Category Archives: Electricity storage

Best use of generated power (cont.)

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In order to illustrate how the combination of battery and immerSUN distributes generated electric power at different levels of generation I created this chart.

For different levels of power generation across the bottom, the chart shows how the power is divided between battery charging (and occasionally discharging), electric car charging, and water heating; which are generally prioritised in that order. My prior post explained the rationale for the 500 Watt switching threshold for the vehicle charger – based on 1.4 kW of mid-value car charging being better value than a mix of 800 Watts of high value battery charging and 600 Watts of low value water heating.

Alternatively you might like to consider that the horizontal axis represents passing time after daylight comes and that the chart shows how diversion changes as the sun reaches its zenith.  You might then view the end of the day as a mirror image of this as the output of the panels ramps down in late afternoon, although at the end of the day there’s the greatest possibility that one or more of the storage devices is/are full and thus the greatest chance of electricity being exported.

Of course all of this assumes that the storage devices aren’t already full, and indeed that the electric car is present at all. As storage devices fill, or indeed if the car is absent, the system automatically switches to the next best value alternative:

  • Battery full – more car charging and/or water heating.
  • Car full or absent – more battery charging and/or water heating depending on power output.
  • Hot water at maximum temperature – this is the lowest priority electricity use so when this is full we don’t currently have another use to divert power to. However there is an unused output on the immerSUN so it would be possible to drive another load. The underfloor heating in the kitchen would be a possibility, although there’s unlikely to be much overlap between days when there’s enough surplus to reach this point and days when kitchen heating is required so it may never repay the cost of fitting the cables.

Best use of generated power

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Over the last few days I’ve been rethinking the best use of generated power.
The prioritisation of battery charging over water heating is clear due to the significant cost difference between day time electricity and any time gas, but the situation on car charging is more complex. It occurred to me that there could be times when prioritising battery charging and water might not always be the lowest cost solution since car charging avoiding mid-price nighttime electricity might be a bigger saving than a lesser amount of high value battery charging combined with low value gas-replacement.

For example, if we look at the lowest level of EV charging that amounts to about 1.4 kW. With our night-time rate of 7.87 p/kWh, 1.4kWh of solar power used for car charging saves 11.0 p of night time electricity.  If the battery is maxed out at 800 VAh that saves 7.34 p of later day time electricity. The water heating using the balance of 0.6 kW saves a further 1.76 pence of any time gas. Thus the total save from 1.4 kWh used for a combination of battery charging plus water heating is 9.1 p, compared to 11.0 p from car charging – so it would appear to be better value to do 100% car charging when a 1.4 kW surplus exists.

A bit of further analysis aimed to establish the point at which it became better value to charge the car, rather than combine battery charging and water heating, even if that involved a small level of mains import. The answer is that, with my energy costs, it makes sense to enable 1.4 kW of charger when 1.3 kW of export would have existed thereby potentially importing 0.1 kW. In practice this 0.1 kW may be supplied by the battery.

Given that the battery has priority by the way it’s wired, and takes up to 800 VA, then I intend to try a 500 W export threshold to start the car charger since 800 VA + 500W ~ 1.3 kW.

A sunny December day

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Today as expected was well above average for December with 8.2 kWh generated so it was a good opportunity to confirm correct prioritisation of battery charging and water heating.

The graph above from the immerSUN clearly shows the green area of electricity being generated from the solar panels, the purple line of the ‘house’ electrical load (including battery charging) rising first (i.e. the priority load) within the green area, and then the blue line representing the water heating rising second. The purple line being at about 1kW is consistent with 200 Watts of house load with the 800 VAp battery charger on top.

Water heating starts to reduce first with battery charging around 14:00. Shortly after 14:00 the house switches to running from an increasing amount of stored energy which lasts until shortly before 20:00.

From the battery’s perspective you can see ‘device power in’ i.e. battery charging rising first, followed by ‘grid power out’ which is actually power available to the immerSUN and should correspond with the immerSUN’s blue line. As the sun goes down power diversion for water heating is reduced first, following by power for battery charging; until finally from about 14:30 the battery switches from charging to depletion ‘device power out’ running the house until shortly before 20:00 by which time it’s supplied around 3kWh.

System data from battery and immerSUN perspective

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Today (briefly!) I watched the battery storage and immerSUN operating alongside each other. As you may recall things are arranged such that the battery should charge first (at up to 800 VA) and then once the battery is drawing full power (in the absence of a car) the immerSUN makes hot water.

From the immerSUN’s perspective as PV output rises the load of the house (which is actually the battery charging) also rises. Once the battery is charging at its maximum 800 VA power then any remaking PV is diverted to making hot water by the immerSUN. Later as PV output drops the battery continues to match the zero PV output discharging the battery to make the house load zero according to the immerSUN until the battery is exhausted – 2 hours of house load in this illustration.

From the battery’s perspective between 10 and 12 it can be seeing hitting its maximum 800 VA, while a varying amount of excess power is seen to be exported. In reality this isn’t true export, its power available to the second priority device (the immerSUN) and corresponds to the immerSUN’s blue line for diversion.

Overall self-use was 96% although admittedly on a low base. Thursday should be interesting as that’s sunny all day on the forecast.

Why I did buy a battery (almost)

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Well, after consideration going back months, I did finally buy a battery (almost). Conceptually I’ve long thought that a time would come when I’d have a storage battery to store excess solar electricity from when I couldn’t use it all, and then use it at a later time; but until now I’ve been put off by various technical constraints and the cost. However a few weeks ago I saw an appeal for homeowners to participate in a battery trial sponsored by a DNO (the companies who own local substations and the cables down the street). After initially being told that the scheme was full, I was subsequently offered a place – presumably because others have dropped out. In exchange for giving the DNO 18 months of usage data (for which a data logger is installed in the house) I get a discounted storage system to keep.

img_0637The battery that I’m being offered is a PowerVault with 4kWh usable capacity. It’s connected on the AC side of my inverter, and can be charged either by the surplus on my solar PV or, when there’s insufficient PV output, on cheap night-time power for later daytime use. It also has a UPS function that could usefully maintain power to my PC, internet, phone etc during a power cut. On the downside it has a relatively low output power so it can’t fully power larger consumers like the oven, car charger etc; although it can sustain lighting, TV, the fridge and a host of smaller loads that make up modern electricity consumption.

This means I now have multiple means to save surplus free power when it’s available and avoid buying power at the time of need which are, in order of financial attractiveness:

  1. Charge the storage battery avoiding later day time electricity purchase.
  2. Charge my electric car avoiding later night time electricity purchase.
  3. Make hot water avoiding later gas purchase.

The system itself is about the size of a slimline dishwasher and is designed to sit under a worktop. I however now have my battery installed in the study, under my desk, a location chosen because of its proximity to the consumer unit and because any waste heat from the battery will (along with my PC) help to heat an otherwise unheated room. It could alternatively have gone in the adjacent utility room, but I’d have needed to sacrifice a cupboard and the utility room has no need of any more heat as it’s home to the boiler already.

One of my concerns was whether the storage battery control system and immerSUN might interact as both sought to divert the same power to their respective storage devices, but I think I’ve worked out how to ensure the storage battery always has priority while allowing the immerSUN to work simultaneously. One could imagine that, for example, at the height of a summer day the maximum 800VA would be going into the battery while the immerSUN managed nearly 3kW into car and/or hot water.

img_0638When it comes to viewing system activity then there’s opportunity for me to see what’s happening or has happened on any day through the cloud servers for both the storage battery and the immerSUN. I’ve not got to grips with the storage batteries’ data logging yet. However, from the immerSUN’s perspective, battery charging looks like additional household load up to 800 VA (but not to the point of exceeding PV output), while battery discharging looks like reducing the household load by up to 1200 VA (but not beyond zero).

And why did I buy a battery (almost)? Well, ignoring the fact that I have a battery but no invoice so far, I won’t have to pay the full installed price because of the subsidy provided by the trial so I’ve arguably only bought some of the battery.

Why I didn’t buy .. a battery

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This week for the first time a salesman ventured into my home to sell me a battery for my solar PV system. His company were not the first to discuss it on the phone, or make an appointment, but this guy actually turned up which is a first.

The principal of such a battery is straightforward – there will (often) be times when you generate more electricity than you can use immediately, so why not store it for later use?

imageFirstly, let’s think about the architecture of the system that was being presented. This system puts a battery and a charge controller on the DC side of the inverter i.e. between the panels and the inverter. This reduces consumption on the generation meter when you charge the battery, but registers on the meter when the battery is discharged – not in itself a problem just a difference.

This architecture creates a system in which the battery can only charge in daylight and only discharge in darkness but to my way of thinking that’s less efficient than it could be. Firstly during my day there are often load peaks that go beyond what the panels can cover. Events like boiling a kettle, or the heating periods on the dishwasher or washing machine often push electricity demand beyond the immediate panel output, and you you might hope that the battery would cover these peaks and then recharge later – but no this system cannot discharge the battery during daylight so you’d end up buying electricity for these peaks. No discharge in daylight also means that activities like cooking in summer evenings on our electric oven and hob would be on imported electricity (even though the battery has sufficient energy) because it’s still daylight. Finally my Economy 7 meter tells me that I regularly don’t use as much as 4kWh overnight but to get pay back you want to empty and refill the battery as many times as possible.

Of course I could shift electrical load to the nighttime to empty the battery, but that’s contrary to trying to move load to the daytime to maximise use of PV directly. Which is better? Hard for the user to tell since no tools are provided to tell the user how much energy is in the battery, or any history of usage. By contrast my immerSUN produces detailed graphs of usage to show you what it’s achieving on my behalf. If this can be done with one device, why not also with a battery costing 20 times more?

How can I make sure the battery is empty in the morning when my normal night time usage would only be 1-2 kWh? Well you might consider running a heater to warm up a critical room or rooms at the start of the day (once you’ve got the equipment the energy is free of course), or in my case I might want to top up the electric car, is there a switched output or other mechanism to signal when the battery is exhausted and switch off such a device? No there isn’t.

So it can be hard to ensure that the battery is empty. What about filling it, how easy is that? Remember that the proposed battery has 4 kWh working capacity and sits on the DC cables between the panels and the inverter. Well, my 4 kW PV system like many is actually 2 x 2 kW systems feeding dual input inverter. That means that only half my panels would feed the battery so to fill the battery I’d need to generate at least 8 kWh in a day (and not use much of that during the day as immediate usage is prioritised over battery charging). My average daily generation between November and February is well below 8 kWh so there are many days in which the battery wouldn’t be charged fully (and may be not at all).

Then there’s the kicker – the price. If I look at a battery with 5,000 cycles life (that’s over 10 years although the brochure also quotes 10 years as the life) where each cycle is 4 kWh (which it will only be when new) that’s a lifetime throughput of 20,000 kWh. Electricity at today’s money is about 12 p/kWh so those 20,000 kWh are worth roughly £2,400 in savings. And the price of the unit? More than double that.

I did a detailed analysis including my actual ability to charge the battery based on real daily generation and usage, declining battery capacity with age and use (down to 80% of original capacity after 5,000 cycles), and energy price inflation; and concluded that energy price inflation would need to be around 30% annually each year just to get my money back over 10 years – without actually generating a return on the investment. While I do expect energy prices to rise faster than inflation in the long term I think 30% annually is unrealistic.

In the next few years I expect battery prices to fall as more cells are made for other purposes like electric cars, and also as used cells/batteries become available from electric cars at the end of their lives. I also expect revenue opportunities to arise through grid stabilisation and support where some access to the domestic battery is made available to support the local network.

However a battery on the DC side of the invertor which only charges in daylight and only discharges in darkness will be of little use to support the grid. I think a battery needs to be on the AC side of the inventor to flexibly interact with the house and grid based on charging to eliminate export and discharging to eliminate import in microcycles (unless the grid overrides that).

So I do expect to get battery storage eventually – just not now, not at this price, and not this solution.