Making use of ‘extra’ electricity off the grid

When someone has grid connected solar panels, they are usually trying to maximize total output, as they can sell all power straight to their power utility through the wires connected to their home. With off-grid systems, there is by definition no connection to the grid, so when you generate your own electricity, you are in a ‘use it or lose it’ kind of situation. As we’ve written elsewhere, solar electricity generation in an off-grid home isn’t about producing the maximum possible amount of power, it is about trying to make sure that you don’t run out. Further, there is great variation in the amount of sunlight available over the course of a year. Here in Ottawa there is four to five times as much energy available from sunlight during the long days of June than in December. On top of that, the cold and dark parts of winter require a lot more electricity to power things like lights and heating systems. Off-grid solar systems need to be designed to cover these winter needs, and so overproduce during all of the warmer and sunnier parts of the year. Putting all of this together, it means that most off-grid homes waste a lot of the electricity that the panels produce.

Up until December of 2018 this was the exact situation for the Manitou Bay house, where we were only using about 20% of the total power that the panels could have made available. During much of the year, our battery bank was full or nearly so, and all of that extra electricity was left unused. Knowing about this waste nagged at us, that we had all of this electricity with no place for it to go. We knew it was possible to do better and so made a plan to fix things, but due to it being somewhat lower priority as well as some unforeseen delays, it ended up taking almost three years to put that plan into action.

We have now installed some new hardware that allows us to utilize much of that wasted power. In principle, all we have done is add an automatic system that switches on some electrical devices anytime there is extra electricity available – on bright and sunny days. These extra electrical loads are called ‘opportunity’ or ‘dump’ loads, and need to be those things that are useful to us even though they only turn on during sunny days. This means that it can’t be things that we absolutely require, or those that we want available at a moment’s notice. We currently have one major need that fits this description, which is supplemental heating. Up until now much of our active heating has been accomplished with propane. The new system will do space heating through all the colder months, and make hot water for showers and the like during all of the summer months. We expect that this could cut our propane use at the house by a third (see below). In the next couple of years we will have one more use of intermittent power, to recharge an electric car, when we replace one of our current gasoline powered ones around 2021 or so.

This principle, of using electricity when it is cheaper or more easily available, is one of the main ideas that will underlie the ‘smart grids’ of the future. Right now most users simply turn on any power needs that they want whenever they want them, but in the future we will be able to set our heating, cooling, charging, and other devices to automatically turn on when power is cheaply available, and turn off when it gets more expensive. This will save everyone money, and will allow more renewable energy into the grid and give greater flexibility to utilities. What we have done with our upgrade is to make our off-grid home something of a ‘smart’ microgrid.

How the relay works (the rest of this post will be a bit more technical). In our situation, we are using the voltage of our battery bank to regulate our intermittent loads. When the batteries are full and the voltage is high the switch turns on, and when the battery voltage falls to about 80% the switch turns off, as we need to keep a healthy amount of power in reserve for all of the normal electricity needs. Our charge controller (system details here) has the components and software built in that can control a switch based on voltage. However, this switch can only control a low power direct current (DC) circuit, not the higher energy alternating current (AC) electricity that is typically used around a home. So this is why we had to add new hardware in December, installing a relay so that the DC circuit could control AC circuits that go out to the heating and plug loads where we have the intermittent power consumers.

How much electricity will we effectively gain by adding the relay? Before adding this relay, the house consumed a bit less than 5 kWh per day of electricity year-round, or about 1500 kWh per year. Based on a power production estimate that we had done when our solar system was installed, which is now doubled since we doubled the number of panels, our solar system could produce up to about 7500 kWh of electricity per year. Now that we have the relay, we will be able to capture much of that excess. From November to April we will put every extra kWh into space heating with a 1500 watt plug-in heater. In December this is only anticipated to be about 2 kWh per day, while in April there should be an extra 20 kWh per day. From May to October, the extra electricity will be routed to a standard electric hot water heater. We estimate that with the number of showers that we take that we will consume around 10 kWh per day of electricity through the water heater, which still will leave a fair amount wasted during the sunniest months of May to August. Add all this extra power up, and we will use around 3600 kWh of additional electricity. We should now be using about 70% of the total output of our system instead of the 20% that we managed before the relay was installed. See the table below for specifics.

MonthDaily extra kWh usedEnd use
January4space heating
February9space heating
March18space heating
April21space heating
May10hot water
June10hot water
July10hot water
August10hot water
September10hot water
October10hot water
November4space heating
December2space heating
Annual total3600all uses

We’ll wrap up by going over the greenhouse gas emission and financial reasoning behind this project. Before adding this relay the home was using about 400 gallons per year of propane. Adding 3600 kWh of electric heat should reduce the propane consumption by around 140 gallons. By avoiding burning all of that propane, we will reduce our CO2 emissions by a full metric ton each year (this is a part of our 5 year plan to reduce our emissions by half). The cost savings for that propane not purchased will be around $350 per year as long as the system keeps running. The relay work cost a bit less than $1000 and the electric hot water tank cost about $500, and so the work will pay for itself in under 5 years. Finally, I ought to mention that even with this increased efficiency, solar electricity off the grid isn’t cheap. Considering all the panels, electronics, batteries and labor, and the fact that most of these components only last for 10 to 20 years, even with the relay our solar electricity still costs around $.50 per kWh. This compares to power from the local utility, HydroQuebec, that comes in at around $.10 per kWh. With the secluded location of the home off-grid was the only sensible choice, but the costs do make one appreciate the value one gets by tying right into the power grid.

Special thanks to Tom Hewitt at NetZero Construction for his assistance with the solar and DC circuit work, and Jean-François at Lamarche Electric for all of his work installing the relay and AC circuits.


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