Figuring battery size

There are a host of considerations to take account of when deciding how many kWh of energy storage are needed for a domestic battery storage system; if I choose too big a battery, the surplus available from self-generation may not be enough to fully charge it, and additionally it will be needlessly expensive; choose too small and it may not have enough storage to meet the functionality I want.

So an early decision in the journey to installing battery storage is to be sure what I want. 

There is a divide based on complexity between two options, with a further divide in the second option; the  technically simplest option is to store energy so it can be used at times of peak demand, thereby reducing drawing from the grid at those times; the more complex alternative is to have that capability plus the capability to back up the house in the event of a grid outage.

The further divide in this latter alternative is whether it is 'whole house back-up' or 'secure outlet back-up'; the distinction between these is important because 'whole house back-up' will allow the Powerspout to continue to feed its output into the house grid when the house is disconnected from the utility grid, whereas a 'secure outlet back-up' only provides a single, switched, power outlet to which emergency loads can be plugged in. In a grid outage, being able to make use of the Powerspout's output is obviously very desirable, so if at all possible, that is what I would like to go for. 

Grid outage back-up of either type requires the battery to be able to store more kWh, and questions immediately flag up - how much energy does my house use, when are the peak times of usage, and how great is the power drawn at times of peak demand. 

With these 'demand-side' unknowns in one pan of the scales and 'supply-side' unknowns in the other, - supply-side questions being: how much surplus energy does my installation produce, and how does this surplus differ between summer and winter months, - a delicate weighing up operation needs to be undertaken to balance the one with the other.

The whole matter becomes rather complicated, and working through to a properly reasoned conclusion can only be done by having data, - the more data the better, and data covering both 'demand-side' and 'supply-side'.

Below are graphs to show my 'supply side' data; they document my hydro and solar generation over the past 6 years: 

The bars in the main graph show energy generated each day of an 'average' year; such a year is a hypothetical one concocted from the averaged data from the 6 years from 2016 to 2021; the individual years are shown in the smaller graphs above and below the main plot, to make the point that real years are never quite the same as an 'average' year.

The graph shows that 'typically' the lowest level of self generation happens in September / October and is around 10 kWh / day. Highest generation peaks at 30 to 35 kWh / day in March / May when solar output is picking up and yet hydro output is still good due to groundwater having been replenished by rain which fell in the previous December to March period.

So much for my 'supply-side' data, which being accumulated from 6 years of record keeping is about as reliable as one can hope for; what about 'demand side' data? 

Below left is a plot of energy consumption from the grid in a month; for our house, all months, winter or summer, look the same; 

note it is not a complete picture of house consumption because it does not include energy consumed which is self generated; to get that full picture, the number of kWh consumed which are self-generated need to be added, and that number comes to, I have established, an additional 10 kWh / day; the graph below right shows this daily addition (in blue), so the graph now represents the complete demand-side picture over one month:

It will be clear from the above graphs that on days (in truth, nights, - so use is made of cheap rate, night-time energy) when my Nissan Leaf is charged, the grid energy taken is in a different league to other days, 20 kWh compared to 3 - 6 kWh; this is important to keep in mind when deciding on battery size; if I rule out any hope of storing sufficient kWh to charge the car from energy stored in the domestic battery, then the battery only needs to be big enough to cover each day's domestic consumption taken from the grid; a battery of 5 - 6 kWh capacity would be plenty large enough for that, whilst to charge the car from stored energy would require a 20 kWh battery.

I have established that the demand each month is much the same in one month to the next, regardless of the time of year, so the pattern of monthly demand can be scaled up to a year pattern simply by repeating it 12 times; it then becomes possible to see how data for 'supply side' and 'demand side' relate to each other if the one is overlaid on the other; as the following graph shows, where blue bars are not overlain by red bars, that represents surplus generation which is available for storing in a battery:

Interpreting the graph further, one can conclude that for the 9 months between December and August, the excess of generation over consumption which is indicated by blue bars not being covered by red bars, amounts to at least 5 kWh/day, and this should make it possible to have enough surplus energy to fully drain and re-charge a 5 kWh battery each day throughout that period; within that 9 month period, there are 7 months from January to July when the margin of surplus is much more generous, at 10 to 15 kWh /day.

Here however, a caveat needs to be remembered: these calculations are founded on 'averaged' data taken over a 6 year period; if the actual data of each of those 6 years, given above, is scrutinised, it will be clear that in any given year, the 'supply side' data is quite variable; the import of this is that in some years things will not work out as anticipated.

...but a battery of 5 kWh seems to be the size for me and it is the size I am contemplating getting; it will be big enough to cover what we usually take from the utility grid each day and it should be enough to cover 'grid outage back-up', whether I go for the option of 'whole house back-up' or 'secure outlet back up'.

Next time, we'll look into those two options.

Towards battery storage

For quite a long time, I have been thinking about adding battery storage to my set up; the recent eye-watering rise in the cost of grid-purchased electricity here in the UK is finally driving me from thinking about it to acting on it.

It is not a simple matter however; choices about battery size, whether to include grid outage back-up capability, where to physically locate the battery and charger/inverter, - all these and much more have to be worked through, and inevitably compromises identified and accepted.

I have persuaded myself that the case for battery storage is unassailable. My reasoning goes like this: my Powerspout is good at delivering energy over long periods at low power levels; batteries are good at delivering energy over short periods at high power levels; by combining the two, advantage can be taken of their respective strengths.

In effect, for a grid connected Powerspout like mine, a battery can be used to address the limited capability of the Powerspout to deliver sufficient power when domestic appliances like a kettle or cooker are turned on; the battery opens the door to saving on the energy which would, without a battery, be drawn from the grid.

In the complicated journey of working out how to configure a battery system, a major 'road block' has been how to integrate the existing diversion device I have, a Solarcache; it harvests surplus energy and diverts it to 3 different heating loads in the house, always maintaining a bare minimum of power to trickle back to the grid; it has been a very valuable component of our system which we have come to rely on as our sole source of domestic hot water in the summer months; I am keen not to have to jettison it with the introduction of battery storage.

The 'road block' was because my Solarcache was an early model which lacked the later firmware which enabled Solarcache to be adjusted so the battery management system (BMS) got first priority of surplus power; 

Happily, yesterday that block was removed; I discovered I had a spare Solarcache with the required firmware update; below is the screen from it which shows how the Solarcache's settings can be adjusted to give the BMS priority.

The settings allow for a threshold level of Watts flowing back to the grid, and a delay time during which that power needs to flow, before Solarcache will start to divert power to the loads connected to it. Discrimination between the settings of the BMS and Solarcache can thus be achieved to ensure the BMS gets first priority.

I was really excited at discovering I could keep Solarcache alongside the battery system; having removed that particular 'block', I now feel free to progress the battery scheme further.

I'll post more in due course.