Having completed a first winter with a Samsung HTQ 8kW straight oil boiler swap out running at 35 to 50 deg C maximum (no MCS grant was asked for or would have been given for my situation) I have achieved the comfort level required and now looking at the economics.
I realised early on that the tariff and battery for load shedding has a much higher effect on HP running costs than trying to reduce SCOP by increasing the existing radiators (an 8% improvement at most)
In October I changed to cosy octopus with a 12/24/36p per kWh model which follows the daily energy cost profile like agile does but is fixed so you can plan load shedding in advance. I worked out that the optimum battery size needed was 6kWh usable (7.2 installed) this bridges the evening 3 hour peak with 2 hours battery and one hour temperature setback. We still keep warm when energy costs are at the highest. (100p on agile for many days in January)
Charging the battery on the other two cheap rates as well results in an average winter cost of 16p kWh or 4.5p kWh for space heating which means my heating costs have reduced by around 50% over oil. A reduction worth ÂŁ 12,500 over the next 20 years and 75 tonnes of CO2 saved.
For my reasonably insulated victorian house with old radiators running at the higher 50C temperature on only the coldest of days, the annual costs for energy is less than a property running on mains gas.
The load shedding batteries make a big difference to the grid infrastructure.
Are my calculations correct? My electricity bill confirms it is so far.
It is a shame the MCS grant scheme cannot include a category for this type of boiler swap installation where the radiator emitters maybe less than perfect for various reasons and an indirect hot water storage tank is already available so no plant room. Much less labour and equipment cost -just the heat pump installation - I have done it and it works.
Surely adding in this approach to the great Heat Pump rollout would achieve the governments net zero targets faster and more cheaply. (hornets nest now torn open!)
I’ve not got batteries yet but have been running some calculations based on my historical (pre and post heatpump) PV export and grid import figures on my emonpi. My PV array is only 2kW due to limited roof space (the rest is traditional derbyshire “grey slate” which I’m not having anyone clumping about on), so this is a minor component.
In my analysis there are 2 factors to consider when sizing battery storage: the maximum working power in/out of the inverter and the battery capacity. For my case, a 3.6kW power inverter and something like 8kWh of usable battery capacity should bring the import at the Cosy Octopus peak times down to ~0, and only a few ÂŁ per month of standard rate import. And that is without holding off heating while cooking. I used a ballpark 80% efficiency figure for AC->DC storage->AC load. I should add that we are quite frugal with the heating.
So yes, although I’m not “live” yet, my calcs seem to agree with your calcs + experience and, with the way battery prices have gone over the last 2 years, the payback period looks to be in the 5-6 year realm.
FWIW, I’m planning to get one of the Fogstar SEPLOS kits which give a bit over 14kWh raw battery capacity. That should be able to cope with load shifting the heat pump in winter + bringing summer PV export to ~0, while allowing for battery degradation, and probably avoiding the need to use a set-back to eke the battery energy during the evening peak/standard rate.
Thanks Adam, It confirms my approach that a 5 to 10 kWh storage has the greatest payback due to the the three battery cycles that you can utilize a day with the cosy tariff. That is the main target for my ROI. The savings you get with solar is a different case but the need to strive for zero import is less important financially when you have three cheap rates spread out over the day. Batteries can be very expensive and resource hungry and that puts people off but even a 7kWh system, provided it can charge and discharge at 3kW, is a great investment for the cosy tariff user and the national grid network.
Yes, the 3 cycles of Cosy is a boon to keeping the capacity down. WAY better than the EV-targetted single (but cheaper) period. Maybe I should anticipate having an EV too?
I too will get an EV in due course but to put it in perspective the average user at 6000 miles a year needs a top up of only 5kWh a day. Of course there are times when a larger top up is required but it is not likely to be worth buying more battery capacity for. The single EV super low rate may seem attractive but it is the winter space heating case that rules the annual budget.
It will depend on the individuals usage case of course.
Another comment for anyone who might discover this thread while musing about the same tech.
One thing which does attract me about some inverter models is the availability of an off-grid backup output. With a proper change-over switch (consult an electrician!) it would be possible to supply the heat pump during a power outage. Given that I, as most people I believe, have frost protection valves fitted, the possibility to keep the HP ticking over and without spilling itself on the ground during a power cut during a cold spell is worth a few quid too.
And an aside: a product which should exist is a HP with integrated LiPO4 battery and inverter to make the most of discount elec periods, and an automatic transfer to off-grid battery power for frost protection in the event of a power outage.
Interesting you should mention backup for power outage. The difficulty with using the inverter UPS as a backup supply directly to the HP is that it can draw very high current when a start up or frost protection is activated. We get power cuts regularly where we live and to get around this on my Solis 3.6 inverter (which comes with a UPS output as standard ) I am always ready to plug just the circulation pump into the UPS on an extension lead. This is enough to reduce freezing of the heat exchanger in extended power cuts and it only draws 60W so it should cover a long outage. It is something that could be automatically operated. This assumes you can keep your house warm by other means or just the stored heat of the building may be enough in most cases.
I use plain “calling for heat” (with some home-brew controller hardware/software), rather than madoka control, so it would be easy to set up just freeze protection circulation, and the 12kW log burner should keep the inside warm. I also use the “power limitation” on my Daikin, so can in principle limit the running to say 700W, although whether the inrush current on the compressor would be a killer?.. I would have thought that these inverter compressor heat pumps would not be as susceptible to large start currents as conventional armature motors (I did once kill a small petrol generator trying to run my freezer off it, having naively compared the running current to the generator max rated output). Correct me if I’m wrong on that point.
I found this article quite helpful. https://www.renewablewise.com/what-size-generator-to-run-heat-pump/
Specifically it states that “For inverter heat pumps, also known as variable speed heat pumps, the starting wattage will not exceed the running wattage range.” which suggests that you are absolutely correct when you say
The risk depend on the location of your home. In 15 years we have not had an electric cut lasting more then two hours and as we are not connected to overhead lines below 33kv, we get very few electricity cuts. My parents had a few electricity cut that lasted a day or more due to having a overhead line connection.
The outside temperature will need to be very cold to activate frost production values in two hours.
However for a holiday home having the backup to the heatpump would be useful as I would not be on site to sort out the heatpump.
Can a heatpump be easily told that it must only use the backup power for freeze protection?