Solar Panels and Battery and Heat Pump Info Dump M0YNG.uk

Created 2024-03-24

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I got solar panels, and a battery, and a heat pump! Be prepared for an info dump.

I've been writing this blog post for a month now, so if there are things missing or questions unanswered ... sorry? Maybe I'll do an update next year.

What do I have, what is it?

It makes and stores electricity, the battery can be charged from and discharge to the grid.

Panels are West facing in Gloucester, UK.

No "Isolation" from grid. Grid goes offline, we go offline.

It's an Air Conditioner, running backwards, and dumping heat into either hot water, or hot water ... for radiators. Some new radiators needed as HP works at lower temperatures than gas boiler - lower delta between radiator and air means increased surface area needed to transfer the same heat.

Why?

Would prefer to avoid murdering the planet.

Cost, I guess? Having our own generation insulates us from the cost (a bit) and heat pump is more efficient than gas boiler.

Getting it

Checking my privilege

Lucky to be able to do this.

Solar

The panels are only on the West roof, we did get a quote including the East roof but it would have been disproportionately more money for little (although not nothing!) extra generation. It wasn't in budget at the time, but maybe we can add more later.

Total cost of install about £11k.

It was installed in October 2023, so I only have about 5 months experience with this, and all of that was in the "low" season.

Heat Pump

Total cost £10,500 - but government "Boiler Upgrade Scheme" paid £7,500 of that = total "out of pocket" £3000 (aka the cost of a new boiler.)

The pump was installed in Mid January 2024, at this point we've had it less than 2 months, although they are the coldest months of the year with my garden temperature not getting above 15 and averaging around 10.

(Note: the BUS is only available for heating, if you want a system that can also cool you in summer - that isn't eligible. Or so I'm told. Although good luck cooling with convection heaters (aka "radiators".))

Living With it

Heat Pump

Boring.

It just works.

Seriously, I barely think about it.

The only reason I do think about it is because it uses electricity and I'm obsessed with optimising the solar + battery usage.

Rooms are warm, water is hot, it blows freezing air even when the air is already freezing.

We've got the thermostat set to 19°C, but it's by the front door and a bit colder than average, which keeps the whole house a comfortable 20°C ish.

One downside of having the hot water tank in the garage is the hot water is now further away from the taps, so you have to wait a bit longer for it to come through. Previously the tank was in the geographic center of the house so there wasn't much distance to travel for the hot water, but now it has to come from the back of the garage, under the floor of the study, and the landing, before it gets to the starting point of the old tank. However, I'm assured that water costs less than electricity and the reduction in efficiency would be significant enough that this works out the better solution.

Sometimes the hot water runs out, unlike the old gas boiler that would wait for the next scheduled time to make more the heat pump will kick in and get heating. This is good, because we have more hot water, but it's bad if we're running low on stored power and it means we draw expensive power from the grid.

The water is set to the higher of the two default presets, due to the longer run, and we could probably save a bit / improve the efficiency by using the lower setting. Maybe in the summer!

Does it work when it's cold? Yes. Obviously. The week we had it installed the canal froze over. We had the hot water enabled first and we could have hot showers that same day, whilst it was cold enough for the canal to freeze from side to side. We had the house heating turned on in the last afternoon and were back to normal internal temperatures that same evening.

Lifetime COP (Coefficient Of Performance) is 3 (so far), for heating and water. For every 1kWh of electricity put in it has pumped 3kWh of heat. Hot water is less efficient than heating - but the combined total is 3.

I've done the maths and it is costing less to run than a gas boiler.

Looking at a bill from December, we used 443.7kWh of gas, at 6.49p/kWh for a total of £28.81. At 90% efficiency (an A-Rated boiler) that's 399.33kWh of heat output. At COP of 3 the Heat Pump would use 133.11kWh of electricity to do the same. We're paying 17p/kWh (on "Cozy Octopus") or £22.63.

Heck, even if the gas boiler was 100% efficient the heat pumt would cost £25.14 for the same heat output. Which is still less than gas!

Admittedly we did use some gas for cooking, but it will not be £6 worth.

We've also scrapped the gas hob and installed induction. We're completely off the gas grid. Not paying the standing charge will save over £95 a year, and nearly £115 with the new price cap come April. https://www.ofgem.gov.uk/energy-price-cap

Stop Press: The April prices have been announced. Gas will be 6p and "cozy" electric will be 11.676p, which is less than 2 times the unit cost of gas. That same example would be £26.62 as gas and £15.54 with the heat pump.

Not to mention we're now not putting all the combustion by-products of gas into the kitchen when we cook.

And on a sunny afternoon we pay £0. Yes, this does happen, even in February. It's worth a reminder that the Heat Pump and Solar are different systems, we didn't get one to support the other. They do overlap and help each other (well mostly the solar helps the pump!) but running costs etc. should be considered in isolation because yes, solar won't produce enough to run the pump when we most need it in winter.

The financial break even point on this is negative, it's already about the same as buying a new boiler and the running costs are less. Admittedly we didn't NEED a new boiler...

The climate break even point is harder to know, but still encouraging. I asked Octopus sales, and the installation team, and the manufacturer, none can tell me the embedded emissions of manufacturing this system and installing it. HOWEVER, given the worst common situation where most of the electricity on the grid is generated by gas - we're still emitting less CO2 by using that electricity to pump heat rather than burn it for heat. I don't know exact numbers, but the losses in the electric system would have to be massive to counteract a 300% efficiency. Plus, we're with Octopus so (at least in theory) 100% of our electricity is renewable, and when the sun shines it's our own solar being used.

I'm a massive nerd so you might expect me to have connected this to my Home Assistant system, but I haven't. The options are all bad, there is no open source integration because the manufacturer has an NDA (Non-Disclosure Agreement) before you get access to the API, so it's impossible to do an open source anything with it. Plus the app has options for logging in with Facebook ... and fuck that, if you're going to even give that as an option I do not trust you with my home heating.

However, I have got a clamp energy monitor on the power supply so I can at least monitor the power separately from the rest of the house, in Home Assistant.

The control panel in the garage does give some data, for example kWh used, kWh heat generated, etc. which is where the figures here have come from.

Checking my privilege again

Given that 9.6 million households in the UK are living on incomes below the minimum income standard and in poorly insulated homes.

And these households have most to gain from more efficient heating.

But the government grant is only available to homes with a valid energy performance certificate for the property which does not recommend installing loft or cavity wall insulation.

And it's not just heating for comfort.

The Building Research Establishment Group (BRE) has calculated that the very worst housing in England – approximately 720,000 homes with a category one hazard of excess cold (typically insulated to bands F or G) cost the NHS £0.5 billion per year in first-year treatment costs alone.

That's probably more than a million people (assuming more than one person lives in a home, on average) who are so cold they end up needing medical treatment.

We urgently need to insulate Britain and roll out heat pumps.

Solar + Battery

I am a massive nerd and obsess over this ... a lot. I suspect that if I'd lived in the past I'd have been the one obsessing over having enough fire wood, and water, etc. for the village.

There are a few different ways to live with solar and storage, one is to basically ignore it and carry on as normal. This is somewhat pointless as you'll inevitably end up using power at different times to when it's being generated. This is mitigated somewhat by having storage, now you can store power for use later, especially important for things that can't really move about much, like cooking dinner. Some care is needed for this to work - the inverter can only provide 3.6kW, which is plenty to boil the kettle, run the oven, dishwasher, etc. but not at the same time and this is where you can start drawing from the grid for example if you boil the kettle whilst cooking, or the tumble dryer at the same time as the washing machine. I've started to get cleaver with this and monitor the energy draw of the washing machine as once it has heated the water it doesn't use much power, then I can turn the tumble dryer on and run them at the same time. It's risky, there isn't any overhead if someone turns on the toaster!

In some ways this is the least impressive thing because I really enjoy the days when it's overcast, even raining, and we're generating more electricity than the house is using. It's great to sit at my desk with music playing, lights on, computers running, and be doing it all from solar. In the rain. In December.

Most of the time we don't worry much about this, and with some care to make the most of the generated solar and charge when it's cheap, and not overdo things when cooking dinner, it's just like a normal house. But with the nice realisation that you're watching TV at night on the electric you generated that afternoon.

The battery has a 9.6kWh capacity, so enough to run the house for 24 hours, although admittedly not with the heat pump! Although in the summer I expect we'll be able to operate without using the grid at all, we've already done in multiple days in October!

It's really intuitive, if the suns out we're making power. It's even possible to have a sense of exactly how much and if it will last, the brighter it is the more we're making, and the cloudier it is or if the weather is blowing in we'll probably not have it for long. Clouds make a real difference, even small fluffy ones can drop generation from 2.2kW to 400w whilst they pass.

Because the panels are all West facing there is a significant delay in the mornings before we start generating the big watts, today (3rd March 2024) generation started around 7am, and by 10am it was still only making 275w. But then the sun was in a position to illuminate the panels directly and generation shot up to over 2.2kW by 12. It will stay high until late afternoon / evening (weather permitting) until it falls off a cliff when the sun goes behind some tall trees across the road. This is noticeable in winter, but a near vertical drop in summer.

Data and control of Solar + Battery

The battery and inverter are made by GivEnergy, I was made a cloud account for the system when it was installed (as in, the engineer handed me my username and password!) but it also has complete local control via the GivTCP software / Home Assistant integration. As a result I have a lot of data and full control of the system.

I also use Forecast.Solar to get predictions of generation which helps me plan usage and charging, especially when combined with the Unofficial Octopus Energy integration.

I have this data almost constantly visible, I've curated a dashboard on Home Assistant which gives me quite a lot of detail, including power flows to/from the solar, battery, grid, house, and heat pump. Forecast generation, "running totals" of generation, usage, import, export, tariff, cost, grid carbon, "benefit" (saving in money/carbon) etc.

I've got an e-ink display on my desk than updates every minute with key bits (plus other house stuff like temperature) and I've also got some widgets on my phone home screen so I can pretty much always know power going to/from the grid, solar, and battery, plus battery charge level, grid carbon, top grid fuel, and outside temperature.

One of my favourite (but also I have complicated emotions about it) is the "cost per kWh generation". It's a simple "sensor" which divides the total system cost by how many kWh we've generated - so far every single kWh of solar generation has cost £18.82 which is quite a lot! This is a kind of "break-even-o-meter" as once this gets to the same cost per kWh as buying from the grid, we've broken even.

I've also got a bunch of notifications that are sent over Matrix.

This one is sent at the start of "cheap times"

Electric import is about to be £0.168739 Target charge 72% (currently 40%, 4.7 kWh solar left today) and/or forcast generation in the next hour is 1.5 kWh starting at 13:00:00 and ending 16:00:00

At the end of the (solar) day we'll get one like this

☀️ production today 13.5kWh 🔋 battery is 88% charged. 🔮 Forcast generation tomorrow 5.897kWh

At midnight the daily summary will be delivered (the numbers often don't add up, I blame rounding)

Saved £3.13 and 3085.34gCO2 today by using the battery (£2.12 | 8.6kWh | 1577.60gCO2) and solar (£1.30 | 11.0kWh | 1612.31gCO2), plus exported 2.5kWh.

Making Money with the Battery

Yeah, really.

Well, kind of.

For a couple of months we were on Octopus' "Intelligent Flux" tariff which takes control of the battery and charges it overnight when electricity is cheap / carbon is low, and then tops it off during the day using solar or the grid. It then pays extra for your exports during peak time, by about 10p per kWh. With a 9kWh battery you can in theory make 90p per day, just by charging and discharging the battery. In reality the charging algorithm they use isn't great, and it never discharged as much or as fast as I thought it could, and I never managed to get someone to talk to me about it.

There are other tariffs that are "manual" but also work in a similar way.

Exporting usually pays about 15p per kWh, which is a lot less than buying it costs, so it's not usually worth charging and then discharging it later on normal tariffs. However, Saving Sessions / Demand Flexibility are times it can be worth it, especially if you've got some solar in the store. I've got my system set to automatically dump the battery to the grid at full power during these sessions. We don't usually draw from the grid during peak times any more but it still gives us points as negative "use" is still a "reduction" on nothing!

We're now on the "cozy" tariff which is specifically for Heat Pump users, it charges the normal amount most of the day, with two 3 hour periods of 40% cheaper power for running the heat hump. BUT we also charge the battery in this time, which means we're saving 40% all day long. This is especially useful because there is also a peak period which costs more, during the early evening. Cooking off battery is saving us paying 45p per kWh during that time! My rudimentary sensor in Home Assistant says we've saved about £35 per month by charging when cheap and running off battery the rest of the day, although admittedly some of that power was from the solar - not much though as this was late January to late February. During the same period we saved nearly £19 by using solar generation straight into the house.

We could possibly save more by using the "Agile" tariff, which changes per half hour. But I've not done anything to work out for sure.

Other suppliers have tariffs that vary by time of day and/or grid capacity so I'm pretty sure most people could save money with just a battery. Even the old Economy 7 tariffs would work here as you could charge the battery up over night and reliably have power during the day.

Stop Press: The new prices for April have been announced and we'll be paid more to export (15p) than import during the "cozy" times (11.676p) so this completely changes the calculations on what's "worth" exporting (everything, all the time.)

Return / Break Even

The financial return on this system depends a lot on how we use it. During planning the prediction was we would generate about 53% of our yearly usage, which doesn't sound great. But cutting an annual bill in half doesn't sound like a bad idea! Our average usage in February was 17.4kWh per day (including the heat pump), and the estimated generation in summer is 17kWh so I suspect we'll be exporting quite a lot too.

17.4kWh per day for a year is 4623.8, at our cozy rate we're looking at a worst case total year cost of £786 (excluding standing charge.)

The design pack predicts 3627kWh generation per year, which is 78.4% of that figure. Or about £616 of electricity we're not paying for every year.

On average days we use up to 10kWh excluding the heat pump, meaning we could be exporting 7kWh a day in summer, netting an amazing £1.19 per day! That's £71 over 60 days of peak generation!

One issue is that peak production is during summer and peak use is winter, BUT Octopus pay us a fixed 15p / kWh, and we pay 17p / kWh on the cozy times, so on average, we're paying 2p / kWh.

Design pack predicts a break even point of 6 years 6 months. But we'll have to see how this adjusts based on our real world usage.

The battery has a guaranteed 10 year life, and the solar panels are predicted to last 25 years before performance degrades below 80%. Even then I expect the battery will keep working, and could be replaced and/or upgraded at much reduced cost in 10 years, and even 80% efficiency isn't that bad. Point being, all of this is way past that 6.5 year prediction.

But we didn't do this for the money, although it is nice to have lower and more predictable expenses.

Like the Heat Pump, it's impossible to know the embedded environmental cost of the system. The panels were made in China and no one knows anything about this process. We even had a visit from Octopus France who are starting to supply solar soon and wanted to see how it was being done here, and I was being told that in France they have a legal requirement to know the whole like carbon cost, and the Chinese manufacturing is causing problems because no one knows! Then there is the issues of lithium cells and the significant issues of the mining of the raw materials used in them. All I can really do here is trust that Octopus have done the work and the entire supply chain is slavery free, low carbon etc. GivEnergy claim the battery is "Ethically sourced and cobalt-free." I did ask, and it's the same story as the Heat Pump, no one knows.

In the last week (in February 2024) we've avoided 4kg of CO2 emissions by using solar directly rather than from the grid. Or 7.2 seconds of Private Jet flight. (See my post on High Draw and Low Carbon Automation for the details of that maths.) At time of writing we've generated 600kWh, which would have prevented 145kg of CO2 emissions at the current generation cost of 234gCO2/kWh. It's not much, but it's part of my rebellion against extinction.

Checking my privilege again again

There was zero chance of my affording this system on my own. We could only do it due to inheritance from my Granddad. I think he's be fascinated by the technology (he was always keen to show me his new computer, laptop, gadget, etc.) and the knowledge that he's helping provide our power - just like he did when he delivered coal back when he was younger than I am now.

I'm also very aware that I live in and own a house we can modify like this, that has a roof suitable, and space in the garage. We're (disappointingly, everyone deserves somewhere affordable and safe to live) very fortunate to be in this position.

Optimising Battery Charge

We're in the most difficult period of the year for optimising this (writing in February/March), as the solar generation isn't very predictable, but it also isn't nothing.

The charging is a bit stupid, if the battery is in "eco" mode any solar we don't use immediately in the house goes to the battery, and if it's full, the spare goes to the grid. But if it's in charge mode it draws from the solar AND the grid, up to a set target. Once it reaches the target it will stop adding more to the battery, even if there is capacity and the solar is generating.

SO, I want to maximise the solar we capture, but also not end up running from the grid when it's expensive. I want to set that target to shove as much solar as possible in there, and top it off with cheap grid, even though we often have to grid charge first. But doesn't put so much grid power in there we run out of storage and send some back to the grid.

This is my current "algorithm" that runs at 59 minutes past every hour.

[100  - (float(states.sensor.energy_production_today_remaining.state) / 9.6 * 90)
      + (float(states.sensor.energy_next_hour.state) / 9.6 * 100)
      | round,
      20] | max

What?

In tandem I have a script which looks at the next period, and if the cost of import is low (below 20p) it will schedule the charge at that cheap time. But it also checks if the current charge is lower than the target, because if it is we don't want to enable "charging" because the solar will go into the grid not the battery. Which is pointless. But it also checks the forecast generation for the hour, if that's very low or nothing we'll still enable "charging" even if it means we just run from the grid for that time, because any high loads (i.e. the heat pump) will quickly drain the battery leaving us with nothing for when the price goes back up.

It's not perfect and I suspect that this is a situation where some sort of self-adjusting model would work well, especially if it can also consider the average or likely demand for power (is it cold? The we'll need heat.) and even the forecast carbon intensity of the grid (can we charge later in the cheap period and save some CO2 emissions?) but I have no idea where to start with this.

Stop Press: With the April prices it's going to be pointless to do any of this, I may as well stuff the battery from the grid during "cozy" time and export everything I generate, getting "paid" 3.324p for the difference.

Energy isn't Fungible

Like money or monkeys, we've dealt with electricity for a long time like it hasn't mattered where it comes from or what it does. But it does. 1kWh from coal isn't the same as 1kWh from wind. And it doesn't matter how green the generation is if the 1kWh is used to mine crypto when it could have been used to heat someone's home.

Ultimately I think we have the tools to mitigate many climate issues, and now I've got a bunch of them in my house. Heat Pumps are more efficient, they cost less to run and emit much less / or no / carbon. Batteries enable us to flatten the curve on demand, meaning we can minimise the most polluting generation and store the least polluting for later.

We need a significant shift in attitudes from everyone, especially politicians, to make this change. But my house shows we can do it.

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