feet of wall space.  Really there’s supposed to be an electrical outlet or two or three on each wall in your house, even if never in use. So depending on how your electrician wires the house you could have a dozen breakers for electrical outlets you never use. Your heavy electric appliances can have misleading breakers, being sized for maximum current draw, but how often do you use all five burners and the oven on your electric stove? It might be on a 60 amp breaker but only use 5 amps of power. Of course it is important to make sure the cable is sized for its maximum continuous output, but when you get back to the electric service panel and count up all the breakers, you may find you are starting to run out of space on the panel. While it is very common to have more than 200 amps of breakers on a 200 amp panel (the current is limited by the main breaker switch at the top), if you run out of breaker slots you may start to think you need another 200 amp panel for the electric service, and to power all the load at once without actually knowing what your load is, you may assume you need a 400 amp electric service.

Most homes, even large homes, do not need 400 amp electric service. The exception being tankless water heaters which use a tremendous amount of power, often 15 to 20 kilowatts. But a typical home is only drawing six to eight kilowatts of power on average, as revealed by inspecting your interval data if you have it. So keep in mind that tankless water heaters are awful for off-grid living because their instantaneous power draw is so large while in use, even if you are only using it for a short period of time. It will spike your load up and drop back down, which is not very good for solar because for starters, your inverter output capacity may not be able to handle it, but also, rapid discharging of any battery type results in efficiency loss. In our application the typical home power draw is four kilowatts which is in the springtime, and here summer is a bit taller but we only get up to around eight kilowatts, so if I put it on a 12 kilowatt inverter I have plenty of headroom for instantaneous power, which is the vertical axis on this charts. Unless I use a tankless water heater at which point my inverter will be insufficient. So what is better for off-grid is to keep your load profile low throughout the day. It’s a lot better to be a short, fat rectangle rather than a tall, skinny rectangle as would be associated with spiky loads. It would be better to have multiple electric hot water tanks continuously operating throughout the day rather than a tankless water heater that spikes, because you would otherwise have to buy a tremendous amount of inverter capacity to power only one device for a short while.

There’s a better water heating option that can exist within your system parameters. On a house I have under construction, we have two electric hot water tanks, with lots of breakers which would normally indicate a 400A service. But keep in mind 200 amps multiplied by 240 volts is 48 kilowatts, so a 400 amp service can actually pull 96 kilowatts of power from the grid continuously. But we are using a 12 kilowatt inverter for cost-effectiveness. So we are using digital controls for load management, which we will talk about later.

For now we’re trying to optimize the offgrid design, and so many will go for gas or wood chip heating instead of electric, but I think that’s cheating because you may simply be trading one cost for another.  At the same time, you can go much further if you manage your electricity, such as having an electric heater running throughout the day rather than just on high for a short period of time. This is where energy efficiency and thermal mass can come into play. But as a starting point you want to get a good survey of all the electrical devices in the home.

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