What a heat battery actually is
Most people's mental model of hot water storage is a cylinder of water that's been heated and slowly cools. The Sunamp works on an entirely different physical principle. Instead of storing energy as sensible heat in water — where temperature rises as energy is added — it stores energy as latent heat in a phase-change material, where temperature stays nearly constant while the material absorbs or releases large quantities of energy by melting or freezing.
The material is called Plentigrade P58 — a proprietary formulation of sodium acetate trihydrate developed over a 12-year collaboration between Sunamp and the University of Edinburgh's School of Chemistry. Sodium acetate trihydrate is the same compound used in reusable hand warmers — the ones where you click a metal disc and the clear gel suddenly solidifies and releases warmth. Sunamp's version is engineered for domestic hot water temperatures, optimised for thousands of charge/discharge cycles, and packaged with the heat exchanger and safety systems needed for a plumbed installation.
The critical physical insight is the energy density advantage. Water stores approximately 4.18 kJ per kg per °C of temperature rise. The phase transition of sodium acetate trihydrate stores approximately 264 kJ/kg as latent heat — without any temperature change at all. This is why the Sunamp 300e — labelled as the "300 litre equivalent" — is physically much smaller than a 300 litre cylinder while storing equivalent useful hot water energy.
Physical size: approximately 600mm × 600mm × 650mm — fits in a standard cupboard
Equivalent 300L cylinder: ~1,600mm × 550mm diameter — requires dedicated space
Standby heat loss (Sunamp): 0.5–0.77 kWh/24hrs
Standby heat loss (traditional cylinder): 1.344–3.336 kWh/24hrs
PCM cycle life tested: 40,000 charge/discharge cycles — equivalent to ~110 years at one cycle per day
Warranty: 10-year warranty on heating element and PCM core
Regulatory status: G3 exempt — no unvented cylinder regulations, no annual inspection required
Why G3 exemption matters for installation speed
This is the detail that made the four-hour emergency installation at Rose Cottage possible. A conventional unvented hot water cylinder — the pressurised type that delivers mains-pressure hot water — falls under G3 of the Building Regulations. G3 requires installation by a qualified G3 engineer, a specific commissioning procedure, pressure relief valves, expansion vessels, annual inspections, and tank cleaning every three years.
The Sunamp contains no stored hot water. It is a heat exchanger through which cold mains water flows on demand. Because there is no pressurised stored hot water, G3 regulations do not apply. Any competent plumber can install one in the time it takes to make two plumbing connections and wire an immersion circuit. No specialist qualification. No commissioning paperwork. No annual inspection obligation. Just hot water.
The PCM has been tested to 40,000 charge/discharge cycles with minimal degradation — equivalent to over a century of daily use. This is not a component you will replace during the life of the house. The chemistry is non-toxic, non-flammable, and the material is fully recyclable at end of life. Sunamp is committed to ensuring full re-use or recycling of every battery component.
The four-hour installation: what actually happened
Rose Cottage's original hot water cylinder failed — a conventional indirect copper cylinder that had served the house for years. The failure was not gradual. Hot water simply stopped being available, the airing cupboard was damp, and the options were: wait days for a like-for-like cylinder replacement through the normal trade supply chain, or install a Sunamp the same day.
The Sunamp 300e was sourced and delivered within hours. Because it required only two plumbing connections — cold mains in, hot water out, with a standard tempering valve at the outlet — and a standard immersion element circuit, a competent plumber could complete the installation in an afternoon. No specialist equipment. No G3 sign-off. No pressurised system commissioning. The unit was running before the working day ended.
By the following morning, the Myenergi Eddi diverter — already installed and managing the solar surplus — had automatically recognised the new load and started diverting. The Sunamp doesn't need to be told it's connected to solar. The Eddi sees available surplus, sends current to the immersion element circuit, and the PCM charges. It is genuinely plug-and-play in the solar context — no additional configuration, no new automation rules, no changes to the Homey setup.
What the solar charging data actually shows
The Myenergi Eddi app provides granular consumption history for each load it controls. For Rose Cottage, Priority 1 is the Sunamp hot water tank. Priority 2 is Tank 2 (a backup load). The data across April 2026 is instructive about exactly how the system operates in practice.
The solar timing tells the real story
Look at when the Sunamp charges in the daily chart: the spike runs from approximately 10am to 12:30pm. This is not a coincidence — it is the Eddi responding to surplus solar generation once the Sonnen battery is full and the house loads are met. The solar array at Rose Cottage peaks between 10am and 2pm in spring and summer. The Sunamp absorbs exactly that peak surplus at the point where it would otherwise start hitting the 6 kW DNO export limit and triggering Enphase throttling.
This is the integration that makes the Sunamp so valuable in a solar-heavy system. Without it, that midday surplus either exports at 15p/kWh, gets throttled to zero by Enphase at the export limit, or — in Homey's management logic — triggers towel rails and secondary loads. With the Sunamp as Priority 1 on the Eddi, it absorbs 3 kW of surplus for approximately two hours every clear day, storing it with minimal losses until the shower is needed in the evening.
// Typical sunny day charge profile — solar surplus to Sunamp
// Peak charge 10:00–13:00 from solar surplus. Sunamp fully charged by midday. Zero grid draw for hot water all day. Evening hot water delivered at zero marginal cost.
The annual economics across both seasons
| Period | Charge source | Rate paid | Est. monthly kWh | Monthly cost | vs grid heating |
|---|---|---|---|---|---|
| March–November | Solar surplus via Eddi | £0/kWh | 130–200 kWh | £0 | Save £36–55/month |
| December–February | Overnight cheap rate (Octopus Go) | 6.67p/kWh | 80–100 kWh | £5–7/month | Save £17–21/month vs grid |
| April 2026 (actual) | Solar surplus — 100% | £0/kWh | 194.4 kWh | £0 | Saved £53.95 vs grid |
| Full year estimate | Solar (Mar–Nov) + overnight (Dec–Feb) | blended ~1–2p/kWh | ~140 kWh avg | ~£2–3/month avg | Save ~£400–500/yr vs grid |
Sunamp vs conventional cylinder: what actually matters
Two people, two rain showers, and the honest truth about capacity
The Sunamp 300e is marketed as a "300 litre equivalent." That figure deserves examination when you have two people who both want a long hot shower under a large-head rainfall shower — the kind with a wide plate head, high flow rate, and genuine mains pressure rather than the apologetic trickle of a gravity-fed system from a vented cylinder.
The good news first: the mains pressure delivery is genuinely excellent. The Sunamp is a heat exchanger through which cold mains water flows on demand. It does not store hot water at all — it heats the water instantaneously as it passes through the PCM block, at whatever pressure the mains supply provides. A large rainfall head at 10–12 litres per minute gets exactly the mains pressure it would get from any other mains-pressure fitting. No pump. No compromised flow. No waiting for a header tank to refill. This is one of the Sunamp's genuine advantages over a gravity-fed vented cylinder — the pressure is simply not an issue.
The capacity reality with high-flow showers
The 300 litre "equivalent" rating describes the volume of hot water at 40°C the unit can deliver before the PCM is fully discharged. In practice, two people having consecutive long showers under large-head rainfall heads — running at perhaps 12–15 litres per minute — will deplete the Sunamp more quickly than the headline figure implies.
A large rainfall shower head at 12 litres per minute over a 10-minute shower uses 120 litres of water. At a 40°C delivery temperature mixed from mains cold and stored heat, the actual draw on the PCM depends on the cold water inlet temperature — colder mains in winter means more heat required per litre, depleting the PCM faster. In summer, with warmer mains and a fully charged PCM, two consecutive 10-minute showers are comfortable. In winter with cold mains, back-to-back long showers under a high-flow head will push the limits of a single charge.
The practical solution is straightforward: charge the Sunamp overnight on cheap rate before a morning shower routine. The overnight charge window on Octopus Intelligent Go (00:30–05:30) means the PCM is fully recharged — from whatever state it was left in — before the first shower of the day. At 6.67p/kWh the full recharge costs approximately 50–60p. The capacity question disappears when the unit reliably starts each morning fully charged.
The fuel gauge: a Morris Minor petrol gauge in a modern device
This deserves plain language. The Sunamp's charge indicator — the lights that tell you how full the PCM is — is, in the experience of real users, only reliably accurate when the unit is completely full. When fully charged, the indicator shows full and it is correct. Below that, the relationship between the light display and the actual remaining hot water capacity is unreliable — the intermediate states are approximate at best and misleading at worst.
This is a known property of phase-change material storage and not entirely Sunamp's fault — the PCM doesn't have a linear relationship between temperature and stored energy the way a water cylinder does. A cylinder cools gradually and a temperature sensor gives you a reasonable indication of how much hot water remains. The PCM sits near 58°C whether it's 90% charged or 30% charged — only near full depletion does the temperature start to fall meaningfully, which makes accurate mid-state indication genuinely difficult.
2–3 lights: Unreliable — somewhere between half and nearly full. Could mean plenty or could mean one shower left. Do not plan consecutive long showers based on this reading.
1 light: Low — act on this. Consider whether a recharge is needed before the next heavy demand.
No lights: Empty — PCM fully solidified. No useful hot water available until recharged.
// The Sunamp is at its most useful when treated as a binary: either fully charged, or schedule a charge.
The practical implication for a household of two with high-demand showers: treat the Sunamp like a smartphone battery, not a petrol tank. Don't try to ration the last 30% based on the indicator lights — you will be wrong. Instead, let the Eddi and the overnight schedule ensure it starts each day fully charged, and enjoy unlimited confidence in the full-charge state. This is exactly how the Rose Cottage system operates: solar charges it opportunistically during the day, the overnight schedule tops it up if needed, and the morning starts with a full PCM every day regardless of the previous evening's shower duration.
Mains pressure performance: genuinely excellent. A large rainfall head at full mains pressure with no compromise — this is one of the Sunamp's strongest practical advantages over gravity-fed systems.
Capacity for two consecutive high-flow showers: adequate when fully charged, marginal when partially discharged. The solution is ensuring a full charge before the morning routine, which the overnight cheap-rate schedule handles automatically.
The charge indicator below 100%: treat it as approximate. It is accurate when full and useful as a low-warning indicator, but the intermediate readings between those two states are not reliable enough to plan back-to-back long showers on. The Morris Minor fuel gauge comparison is apt — both devices are honest only at the extremes.
The Sunamp doesn't operate in isolation. It sits as one layer of a carefully designed energy cascade in which every component knows its role and the whole system is coordinated by the Myenergi ecosystem and Homey automation.
The priority hierarchy is set in the Eddi app: Hot water tank (Sunamp) first, Tank 2 second. When the solar array generates surplus above household demand and above what the Sonnen battery can absorb — which happens from approximately 10am on most spring and summer days — the Eddi begins diverting to the Sunamp at whatever rate of surplus is available, from a minimum of around 1.4 kW up to the full rated element power.
The Sunamp's relationship with the 6 kW DNO export limit is important. Without the Sunamp, surplus above 6 kW would trigger Enphase microinverter throttling — real generation physically prevented from happening. With the Sunamp as a priority load, the Eddi absorbs 2–3 kW of midday surplus that would otherwise push against the export ceiling. This means the Enphase array runs at full output for longer, capturing more energy that is then stored as usable heat rather than wasted at the inverter level.
In November to February when solar generation is insufficient to charge the Sunamp during the day, Homey schedules an overnight charge during the Octopus Intelligent Go cheap rate window (00:30–05:30). The effective cost of hot water in winter is 6.67p/kWh — approximately £5–7 per month for complete hot water provision, versus approximately £25–35/month on a standard grid tariff. Gas is not involved at any point in the year.
The hot water separation advantage for the planned air-to-water heat pump
This is worth emphasising for anyone planning a heat pump installation. A conventional heat pump system uses the heat pump for both space heating and domestic hot water — running the heat pump at 55–60°C flow temperature for legionella-safe hot water storage, significantly above the 37–42°C optimal range for space heating. This hot water efficiency penalty reduces the whole-system seasonal SPF by approximately 0.3–0.5 points.
By having the Sunamp handle hot water entirely — solar from March to November, cheap-rate electricity in winter — the planned air-to-water heat pump at Rose Cottage will run exclusively in its optimal 37–42°C space heating regime. It never needs to produce hot water. It never needs to run at the higher flow temperatures that drag down efficiency. The whole-system SPF benefit of this separation is real and has been factored into the air-to-water sizing and specification.
The Sunamp is not just a hot water device in this system — it is an active component of the solar optimisation strategy. It absorbs midday surplus that would otherwise hit the export limit, prevents Enphase throttling, stores energy with low losses, and delivers it as free hot water on demand. In April 2026 it absorbed 194.4 kWh of solar energy. At the avoided grid import rate of 27.75p/kWh that is £53.95 of hot water for free in a single month.
And by handling hot water independently, it allows the future air-to-water heat pump to operate at its peak efficiency for space heating alone — the design decision that is worth approximately 0.3–0.5 points of whole-system SPF over the heating season.