Our house produces more electricity than we use, so in theory it would be very easy to unplug from the grid and become self sufficient. We don't do this for three reasons:
First, being connected to the grid means that we have electricity when the sun is not shining, its rays are blocked by heavy clouds, or by snow on the roof. We don't need to worry about batteries or generators because the grid is our back-up power supply.
Second, since we produce more energy than we use, we can supply energy to the grid and contribute electricity to the community. We wanted the house to produce more energy than it consumes, and we like to feel that the extra energy is being used and making a difference.
Third, they pay us for any electricity that we supply. They pay us very well: about twice what we pay for day-time electricity and five times the amount for night-time electricity. This is similar to the second reason, since we can see from the negative bills that our electricity is making a difference. We can safely assume that there is more demand and less supply in the day time, so we are filling some kind of need by selling our electricity. It's less safe to assume that our electricity is worth twice as much as their electricity, and easier to see the feed-in-tariff as a boost to the solar industry. Even then it is probably a good thing as the renewable energy industry and its exploitation of a resource that literally falls from the sky still seems to be getting less subsidy than exploiting fossil fuel reserves, and if we are to transition from fossil fuels we will need solar panels.
Regardless of the politics, the highly tangible and easily countable financial considerations mean that we try to sell as much of our day-time electricity as possible, and use their night-time electricity instead. Looking just at energy use this is a bad idea. Our main power consumption is for heating water, which we mostly use in the evening. Currently we are heating hot water at night and it is sat in the tank steadily losing heat for most of the day. Also, the tank is heated by an atmospheric heat pump, getting heat from night-time air, which is colder than the daytime temperature by something like 10 degrees at any time of the year. If we were using electricity in the day time from our own panels, then the heat pump would do a lot less work to get the heat from the outside temperature up to the temperature in the water tank, and the hot water tank would be losing a lot less heat before we use it. This could save us as much as 25% of our electricity, but we don't do it because using our electricity in the day time is over 300% more expensive.
Our contract for selling electricity runs out after ten years and we certainly will not be able to get the same price, but it's not clear yet what the financial calculation will be. If we were to start using daytime electricity, we would also think about trying to use the hot air under the solar panels, which would be even hotter and need even less work to provide us with hot water, but that's another blog post.
Back to the question in the title: If we were to disconnect from the grid and wanted to get a battery to keep us in power, how big would the battery need to be? I have seven years of generation and consumption data to give me an answer.
When I said that we produced more power than we consumed, this has been true for every year and every month. The lowest producing month was October 2017 (670 kWh), which was the least sunny October since 1917 with only 100.9 hours of sunlight. September 2018 had even fewer hours of sunlight (94.4), but we made 800 kWh. That's the same as our highest monthly consumption, 800 kWh, in February 2013.
The longest period when generation stayed above consumption every day was 153 days from 20th April to 20th September, 2016.
The longest period where consumption stayed above generation was for five days between 12th and 17th October, 2017.
If we need a battery to cover all our energy needs, then it may be for these five days. In the simplest calculation, we need a battery of 33.1 kWh (the shortfall between the 70.5 kWh consumption over those five days and the 37.4 kWh generated). That's one or two Nissan Leafs.
There were five times when the consumption stayed above the generation for four days: from 14th January, 18th June, and 23rd October, 2013, from 6th September, 2015 and from 19th October, 2017. Many of these grey-outs are in September or October, when consumption is at its lowest. The snowy days in the middle of January 2013 were at a time of much higher consumption, and for those we would have needed to store 53 kWh to make up the gap between 61 kWh generate and 114.2 kWh consumed. The Teslas have 60kWh batteries.
Although the meteorological data confirms September and October as the months most prone to sunlight shortages, when the roof is covered with 22 cm of snow, our heating needs may also peak.
So the short answer is, we would need a 53kWh battery. Anything smaller and we are still going to need to rely on the grid and pay the monthly connection charge, or we would need some other backup, so the value of a smaller battery is limited.
Since most of our energy is for heating, it may make sense for us to look at storing heat rather than electricity. Phase-change materials may be useful for this.
Also, a more thorough answer would look at charging efficiency, discharging efficiency and electricity leakage. The figures above assume 100% of the electricity goes into the battery, 0% of the charge is lost over time, and 100% of the charge comes out.
First, being connected to the grid means that we have electricity when the sun is not shining, its rays are blocked by heavy clouds, or by snow on the roof. We don't need to worry about batteries or generators because the grid is our back-up power supply.
Second, since we produce more energy than we use, we can supply energy to the grid and contribute electricity to the community. We wanted the house to produce more energy than it consumes, and we like to feel that the extra energy is being used and making a difference.
Regardless of the politics, the highly tangible and easily countable financial considerations mean that we try to sell as much of our day-time electricity as possible, and use their night-time electricity instead. Looking just at energy use this is a bad idea. Our main power consumption is for heating water, which we mostly use in the evening. Currently we are heating hot water at night and it is sat in the tank steadily losing heat for most of the day. Also, the tank is heated by an atmospheric heat pump, getting heat from night-time air, which is colder than the daytime temperature by something like 10 degrees at any time of the year. If we were using electricity in the day time from our own panels, then the heat pump would do a lot less work to get the heat from the outside temperature up to the temperature in the water tank, and the hot water tank would be losing a lot less heat before we use it. This could save us as much as 25% of our electricity, but we don't do it because using our electricity in the day time is over 300% more expensive. Our contract for selling electricity runs out after ten years and we certainly will not be able to get the same price, but it's not clear yet what the financial calculation will be. If we were to start using daytime electricity, we would also think about trying to use the hot air under the solar panels, which would be even hotter and need even less work to provide us with hot water, but that's another blog post.
Back to the question in the title: If we were to disconnect from the grid and wanted to get a battery to keep us in power, how big would the battery need to be? I have seven years of generation and consumption data to give me an answer.
When I said that we produced more power than we consumed, this has been true for every year and every month. The lowest producing month was October 2017 (670 kWh), which was the least sunny October since 1917 with only 100.9 hours of sunlight. September 2018 had even fewer hours of sunlight (94.4), but we made 800 kWh. That's the same as our highest monthly consumption, 800 kWh, in February 2013.
The longest period when generation stayed above consumption every day was 153 days from 20th April to 20th September, 2016.
The longest period where consumption stayed above generation was for five days between 12th and 17th October, 2017.
If we need a battery to cover all our energy needs, then it may be for these five days. In the simplest calculation, we need a battery of 33.1 kWh (the shortfall between the 70.5 kWh consumption over those five days and the 37.4 kWh generated). That's one or two Nissan Leafs.
There were five times when the consumption stayed above the generation for four days: from 14th January, 18th June, and 23rd October, 2013, from 6th September, 2015 and from 19th October, 2017. Many of these grey-outs are in September or October, when consumption is at its lowest. The snowy days in the middle of January 2013 were at a time of much higher consumption, and for those we would have needed to store 53 kWh to make up the gap between 61 kWh generate and 114.2 kWh consumed. The Teslas have 60kWh batteries.
Although the meteorological data confirms September and October as the months most prone to sunlight shortages, when the roof is covered with 22 cm of snow, our heating needs may also peak.
So the short answer is, we would need a 53kWh battery. Anything smaller and we are still going to need to rely on the grid and pay the monthly connection charge, or we would need some other backup, so the value of a smaller battery is limited.
Since most of our energy is for heating, it may make sense for us to look at storing heat rather than electricity. Phase-change materials may be useful for this.
Also, a more thorough answer would look at charging efficiency, discharging efficiency and electricity leakage. The figures above assume 100% of the electricity goes into the battery, 0% of the charge is lost over time, and 100% of the charge comes out.
