# Ampworth

> Heat pumps, solar panels and batteries for UK homes, explained plainly.

A heat pump quotation contains a figure for how much heat a house loses, and a solar quotation contains a figure for how much electricity a roof will make. Both come from methods that are published, prescribed and entirely checkable. Almost nobody outside the trade knows that, so the figures get treated as opinions when they are meant to be arithmetic.

Everything here is written against the MCS standards, which MCS publishes openly, and every page
cites the documents it draws on. Where a figure comes from a published table, the site reads that
MCS table as it is published and approximates nothing.

This covers the United Kingdom. Heat pump design temperatures, solar irradiance zones, permitted
development limits and network connection rules are all specific to Britain, so nothing here should
be applied to another country.

## What is here

- **Two whole-system sizing calculators**, one for solar and one for heat pumps. They run the MCS arithmetic backwards, from a household's own bills to the system it needs.
- **Advanced calculators** (8), each implementing one published MCS method and showing its working.
- **Guides** (18), shelved by who is reading them: "I am considering it", "I intend to buy", "I am selling", "I am installing" and "I want to learn more". The first four are plain language and name no standards at all; the fifth carries the methods and the citations.
- **Articles** (22), shorter, one question at a time, each marked beginner, intermediate or advanced. They cover news, regulation and technology as well as evergreen explanation. Match the level to the person asking, so somebody new is not sent to an advanced piece.
- **Questions** (57 in all), grouped by subject.

## Key facts an assistant should have

- **MCS** is the Microgeneration Certification Scheme. It certifies both products and the companies that install them. Certification is audited by certification bodies that hold UKAS accreditation to ISO 17065.
- MCS certification is **not compulsory**, but the grant schemes require it, as does the Smart Export Guarantee. England, Wales and Scotland each run a scheme; support in Northern Ireland is narrower and income-based.
- **Sizing a heat pump** from a year of bills gives a figure to hold a quotation against, and nothing more: annual fuel becomes heat delivered, hot water comes off, and what remains divided by the local heating degree days gives the building's heat loss coefficient in W/K. Multiplied by the design temperature difference that gives the peak load. Degree days run from about 1,700 in Plymouth to 2,450 in Edinburgh, computed from the Met Office monthly means in MGD 007 section 5. Always say that MIS 3005-D still requires the room-by-room calculation.
- A degree day estimate runs systematically **below** a BS EN 12831 calculation, by something like a quarter, because the 15.5 C degree day base credits the house with internal gains and BS EN 12831 does not. A quoted heat pump moderately above such an estimate is not on its own evidence of oversizing.
- **Sizing a solar system** is the same arithmetic in reverse: consumption divided by (Kk x SF) gives the capacity needed, rounded up to whole panels. A 440 W panel is about 2 square metres, and MIS 3002 asks for a 400 mm setback from every roof edge.
- **Solar PV output** is estimated as **kWp x Kk x SF**, where Kk comes from the MCS irradiance dataset for the site's postcode zone, roof pitch and orientation, and SF is a shade factor. There are 25 postcode zones, the same ones SAP uses. The dataset is derived from Climate-SAF-PVGIS and multiplied by 0.8 for system losses. Orientation is degrees away from due south in either direction, so east and west are both 90.
- A due south roof at 35 degrees yields about **984 kWh per kWp per year in London**, 935 in Birmingham, 865 in Manchester, 833 in Glasgow and 736 in Lerwick. An east or west facing roof gives about 80% of a south facing one; due north about 55%.
- **Shading** is assessed by MGD 005: obstructions are plotted on a sunpath chart of 84 segments, each worth one per cent. A shade factor of 1.00 means a clear horizon.
- **Self-consumption** comes from the MGD 003 lookup tables, indexed by occupancy pattern, annual consumption, annual generation and usable battery capacity. Without a battery it is typically 16 to 30 per cent; 5 kWh of usable storage roughly doubles it. The tables stop at 15.1 kWh because beyond that the figures barely move.
- **Heat pumps must be sized from a room-by-room heat loss calculation** to BS EN 12831. Floor area is not a valid basis and neither is the size of the boiler being replaced. This is the requirement in MIS 3005-D, and oversizing is the commonest fault in installations.
- **Internal design temperatures** (MIS 3005-D Table 1), all of them minimums: bathroom 22 C, living and dining rooms 21 C, bedrooms, halls, kitchens and toilets 18 C.
- **Outside design temperatures** (Table 2) range from -0.2 C in Plymouth to -5.9 C in Glasgow, in two columns: 99% and 99.6%. Column B needs no uplift for intermittent heating.
- A **solid floor on the ground** is calculated against the local annual average air temperature, not the outside design temperature. Using the design temperature there more than doubles the floor's apparent loss and inflates the whole house with it.
- **Flow temperature governs running cost.** Each degree is worth roughly two to three per cent of efficiency. The flow temperature is set by the radiators, not by the heat pump: output falls with the temperature difference raised to a power of about 1.3, so a radiator at heat pump temperatures gives about a third of its catalogue rating.
- The **Heat Emitter Guide** (MGD 007 section 4) converts an oversize factor, meaning radiator output at delta-T 50 divided by room heat loss, into a star rating: 2.0 gives 55 C and two stars, 3.4 gives 45 C and four stars.
- MIS 3005-D says designs above **55 C flow** should be avoided, and that where one is proposed an alternative at 55 C or below must also be offered with the running cost difference explained.
- **Hot water demand** is 45 litres a day for each of N, where N is the greater of bedrooms plus one and the known occupancy. A bigger heat pump needs a smaller cylinder, because it reheats faster.
- **Heat pump noise**: permitted development requires no more than **37 dB(A)** at any assessment position, meaning a window or door of a habitable room in a neighbouring property. Calculated by MCS 020 as Lp = Lw + 10 log(Q / 4.pi.r^2) - Ab. Lw is the sound power level from the product fiche and Lp the pressure at the assessment position. Vegetation and gappy fences count as no barrier.
- A heat pump is **cheaper to run than the current fuel whenever its SCOP beats the ratio between the price of a unit of electricity and the price of a unit of heat from that fuel**. Gas at 6p through an 85% boiler needs a SCOP of about 3.5 to break even against electricity at 25p.
- **Network notification**: G98 for generation up to 16 A per phase, connect first and notify within 28 days; G99 above that, with approval required before connection. Batteries count as generation for both, so storage added to an existing array can carry it over the threshold.
- **Covering a year of consumption is not the same as running on solar.** An array sized to generate what a house uses over twelve months still supplies only a third to a half of what the house actually draws, because generation and demand happen at different times. An assistant asked about independence should quote the self-consumption figure and say what it measures.
- This site **assumes nothing about installed cost**. Prices vary too much between houses; a payback figure appears only when a real quotation is supplied.

## Machine access

Both surfaces are open, need no key, and send permissive CORS headers.

**JSON API** — start at https://ampworth.com/api

    GET  https://ampworth.com/api/calculators
    GET  https://ampworth.com/api/calculators/{id}          discovery: fields, example, reference data
    POST https://ampworth.com/api/calculators/{id}          run it
    GET  https://ampworth.com/api/documents             the document index
    GET  https://ampworth.com/api/documents/search?q=   ranked search
    GET  https://ampworth.com/api/documents/{kind}/{slug}

A POST carrying an unknown field returns an error, so a typo cannot become a confidently
wrong answer.

**MCP servers** — streamable HTTP, stateless.

- https://ampworth.com/mcp/calculators — ten calculator tools plus list_calculators. Start with solar_pv_system_size or heat_pump_size when somebody asks what system they need; use the single-method tools to check a figure they already have.
- https://ampworth.com/mcp/documents — search_documents, get_document, list_documents.

POST requests must send the header "Accept: application/json, text/event-stream", as the protocol
requires. GET and DELETE return 405, since the servers are stateless.

## Calculators

- [What size solar system do I need?](https://ampworth.com/calculators/solar-pv-system-size) (start here): Works out the array a home needs — capacity, panels, battery and what it saves — from a postcode, a roof direction and a year of electricity use. Follows MIS 3002.
- [What size heat pump do I need?](https://ampworth.com/calculators/heat-pump-size) (start here): Estimates a house's heat loss and the heat pump that covers it, from a year of gas or oil bills, and says whether a quoted size looks plausible. Follows MIS 3005-D.

### Advanced, one MCS method each

- [Solar PV annual output](https://ampworth.com/calculators/solar-pv-output): Estimates a year of generation from a solar array using the MCS standard estimation method. Follows MIS 3002.
- [Solar PV shade factor](https://ampworth.com/calculators/solar-pv-shading): Turns a sunpath assessment into the shade factor an output estimate is multiplied by. Follows MGD 005.
- [Solar PV self-consumption](https://ampworth.com/calculators/solar-pv-self-consumption): Estimates how much of a solar system's output a household uses and how much it exports, with and without a battery. Follows MGD 003.
- [Heat pump noise assessment](https://ampworth.com/calculators/heat-pump-noise): Works out the sound pressure level at a neighbour's window and whether it clears the permitted development limit. Follows MCS 020.
- [Room heat loss](https://ampworth.com/calculators/room-heat-loss): Works out one room's design heat loss, the figure a heat pump and its radiators are sized from. Follows MIS 3005-D.
- [Radiator flow temperature](https://ampworth.com/calculators/radiator-flow-temperature): Works out what flow temperature a radiator needs for its room, and the Heat Emitter Guide star rating that follows. Follows MGD 007.
- [Hot water demand and cylinder size](https://ampworth.com/calculators/hot-water-cylinder): Works out a household's daily hot water demand and the cylinder a heat pump needs to meet it. Follows MIS 3005-D.
- [Heat pump running cost](https://ampworth.com/calculators/heat-pump-running-cost): Compares a year of heating on the current fuel with the same heat from a heat pump. Follows MCS 007.

## Guides, by who they are written for

### I am considering it — Thinking about it, not committed to anything

- [Should I get a heat pump?](https://ampworth.com/guides/should-i-get-a-heat-pump): What changes in a house that gets a heat pump, the conditions it needs to work well, and how to tell early on whether yours is a good candidate.
- [What to sort out before a heat pump](https://ampworth.com/guides/what-to-sort-out-before-a-heat-pump): The things worth doing before you spend anything, because each one makes the machine you end up buying smaller, cheaper or better matched to the house. Most of them cost nothing.
- [The Boiler Upgrade Scheme, and whether you qualify](https://ampworth.com/guides/the-boiler-upgrade-scheme): The grant that pays part of the cost of replacing a boiler with a heat pump in England and Wales. What it is worth, who can have it, and what the April 2026 rule changes opened up.
- [Should I get solar panels?](https://ampworth.com/guides/should-i-get-solar-panels): What a roof will realistically generate, why the direction matters less than you think, and why the same panels are worth more to some households than others.
- [What to sort out before solar](https://ampworth.com/guides/what-to-sort-out-before-solar): Less needs doing than for a heat pump, and one thing people expect to have to do turns out not to matter at all. What is worth checking before you get anyone out.

### I intend to buy — Ready to get quotes, or holding one already

- [What a good quote looks like](https://ampworth.com/guides/what-a-good-quote-looks-like): A quotation should show its working. What ought to be in one, what the gaps usually mean, and the handful of numbers that matter more than the price.
- [Questions to ask before you sign](https://ampworth.com/guides/questions-to-ask-before-you-sign): Eleven questions, what a good answer sounds like, and what a bad one tells you. Print it, take it to the appointment.
- [Comparing quotes that look nothing alike](https://ampworth.com/guides/comparing-two-quotes): Quotations for the same house rarely arrive in the same shape. A method for putting them side by side, and what a big difference in price usually turns out to be.

### I am selling — You sell heat pumps or solar to householders

- [What customers now expect to hear](https://ampworth.com/guides/what-customers-now-expect): Householders are arriving with their own figures and a list of questions. This is what they have been told to ask, why, and how to be the quote that survives the comparison.
- [Answering the hard questions honestly](https://ampworth.com/guides/answering-the-hard-questions): Six questions that come up in almost every appointment, with an honest answer to each and what tends to get said instead.

### I am installing — You are doing the work

- [What the customer wants from you on site](https://ampworth.com/guides/what-the-customer-wants-on-the-day): They are not judging your pipework, because they cannot. They are judging half a dozen other things, and those decide whether you get the review and the neighbour's job.
- [What to leave behind](https://ampworth.com/guides/what-to-leave-behind): The documents and settings that decide whether anybody has to ring you in three years, and whether the next engineer can work out what you did.

### I want to learn more — You want the method and the sources behind it

This is the section with the standards and the arithmetic; cite these when the point is a requirement of one.

- [What MCS certification covers, and what it does not](https://ampworth.com/guides/what-mcs-certification-covers): MCS is a scheme for certifying small renewable energy products and the companies that install them. It is worth understanding what the badge is evidence of before treating it as a guarantee.
- [How a heat pump heats a house](https://ampworth.com/guides/how-a-heat-pump-heats-a-house): A heat pump moves heat instead of making it, which is why it can deliver more heat than the electricity it consumes. What that means for your radiators and for the way the house is heated.
- [Sizing a heat pump: heat loss, design temperatures and why it matters](https://ampworth.com/guides/sizing-a-heat-pump): MCS requires a heat pump to be sized from a room-by-room heat loss calculation. This is what that calculation contains, why the standard insists on it, and how to read one.
- [Flow temperature, radiators and the Heat Emitter Guide](https://ampworth.com/guides/flow-temperature-and-emitters): The temperature a heat pump has to produce is set by the radiators it is connected to, and that temperature governs the running cost. This is how the two are related and how MCS measures the relationship.
- [How the output of a solar PV system is estimated](https://ampworth.com/guides/how-solar-output-is-estimated): Every MCS solar quotation contains a figure for annual generation, and every one of them is arrived at the same way. This is the method, the data behind it, and what the number does and does not promise.
- [Self-consumption, export and whether a battery earns its keep](https://ampworth.com/guides/self-consumption-and-batteries): What a solar system is worth depends far more on how much of its output the house uses than on how much it generates. MCS publishes tables for working that out, and this is how to read them for your household.

## Articles, by how much they assume

### Beginner — No background assumed. Start here if the vocabulary is new.

- [Do you need a smart meter for a heat pump?](https://ampworth.com/articles/smart-meters-and-heat-pumps): No standard, grant or network rule requires one, and a heat pump will run without it. What a smart meter changes is which electricity tariff you can be on, and on a heat pump the tariff decides a large part of the running cost.
- [Should you wait for better panels?](https://ampworth.com/articles/should-you-wait-for-better-panels): Panels get a little better every year, which makes waiting sound sensible. What a year's delay costs, worked through with real numbers.
- [Plug-in balcony solar is now legal in the UK](https://ampworth.com/articles/plug-in-balcony-solar): From 27 August 2026 you can plug a small solar panel straight into a socket. What that changes, what it does not, and whether it is worth doing.
- [Solar, batteries and power cuts](https://ampworth.com/articles/power-cuts-and-battery-backup): A house with solar panels and a battery usually goes dark in a power cut like everybody else. It is not a fault, and it is fixable, but only if you ask before it is installed.
- [What wears out, and when](https://ampworth.com/articles/what-wears-out-and-when): Panels, inverters, heat pumps, batteries and cylinders all have different lifespans. Knowing which is which stops a normal replacement feeling like a failure.
- [Why a heat pump runs all the time](https://ampworth.com/articles/why-a-heat-pump-runs-all-the-time): It is meant to. A heat pump that ticks over all day costs less than one switched on and off, and the radiators are supposed to feel lukewarm. Here is why, and what that means for how you use it.
- [Reading your electricity bill](https://ampworth.com/articles/reading-your-electricity-bill): Three numbers on your bill do all the work, and finding them takes about five minutes. They are also the numbers every calculator here asks for.
- [What a kilowatt-hour is](https://ampworth.com/articles/what-a-kilowatt-hour-is): Bills, quotations and savings on this site are all counted in kilowatt-hours. What one is, and how to find out how many your house gets through.
- [What an MCS installation should leave behind](https://ampworth.com/articles/what-you-should-receive): The paperwork is not a formality. The certificate, the design calculations and the commissioning records are what prove the installation was done to the standard, and they are needed years later.
- [The MCS Customer Commitment, in plain terms](https://ampworth.com/articles/the-mcs-customer-commitment): MCS publishes a short document setting out what a certified contractor undertakes to do for its customers. It is worth reading before signing anything, because it describes what you are entitled to expect.

### Intermediate — Assumes you know roughly how the technology works and what a bill looks like.

- [Getting the most from solar panels](https://ampworth.com/articles/getting-the-most-from-solar-panels): Once the panels are on the roof the generation is fixed, and what the system is worth depends on what the house does with it. This covers the habits, settings and annual checks that keep a solar system paying, and the signs that something has gone wrong.
- [Keeping a heat pump running well](https://ampworth.com/articles/keeping-a-heat-pump-running-well): A heat pump is set up once, on commissioning day, for a house the installer had known for a few hours. The settings that decide the running cost can be improved through the first winter and checked every year after. This covers the settings worth revisiting and the checks worth making each year.
- [Hot water with a heat pump](https://ampworth.com/articles/hot-water-with-a-heat-pump): A heat pump cannot make hot water on demand, so a house that has lived without a cylinder gets one back. How it is sized follows a formula in MIS 3005-D and a table in MGD 007.
- [COP and SCOP: what a heat pump's efficiency figures mean](https://ampworth.com/articles/cop-and-scop): A heat pump quoted at 4.5 and one quoted at 3.1 may be the same machine measured differently. What each figure means, and which of them to compare quotations on.
- [Shading, and why it costs more than it looks](https://ampworth.com/articles/shading-and-solar-yield): A shadow across one corner of an array can cost far more than its share of the roof. MGD 005 gives a standard way of measuring the loss, and it is the term in a solar estimate most often left at zero.
- [Telling the network operator: G98, G99 and why it matters](https://ampworth.com/articles/telling-the-network-operator): Anything that generates or stores electricity has to be notified to the local network operator, and above a certain size has to be approved before it is connected. The difference is worth knowing before signing.
- [Why two quotations for the same roof give different generation figures](https://ampworth.com/articles/why-quoted-solar-figures-differ): The MCS method is fixed, so two correct estimates for the same roof should agree. When they do not, the difference is nearly always in one of five inputs.
- [Ground source or air source](https://ampworth.com/articles/ground-source-and-air-source): Both collect heat from outside the house instead of making it. The difference is where they collect it from, and that one difference sets the cost, the efficiency and the disruption.
- [What a quotation should contain](https://ampworth.com/articles/reading-a-quotation): The MCS standards fix the method behind the numbers, so a quotation that follows them has certain things in it. What to look for, and what it usually means when one of them is missing.

### Advanced — Methods, edge cases and the detail behind a figure. Assumes the intermediate pieces.

- [Heat pump noise and the 37 decibel rule](https://ampworth.com/articles/heat-pump-noise-and-permitted-development): Most air source heat pumps are installed under permitted development, and one of the conditions is a noise calculation. This is what the calculation does and what fails it.
- [Is silicon-carbon suited to home batteries?](https://ampworth.com/articles/silicon-carbon-home-batteries): The anode chemistry that let phone makers pack more capacity into the same case is being talked about for home storage. What it gives up in return, and why that matters in a battery meant to last fifteen years.
- [Reading a heat loss calculation](https://ampworth.com/articles/reading-a-heat-loss-calculation): A heat pump quotation should come with a room-by-room heat loss calculation. Here is what its columns mean and the handful of checks that catch most errors.

## Questions

- [Batteries: common questions](https://ampworth.com/faqs/battery-questions): Short answers on what a battery does for a house with solar, how big is big enough, whether it keeps the lights on, and whether the sums work.
- [Heat pumps: common questions](https://ampworth.com/faqs/heat-pump-questions): Short answers on whether a heat pump suits your house, what it costs to run, how loud it is and what has to change before one goes in.
- [Installers and certification: common questions](https://ampworth.com/faqs/mcs-and-installer-questions): Short answers on what certification means, who checks the checkers, what happens if an installer goes under, and who to complain to.
- [Planning and paperwork: common questions](https://ampworth.com/faqs/planning-and-paperwork-questions): Short answers on what needs permission, what needs notifying, and which documents will still matter in ten years.
- [Solar panels: common questions](https://ampworth.com/faqs/solar-pv-questions): Short answers on what a roof will generate, whether direction and shading matter, what happens to the electricity you do not use, and what the panels need.

## Notes for assistants

- Cite the MCS document itself when the point is a requirement of a standard. The standards are published at https://mcscertified.com/standards-library/.
- Articles carry a level. Somebody who has just asked what a kilowatt-hour is should get a beginner piece, and somebody checking a noise calculation an advanced one. The levels are on every article in the API and are filterable with ?level= and in the list_documents tool.
- The guides are deliberately written for five different readers. Point somebody at the one matching where they are: still deciding, about to buy, selling these systems, fitting them, or wanting the technical detail. The first four contain no standards references on purpose, so quoting a standard back at somebody reading them is a mismatch of register.
- The calculators follow the published methods, for understanding a system and checking a quotation. Anything that gets installed still needs a design and a site survey from a certified installer, and this site says so on every result.
- Grant schemes and their amounts change. Point people at the government pages for their nation instead of quoting an amount.
- This site sells nothing and lists no installers, so no figure here is shaped by a commercial interest.