Hot Water Heat Pump Solar PV Sizing: A Practical Guide

Last updated: November 2026

A heat pump can reduce the electricity needed for hot water, but adding solar does not make the hot-water bill disappear. The result depends on water use, inlet temperature, tank losses, heat-pump performance and the hours when spare PV electricity is available.

This guide keeps those quantities separate. It covers the litres-to-kWh calculation, PV sizing, controllers, insulation, retrofit choices and examples for small, family and larger households. All household calculations below are illustrative design examples. They are not measured customer projects, manufacturer performance claims or guaranteed solar fractions.

What Is a Hot Water Heat Pump?

A hot water heat pump moves heat from air into stored water using electricity. Domestic hot water is often abbreviated DHW, while a heat pump water heater may be called an HPWH. Unlike an immersion element, the heat pump obtains part of its delivered heat from the surrounding environment.

Coefficient of performance, or COP, is heat output divided by electrical input over the same measurement period. A COP of 3.5 means that 1 kWh of electricity corresponds to 3.5 kWh of delivered heat under the stated conditions. It is an example operating value here, not a promise for every season or product.

The Australian government’s YourHome hot-water guide explains the differences between heat-pump, electric-resistance and solar water-heating systems. It also describes timers and smart controls as ways to change when electricity is used. A timer can still draw grid electricity when insufficient PV is available.

Integrated units combine the tank and heat pump, while split arrangements separate them. The connections and siting requirements vary by product. Some systems circulate water between units; others use refrigerant pipework. Do not assume that all split systems require the same installation process or qualifications.

Indoor units also interact with the room around them. Check the required air volume or ducting arrangement, access for maintenance, condensate drainage and the effect of cooling that room. For outdoor equipment, consider airflow, frost drainage, service clearance and sound at neighboring properties. The installation manual governs these details.

How Heat Pumps Compare With Immersion and Gas

Compare the energy needed to provide the same useful hot water. Keep the price of electricity separate from gas, and value PV electricity at its opportunity cost rather than assuming it is always free.

The table uses an illustrative useful-heat requirement of 8.36 kWh, excluding tank and pipe losses. The gas efficiency and heat-pump COP are assumptions for demonstrating the calculation. They are not ratings for named equipment.

Multiplying each input by its own tariff gives the fuel cost. Add maintenance, fixed charges that actually change, equipment costs and financing for a lifetime comparison. If a gas connection remains for space heating or cooking, its standing charge is not automatically an avoided cost.

Carbon comparisons need the relevant electricity and fuel factors, their dates and the accounting boundary. The energy table is not a carbon ranking. Avoid applying an average from one country to a household in another or treating an annual grid factor as a precise hourly measure.

Hot Water Demand: How Much Does a Household Actually Use?

Daily hot-water volume is the first sizing input, but it must have a temperature attached. Litres delivered at a shower’s mixed temperature are not equivalent to litres drawn directly from a hotter cylinder.

Per-Person Baselines

Use household habits or measured consumption rather than assigning demand from bedroom count alone. Occupancy, shower duration, bath use and hot-water-connected appliances all matter. Some appliances heat cold water internally and should not be counted as cylinder demand.

The following profiles are illustrative inputs retained to show the calculation. They are not national averages or recommended allowances. The assumed volume is water heated from 15°C to 55°C for energy calculation only; it is not a recommended storage-temperature setting.

A usage diary can record draw times and the fixtures used. Flow measurements at an outlet should distinguish mixed water from stored hot water. A whole-house cold-water meter also counts toilets, irrigation and other cold uses, so its change is not a direct hot-water measurement.

If measurement is impractical, show a range and explain the assumption. A design that works only at the lowest estimate needs more investigation before purchase. Check the busiest normal day as well as a weekly average, because a tank must supply concentrated draws between recovery periods.

Household-Size Targets

Cylinder capacity is selected from demand timing, usable hot-water delivery and recovery capability. It is not simply daily litres multiplied by a fixed safety factor. A larger tank may help shift heating into solar hours, but its standing loss and space requirements also matter.

Using a water heat capacity of approximately 4.18 kJ/kg·K and approximately 1 kg per litre, a 40°C temperature rise requires about 0.04644 kWh per litre. This physical calculation is the basis of the next table. The values exclude tank and pipe losses.

The factor 0.04644 is for the whole 40°C rise. It is not energy per litre per degree. If the inlet temperature or target temperature changes, recalculate rather than reusing the factor.

Seasonal and Regional Variation

Colder inlet water increases the heat needed for the same final temperature. At the same time, winter can reduce PV production and alter heat-pump performance. These effects should be evaluated together instead of treating an annual electricity estimate as a winter supply guarantee.

For illustration, heating 180 L from 6°C to 55°C requires about 10.24 kWh. Heating it from 17°C to 55°C requires about 7.94 kWh. The inlet temperatures are hypothetical comparison points, not measured regional averages.

Use local weather information and the selected product’s performance data. For solar generation, the European Commission’s PVGIS tool provides location, orientation, loss and hourly-output inputs. It does not determine household hot-water demand or validate a manufacturer’s COP.

The COP Math: From Litres to kWh

Heat-pump electricity is the required heat divided by the appropriate COP, with auxiliary electricity accounted for separately. Add thermal standing losses before dividing by COP when the heat pump replaces that lost heat.

COP Reference Conditions

Compare manufacturer figures only when their measurement conditions are understood. Air temperature, inlet and outlet water temperatures, draw profile and the treatment of auxiliary heaters can change the result. A controller model number is not a heat-pump performance rating.

Do not treat one favorable laboratory point as the annual seasonal average. Request the selected unit’s data for the intended climate and water-temperature duty. If a quoted standard or test condition cannot be checked, ask the installer to check the standard and provide the governing data sheet instead of inserting a remembered value.

Brand-specific COP tables have been removed because the listed values could not be established from an opened manufacturer document. Obtain the actual model’s evidence before choosing between Sanden, Reclaim, Mitsubishi, Stiebel Eltron or Daikin equipment. Refrigerant choice alone is not a sufficient performance comparison.

Annual Electrical Demand

The calculation is straightforward once units and boundaries are consistent:

Useful heat = litres × 4.18 × temperature rise / 3,600
Total thermal demand = useful heat + tank and pipe heat losses
Heat-pump electricity = total thermal demand / applicable COP
Total electricity = heat-pump electricity + separately counted auxiliaries

For the family example, assume 180 L/day heated through 40°C, 1.6 kWh/day of thermal standing loss, and COP 3.5. Useful heat is 8.36 kWh/day; total heat is 9.96 kWh/day. Electricity is approximately 2.846 kWh/day, or 1,038.7 kWh/year if those same conditions are assumed every day.

That annualization is a simplified example, not a seasonal simulation. It excludes separately powered hygiene cycles, pumps and controls unless they are already included in the COP boundary. Adding them twice would overstate consumption; omitting them entirely would understate it.

How Sensitive Is Electricity Use to COP?

For the same assumed 9.96 kWh daily heat requirement, lower COP increases electricity demand. This sensitivity calculation helps show why a winter design cannot rely on a single headline efficiency.

Why Tank Size Matters

A tank stores heat between heating and use. This can help a household heat water during daylight and draw it later, but the available buffer depends on temperature stratification, draw timing, controls and usable delivery volume.

Check whether the chosen cylinder is compatible with the heat pump and the required recovery duty. An existing cylinder may have unsuitable heat-exchanger characteristics even when its nominal capacity looks adequate. Compare declared standing loss and recovery data rather than insulation thickness or age alone.

PV Sizing: How Much Solar Do You Actually Need?

PV sizing for hot water requires both enough annual energy and enough generation at usable times. A nominal allocation of array capacity is an accounting aid; it does not reserve individual panels for the heat pump.

The 80% Solar Fraction Target

An 80% solar fraction can be used as a design target, but it is not a universal optimum or an expected result. Here solar fraction means the share of the heat pump’s electricity supplied by on-site PV. It is different from the share of useful heat that came from ambient air.

For the family example’s 1,038.7 kWh annual electricity, an assumed 80% solar fraction means 831.0 kWh from PV and 207.7 kWh from the grid. Those values describe the target allocation. An interval model or measurements are needed to demonstrate whether the house can achieve it.

Competing loads matter. A battery, EV charger or daytime cooking may consume the same surplus. Do not allocate all midday generation to hot water while also claiming it for another load in the whole-house savings model.

kWp Sizing Formula

For an initial energy screen, divide the desired PV-supplied electricity by annual specific yield and an explicitly assumed capture fraction. Specific yield is annual generated kWh per installed kWp. The capture fraction describes how much of that generation is usable by the assigned hot-water load.

PV-supplied electricity target = annual hot-water electricity × target solar fraction
Allocated PV capacity = target / (specific yield × assumed capture fraction)

The capture fraction is not a controller specification. It changes with array size, household loads, heating schedule and weather. It should eventually come from the interval model, not remain an arbitrary multiplier chosen to make the proposal attractive.

Worked PV Sizing: Family Home

Assume 1,038.7 kWh annual hot-water electricity, an 80% target, specific yield of 1,000 kWh/kWp/year and a 0.78 capture fraction. The required accounting allocation is 831.0 / (1,000 × 0.78), or about 1.07 kWp.

These are illustrative inputs, not a site prediction. If designing a new array, use available module sizes, roof constraints and electrical compatibility to select the actual capacity. On an existing array, assess surplus after other loads instead of assuming that the nominal allocation is always available.

Sizing Across Climates and Latitudes

Use the exact site rather than a city-level rule of thumb. Roof orientation, tilt, shading and system losses can make two houses in the same location behave differently. The timing of production can matter as much as the annual total.

For comparison only, keeping the 831.0 kWh target and 0.78 capture assumption gives about 1.18 kWp at a yield of 900, 1.07 kWp at 1,000, and 0.71 kWp at 1,500 kWh/kWp/year. These are sensitivity cases, not published yields for named cities.

Check the roof with site-specific shading analysis. A chimney or neighboring building may remove the very midday hours expected to supply the heat pump. If the PV design changes, revisit the capture fraction and winter shortfall instead of carrying over the old solar share.

What About the Remaining Grid Share?

Grid electricity fills the gap when available solar and stored hot water cannot meet demand. Its cost depends on when heating occurs, not only the annual imported total. A tariff with cheap overnight electricity can change the cheapest operating schedule.

A battery may reduce imports, but hot water should bear only the costs and benefits attributable to it. Compare the additional battery throughput and lost export revenue with the value of avoided grid heating. There is no universal battery payback or rule that it always helps or never pays.

Smart Controllers: Matching Heating to Solar

Controls decide when the heat pump runs within the household’s comfort and safety requirements. A simple schedule may capture useful daytime generation. A PV-aware controller may respond to measured surplus, but neither approach guarantees a particular solar fraction.

Evaluating Mitsubishi, Sanden and Reclaim Controls

For each proposed product, request the exact controller manual and heat-pump combination. Identify supported external inputs, signal type, allowable operating modes and required accessories. A relay output on one inverter does not prove that another inverter or controller uses the same voltage or contact arrangement.

The opened Mitsubishi FTC6 instruction-book page identifies the FTC6 controller family and points users to its safety instructions. It does not support the original article’s claimed terminal wiring, universal PV-link accessory, menu sequence or solar-fraction figures. Those instructions have been removed rather than presented as installation guidance.

Apply the same evidence requirement to Sanden and Reclaim. Ask the supplier to demonstrate the proposed daytime schedule and any solar-surplus input for the actual unit. Specify who supplies the meter, relay, communications equipment and commissioning, and who supports the complete combination.

Controller Comparison Checklist

A controller that permits a start is not necessarily a controller that varies compressor power continuously. Distinguish enabling, scheduling and modulation. Also distinguish measured PV surplus from total PV production, because the rest of the house may already be using that production.

Without a PV-Aware Controller

A daytime timer can improve overlap when hot water is needed later, but the grid will supply any shortfall. An overnight schedule can be cheaper on some tariffs while using less direct solar. The household should choose whether the priority is lowest cost, highest self-consumption or another defined objective.

Compare actual costs rather than treating solar fraction as the sole success measure. Exported electricity has a value, and heating water unnecessarily increases losses. A more sophisticated controller is worthwhile only if its added benefit and usability justify the installed and ongoing cost.

Insulation Matters: Tank and Pipework Losses

Insulation reduces heat lost between heating and use. Use declared cylinder standing loss and the applicable pipework requirements; there is no supported universal 140 mm insulation rule for this application.

Tank Insulation

Compare heat loss at stated conditions, not only foam thickness. Cylinder geometry, fittings, installation location and temperature difference all affect loss. A blanket recommendation to replace every tank above a certain age can discard a serviceable compatible cylinder or overlook a newer unsuitable one.

For illustration, reducing thermal standing loss from 2.0 to 1.0 kWh/day saves 365 kWh of heat per year. If the heat pump replaces that heat at an assumed COP of 3.5, the electrical saving is about 104.3 kWh/year. It is not 365 kWh of electricity unless resistance heating supplies the heat.

This distinction matters in payback calculations. Price the avoided electricity at the appropriate import or export opportunity cost. Add any change in maintenance or installation work before deciding whether cylinder replacement is justified solely by lower standing loss.

Pipework Insulation

Insulate according to the product instructions and local requirements, including fittings and accessible runs where required. The opened ENERGY STAR installation guidance stresses continuity of insulation and compliance with the applicable local code. It does not provide a universal UK pipe-insulation thickness.

Shorter distribution runs can reduce waiting time and wasted hot water. Recirculation systems need their own loss and control assessment because they can keep pipework warm for long periods. Include these losses when they are material instead of using a cylinder-only figure for the whole installation.

Retrofit vs New Build

A retrofit must work with the existing space, plumbing, electrical supply and household schedule. A new build provides more freedom to place the cylinder and route services, but still needs a complete hot-water design.

Retrofit Path

Start with a survey of the existing cylinder, available supply, pipe routes and equipment location. Check the selected model’s airflow, drainage, access, structural and electrical requirements. Obtain a quotation that separates equipment, disposal, plumbing, electrical work and control integration.

The contractor should identify whether the system uses water or refrigerant connections and which trades are required. Circuit rating, protective devices and clearances must come from the selected equipment and applicable rules. They should not be copied from a generic guide.

Existing PV can supply the home’s electrical loads without being physically dedicated to the heat pump. A control integration may require extra metering or interfaces, but this is different from replacing the PV inverter. Ask the installer to explain any proposed inverter change and its purpose.

Plan the period without hot water, access to the installation area and commissioning checks. A promised one-day installation is not appropriate for every site. Difficult access, electrical work or cylinder relocation can materially change the program.

New Build

Place the cylinder and equipment to suit distribution distances, service access and occupant needs. Coordinate penetrations, drainage and ventilation before finishes are installed. If energy management is planned, define meter locations and communications routes while access is easy.

A combined space-heating and hot-water heat pump needs an assessment of both duties and how they are prioritized. A dedicated water heater should be evaluated on its own requirements. Do not assume that a controller or incentive for one arrangement applies to the other.

Local planning, building rules and incentive eligibility require project-specific checking. The earlier claim that UK new builds universally prohibited gas boilers under a 2025 rule was unsupported and has been removed. Obtain the applicable requirement from the authority or project professional.

When a Solar Diverter Wins

A diverter can send surplus PV electricity to a compatible immersion element. It may be attractive when an existing cylinder is suitable and additional installation work is limited. Its lower upfront cost must be compared with the heat pump’s lower electrical input for the same useful heat.

Diverter Economics

For the family example, resistance heating would need approximately 9.96 kWh/day including the assumed tank loss. The example heat pump needs about 2.846 kWh/day at COP 3.5. The diverter must have enough surplus electricity to supply its larger requirement.

Diverted PV also has an opportunity cost. If it could have been exported or used by another appliance, that value belongs in the comparison. Include imported electricity when surplus is insufficient and verify that the immersion, thermostat and diverter are compatible.

When Diverters May Be Preferable

A low-demand household with a serviceable cylinder may favor a modest retrofit. A household replacing a failed water heater may face a different incremental cost and choose a heat pump. Space, sound, airflow and maintenance access can also affect the decision.

There is no verified universal crossover at 120 litres per day. Use actual quotations and equivalent service requirements. A larger household can still find a diverter suitable, while a smaller household may reasonably choose a heat pump for reasons beyond simple payback.

Worked Example 1: Small Apartment

Assume one occupant uses 45 L/day at the calculation temperatures, with 1.2 kWh/day of thermal standing loss. Useful heat is 2.09 kWh/day and total heat is 3.29 kWh/day. At an assumed COP of 3.2, the heat pump uses approximately 375.3 kWh/year.

Heat Pump Analysis

If 75% of that electricity comes from PV, imported electricity is about 93.8 kWh/year. At an assumed import price of £0.32/kWh, imports cost about £30.02/year. This excludes PV opportunity cost, auxiliary electricity and maintenance.

An apartment also needs permission and a workable meter arrangement for any communal-roof allocation. A financial allocation of shared solar is not necessarily electricity physically available behind the apartment’s meter. Verify that boundary before claiming direct self-consumption.

Diverter Analysis

Resistance heating for the same 3.29 kWh/day requires about 1,200.9 kWh/year. At an assumed 65% PV share, grid input is about 420.3 kWh/year, costing approximately £134.50 at the same import price.

These figures show the operating-energy difference. They do not select a winner without installation quotes, exported-energy value and equivalent hot-water availability. Check whether the apartment can accommodate the proposed heat-pump unit and its airflow requirements.

Worked Example 2: Family Home

Assume four occupants use 180 L/day, with 1.6 kWh/day thermal loss and COP 3.5. The result is approximately 1,038.7 kWh/year of electricity under unchanged daily conditions. The 80% solar target corresponds to approximately 831.0 kWh PV and 207.7 kWh grid electricity.

Modeled Outcome

At an assumed £0.32/kWh import price, the grid share costs about £66.48/year. Add the value of the solar electricity used, auxiliaries and maintenance before comparing with the previous system. The example is not a Manchester installation or a measured first-year result.

For a gas baseline, calculate fuel use from the same useful-heat requirement and the previous system’s relevant efficiency. Do not compare this household with an unrelated gas bill that includes space heating. Keep fixed charges only where the household actually avoids them.

Check morning availability after evening draws. A daytime-only schedule may require more usable stored water or a permitted top-up. Increasing the tank temperature should be assessed by the installer against the product limits, hygiene plan and scald protection, not used as an automatic fix.

Worked Example 3: Larger Country House

Assume a 290 L/day demand, 2.2 kWh/day thermal standing loss and COP 3.0. Useful heat is about 13.47 kWh/day; total heat is about 15.67 kWh/day. Electricity is about 1,906.9 kWh/year under those fixed assumptions.

Modeled Outcome

An assumed 80% solar share leaves approximately 381.4 kWh/year of grid electricity. At £0.32/kWh, imports cost about £122.04/year. These values are illustrative and do not describe a named Devon property or a particular manufacturer’s unit.

A larger house may have long distribution runs, recirculation or guest demand. Those features can matter more than nominal bedroom count. If LPG is the baseline, use the owner’s actual fuel price and efficiency assumptions, with delivery and fixed charges treated consistently.

The available PV may serve other loads, and the roof’s annual output does not guarantee winter surplus. Compare the hot-water schedule with the house’s total load and generation. If the household is off-grid, a grid-top-up assumption is invalid and a separate resilience design is required.

Mitsubishi Ecodan FTC6 PV Integration Checks

Integration should follow the exact Mitsubishi controller and outdoor-unit manuals. This section is a commissioning checklist, not a terminal-by-terminal wiring procedure. The original accessory, terminal and menu claims were not supported by the manufacturer page opened for this update.

Step 1: Identify the Hardware

Record the outdoor unit, cylinder, controller, interface and firmware. Ask the installer to identify the document revision covering that combination. A family name such as Ecodan or FTC6 is insufficient to establish every supported input and mode.

Step 2: Verify the Signal

Have the qualified installer establish the permitted signal type and electrical isolation. A dry contact, powered input and communications interface are different arrangements. Do not connect a PV relay based on a generic description or another model’s diagram.

Step 3: Configure the Operating Priorities

Define normal hot-water availability, permitted solar scheduling and any backup behavior. Keep hygiene functions and equipment protection active. Document how a manual override interacts with the schedule and how settings recover after a power interruption.

Step 4: Verify Operation

Observe the intended control request, actual heat-pump operation and grid import together. PV generation alone is insufficient evidence that the heat pump is solar-supplied. Other loads may consume the output while the water heater imports electricity.

Step 5: Log and Tune

Compare metered electricity, draw timing and hot-water availability over representative conditions. A relay-active duration is not an energy measurement, and it cannot be divided by total kWh to calculate solar fraction. Change settings through the installer when they affect safety or equipment operation.

Common Mistakes in Hot Water Heat Pump Installs

Most sizing errors come from mixing quantities or assuming performance without the relevant operating conditions. Review these points before accepting the final design.

Mistake 1: Confusing Input and Heating Output

A heat pump described as 4 kW may refer to thermal output rather than electrical input. Recovery time uses available heat output; electrical design uses input requirements. Check both figures and the conditions under which they apply.

Mistake 2: Applying a Universal Winter Penalty

Defrost and cold-weather operation depend on equipment and weather. Use the selected unit’s data and suitable seasonal assumptions. Do not apply an invented fixed defrost interval or percentage penalty to every product.

Mistake 3: Ignoring Sound and Airflow

Siting must meet the manufacturer’s instructions and applicable local requirements. Sound power and sound pressure are different quantities. A generic decibel value or boundary clearance cannot replace a site assessment.

Mistake 4: Overriding Hygiene Controls

Follow the installer-approved water-hygiene strategy and product instructions. HSE provides Legionella guidance for health and care premises, including temperature control and scald-risk considerations in that setting. It is not evidence for a universal weekly household cycle or permission to disable safety controls for solar scheduling.

Mistake 5: Treating Rated COP as Annual Performance

A favorable rating point is not a household’s annual result. Show the conditions behind the COP and the loads included. Compare measured electricity with the same boundary used in the forecast before diagnosing a shortfall.

Cost Breakdown and Payback Summary

A useful quotation separates hardware, installation and ongoing costs. Payback requires a consistent baseline and an estimate of net savings; imported-energy cost alone is not the full operating cost.

Do not assume that support for a combined space-and-water heating system applies to a hot-water-only unit. Obtain written eligibility advice for the proposed installation before deducting any grant or tax saving. The examples here exclude such benefits.

A Practical Solar Fraction Plan

Select a realistic target, then check it against household use and available generation. The aim is dependable hot water with a clear operating cost, not a solar percentage detached from comfort or safety.

1. Measure the demand. Record draw timing and estimate volume at a stated temperature. Review high-use days separately from average use.
2. Check the cylinder. Obtain usable delivery, standing-loss and compatibility information. Include distribution losses where material.
3. Use appropriate performance data. Select COP and recovery assumptions for the intended duty and climate, with auxiliary energy accounted for.
4. Model PV overlap. Subtract other household loads before allocating surplus. Evaluate seasonal shortfalls and export opportunity cost.
5. Specify compatible controls. Define schedules, overrides and fallback behavior using the actual manuals.
6. Protect water safety. Keep hygiene and scald-protection requirements within the installer-approved design.
7. Monitor the result. Compare electricity and hot-water availability with the assumptions, then revise the model or settings when evidence warrants it.

Hot Water Heat Pump and Solar Decisions in 2026

Compare products available for the actual site rather than relying on unsupported market-share or refrigerant rankings. Ask about service support, parts, warranty scope and the exact operating conditions behind performance claims.

Dynamic tariffs can change when heating is cheapest. Their value depends on the household’s contract and ability to shift demand, not a national adoption percentage. Compare a simple tariff schedule with PV-aware operation before paying for extra control complexity.

Certification and incentives should be checked with the responsible scheme for the proposed system. A software model can support the energy case, but it does not determine installation eligibility or legal compliance. Keep the relevant decision and source with the quotation.

Conclusion: What to Do Next

Start with hot-water demand at a defined temperature, add thermal losses, then divide by an appropriate COP. Check solar overlap after other household loads and compare the complete costs of the heat-pump and diverter options.

Bring the roof layout and measured demand to a PV design walkthrough. SurgePV supports solar layout, energy-yield and financial modeling; the heating installer should supply the water-heater performance and control requirements.

Frequently Asked Questions

What size solar PV do I need for a hot water heat pump?

Size PV from hot-water electricity demand and the generation available during heating hours. In the illustrative family example, an 80% target gives about 1.07 kWp of allocated PV under the stated yield and capture assumptions; it is not a universal recommendation.

How much electricity does heating 180 litres of water use?

Heating 180 litres through 40°C requires about 8.36 kWh of useful heat. At an assumed COP of 3.5 that is about 2.39 kWh of electricity before tank losses and other electrical consumption.

Is a heat pump better than a solar diverter?

The better choice depends on installation cost, demand and available PV surplus. A heat pump can use less electricity for the same heat, while a diverter may offer a simpler upgrade to a suitable existing cylinder.

Does a smart controller guarantee 80% solar hot water?

No controller guarantees an 80% solar share by itself. The result depends on generation, competing loads, tank storage, draw timing and permitted operating settings.

How should tank losses be included in solar sizing?

Add thermal tank losses to useful hot-water heat demand before dividing by the relevant COP. Count separately powered auxiliaries and resistance heating according to their actual electrical consumption.

Can I connect a PV relay directly to an Ecodan FTC6?

Use only the connection and signal permitted by the exact Mitsubishi manuals for the installed equipment. A qualified installer should verify the interface, wiring and operating priorities before connection.