Battery prices continue to fall so their time will come and with enhanced export rates from ‘Axle’ it’s becoming tempting. If you are installing solar now, you’ll have to choose between micro-inverters, a direct to AC inverter or a battery based inverter/charger. For what it’s worth I’d go for Enphase micro-inverters and leave batteries for later.
Why not batteries now? I suspect these sums will seal the debate.
Let’s work around a hypothetical 10kWh battery costing £3,000. We’re assuming no losses so everything will be worse than shown by this basic maths.
Solar storage
Base line, use the free battery provided by the grid; export for 12p and buy back for 25.5p Here the free battery is charging a holding cost 13.5p per kWh. Think of it as a rental charge for an infinitely large battery that holds your inputs for as long as you like. Zero capital cost to you though so £3,000 in pocket; stick that in premium bonds for 10 years instead? Whatever you do, your battery project has to compare as an improvement over the virtual battery that is ready to use whenever you like.
Solar + Battery; input free, profit £25.5p so 12p better than the free grid battery solution. For 10kWh that’s £1.20 saving per day, so payback takes just under 7 years of surplus energy every perfect sunny day. So, in reality, about 15 years then. Is that fair? What about storing free energy and getting 25.5p worth later for a 3 year payback. Nice idea but that assumes 10kWh every day of the year, even the 200 days of winter. You’d also be directly consuming some of your solar production (practically all of it in winter) so you can more than double this estimate. 7 years? Possible but unlikely. And, sorry to rub this in, there’s still the grid battery available to you so these optimistic sums are based on a fragile platform.
Other interesting ideas
Off-peak storage sold to grid – 7.5p in 12p out – profit of 4.5p. For 10kwh that’s 45p/day. To pay back£3,000 would take 18 years but then more really because the batteries would die before that.
Off-peak storage used next day – 7.5p in 25.5p worth out – a saving over the grid of 18p. For 10kwh that’s £1.80/day. To pay back £3,000 would take 5 years. After that, the next 5 years drops £3,000 into your pocket but then the batteries need changing so it’s gone again. Close! But not overly compelling.
A bit of solar and a bit of off-peak
There’s a seasonal split here for sure. In Summer there is little demand for electricity and in practice, even with no battery at all, bills are pretty much matched by exports. In winter there is significantly reduced solar so the story is all about off-peak storage and we’ve covered that above.
Unfortunately, we have failed yet again to make a compelling case for battery storage. If that leaves you with £3,000 burning a hole in your pocket just consider that the payback time on solar panels is hovering around 3 years so make sure you have enough of them. Your money will do much more for you here.
Here’s a rough example for PV evaluation. Scale up to suit.
£1,400 buys 4 panels with microinverters. (Includes £500 for fitting)
Total 2000Wp
This makes 1,600 kWhrs a year
That’s worth £400
Payback is 3.5 years.
So, there you go. That £3,000 would buy you a nice solar rig and not a battery in sight.
Plugging in the new April price caps and prevailing war driven spikes this is roughly how many kW hours £1,000 buys you. Gas and electricity will be fixed until July but oil and LPG will certainly remain volatile. Oil, for example, has shot up to £1.31 a litre (good chart on Boilerjuice) so if you are living in a big old leaky house, like the red one below, your heating bill will be over £4,000. Extrapolating from the bar of your particular fuel will give a good bills prediction with the orange house giving targets for most people using around 25,000 kW hours a year. As usual, at the extremes, a direct electric resistance heater is the worst thing to turn on (even half as good as oil) while the best strategy is to harvest cheap rate energy with a heat pump and store it in a tank for later use. While that heat pump trumps everything it is notable that off-peak electricity is currently better than oil or LPG; maybe night storage heaters are due for a comeback.
Trying to find a filling station that had any diesel left at all was a bit of a wake-up call; an electric car, with attendant cheap night rates, could lower heating and fuel costs while giving some protection from energy crises. Bi-directional chargers? Still ‘on the way’ but could be just months now so choose your car with care.
If these prices persist into next winter the Government will no longer be able to support the price cap so expect some unpleasant changes.
If this sudden price shock is prompting you to take some action you might take a look at ‘Absolute ultimate heat pump system’ where there may be one or two useful ideas for your heating strategy.
Meanwhile, solar panel prices continue to fall and make even more sense. They pair especially well with a mini-split heat pump which is not only cheap to install (£2,000 ish) but gives a welcome addition of air conditioning during these hot summers.
Cheaper batteries are worth considering although payback times still look long. While export rates are falling the idea of giving access to your battery when the grid is stressed is looking tempting. Check out ‘Axle’ who will pay you £1 for every kW hour they take. For that money they can hammer your battery as much as they like. N.B. Check out their compatible inverter/chargers before you install your PV.
High temperature heat pumps are a game changer. How about a sub £500 heating bill for the whole winter? Higher temperatures present a new opportunity to store cheap energy in a heat store (a large buffer tank).
Off-peak electricity costs a third of the day rate and that allows heat pump energy multiplication to produce unbelievably low-cost heating (like 2.5p/kW.hr). A heat store tank can time shift the night rates to the following day with that energy simply dispensed to underfloor heating and fan-coils by blending it back down to the required temperatures. With energy this cheap the inefficiencies of large buffer tanks and blending are hardly relevant.
Large tanks are better for high temperature heat pumps which don’t like short cycling and prefer a large volume of water to chew on so that’s another win.
For example: Over 5 hours a 12kW heat pump can place 60kW.hrs in 1,476 litres of water raised from 30c to 65c. As the model lower down shows, that costs just £300 over 200 days of winter. Similarly, a 1,000 litre tank, plus a large hot water cylinder and a heated floor slab make a good combination to store this night time bounty ready for the next day.
Cheaper than a battery
A 1,000 litre tank can store 40kW hours and costs about £1,500. A battery that can make 40kW.hrs (via a heat pump) would cost twice as much and could wear out after 10 years. Water lasts forever so works well as a storage medium; it doesn’t even ignite and burn your house down. Having said that, batteries are still very useful and will become strategically important when you access the very large one in your car. The battery is the top performer in the chart below because the heat pump will run a better COP during the day.
N.B The 7.5p off-peak rate is for electric cars but economy 7 is a workable alternative.
A much bigger heat pump
In order to bank the entire heating load over just a few night hours the heat pump will have to be significantly bigger than normal. 12kW (delivered) is a good place to start especially if only single-phase electricity is available. The Samsung EHS Gen7 R290 12kW is a good example.
Additional power
The calculated heat loss of the house must be matched by the heat pump power (to get the grant) and, on paper, a big heat pump will easily meet that requirement but the stored energy may not always meet the actual demands of the day. Cheap power is time limited rather than power limited.
If there is the occasional shortfall the heat pump is still there to give a powerful boost on that exceptionally cold day – however there is a better way.
Part of a sensible strategy is to have another smaller heat pump (like a mini-split air to air) that runs mainly free off solar panels. Obviously this helps to keep the electricity costs down but also gives the option of topping up when necessary at minimal cost. The blown air of a mini-split is immensly useful for laundry drying and the cooling feature gets the air-con requirement largely sorted too. Forget the daft headline costs of regular heat pumps – mini-splits can be bought for around £600 and you’d get one fully installed for under £2,000. With a typical consumption of around 1kW the mini-split will nearly always run free off the panels whereas the big heat pump would cost precious pennies to fire up.
Domestic hot water
To store off peak energy the 300 litre domestic hot water cylinder has standard 3msq coils for heat pump use. It may require a trickle charge during the day and instead of repeatedly firing up the heat pump an EDDI solar diverter tops it up via a Willis remote immersion heater (better for stratification and servicing if you were wondering). The EDDI captures even tiny excesses throughout the day to produce a higher temperature and the lost exports are about equal to the cost of running the heat pump.
System benefits
Water is usually hot enough to make towel rails work properly.
A large domestic hot water cylinder needs fewer daytime top ups if any.
The big heat pump gives faster DHW recovery times (if needed).
Fan-coils take from the hotter level of the heat bank.
Fan-coils can tolerate high temperatures so a boost mode is possible. Eg. 5 mins of full power on startup to give fantastic response times.
Heating can be zoned without complications.
Off-peak electricity is mainly renewable so this system is as green as Kermit.
Daily characteristics
In the morning the tanks will be hot and ready. The bathrooms will be toasty with the towel rails on full pelt and the floors heated. The bedroom fan-coils instantly transform a cold room into a warm one and there’s loads of hot water for showers. Downstairs the floors are already warm.
The towel rails stay on for a while but all else takes a rest until the floor slab downstairs calls for a top up from the big tank. With a bit of sunlight the EDDI starts to reheat the hot water tank and the mini-split might be able to run free.
At this point the big heat pump has not run at all, at any time, during the day. However, by the evening the temperature is getting low in the big tank, the floors can continue to keep warm even with water down to 25c but the fan-coils may need a last-minute boost from the big heat pump. UFH in bathrooms is a good counter to the possible end of day shortfall. Note that this is all fine tuning to avoid costs; if more heat is required then the system can perform just like any other and in fact even better because it’s so powerful.
Controls
Everything is much simplified with no interlinking of controls.
The heat pump is allowed windows of operation on a timer and tank thermostats.
The night time set point would be 67C and 45C for the day – settable on the heat pump.
The UFH is timed and controlled by a programmable thermostat – an ESBE mixer adds weather compensation.
Fan-coils (OriginalTwist DIY units of course) are timed and thermostatically controlled locally.
Simplicity
There are many options to add sophistication to this system; weather compensation, ESBE electronic temperature reducers, blending down for fan-coils, fan-coils boost feature, air source GSHP boosting (as per the OriginalTwist hybrid concept), batteries.
All should be weighed by the PV test: would the money be better spent on more PV panels?
ZERO COST HEATING
As you increase the size of a solar array so the energy bills fall and the export tally rises. For some well insulated houses the income will pay the electricity bills – ZERO COST heating has just become easier.
From the chart below you can work out how many kW.hrs your current system requires over the winter. Just compare each £1,000 bar with your own bills. 25 -30,000 kW.hrs would be normal. Ultra insulated homes are nearer to 15,000 and only about a third of the way up a £1,000 bar. Note how the off-peak system is easily twice as good as anything else. Want a laugh? Compare direct electric resistance heating with the off peak system.
Just how cheap for 200 days of winter?
The Original Twist model of this entire system looks something like this:
A 12kW ASHP running for 300 minutes a night charges a 1,000 litre tank, a 300 litre hot water cylinder and the floor slab.
Over 200 days of winter this makes 12,000kW.hrs for £300. No, that’s not a mistake, £300 for 200 days!
The 3kW mini-split adds 1,000 kW.hrs for £125 (half from PV).
Total cost for 13,000 kW.hrs is £425 and less with large PV.
The same energy would cost £570 more with conventional daytime heating.
You’ll see on the chart below how night time use of an ASHP (blue line) is particularly disadvantageous compared to a GSHP (black line) which doesn’t really see any significant changes in source temperatures. From that point of view it might seem that this system will be optimised by using a GSHP for the night runs and an ASHP for the day time back up.
The GSHP will not suffer the defrosting cycles which will eat up into the precious off-peak time and the COP will be excellent particularly if the Originaltwist air/ground hybrid idea is incorporated..
You won’t find an R290 high temperature GSHP: that’s because they are fitted indoors and spilled R290 is toxic. The 10kW IVT E11 GSHP might be a contender though. The maximum temperature is lower though so you’d need more water storage; probably a pair of tanks.
Conclusion
This idea can be scaled and tweaked to suit individual requirements but the underlying message is convincing.
Off-peak electricity can be time shifted with large amounts of water to dramatically cut heating costs.
A concrete slab floor also stores heat usefully so a boost at the last hours of off-peak could be part of the mix.
N.B. If you want the grant, with it’s strict rules, you have to ‘design’ the system for normal efficient heat pump systems and then run it ‘inefficiently’ at night when all the techies have paid up and gone.
Note that a heat pump costs about £5,000 so when you are quoted £15,000, guess who really gets the grant money!
Original Twist 