How the Home Energy Model Changes What Solar Panels Are Worth on an EPC
How the Home Energy Model Changes What Solar Panels Are Worth on an EPC
The short version
Your EPC’s maths is being rebuilt. The Home Energy Model replaces the monthly averages in SAP and RdSAP with half-hourly simulation, so it can finally see when your solar generates, when you use it, and what a battery does in between. The government has pushed HEM-based EPCs back to the second half of 2027, so nothing changes on your certificate today. When the switch does happen, expect solar-only homes to gain modestly, solar-plus-battery homes to gain most, and export income to stay outside the rating. Existing certificates stay valid; new assessments pick up the new rules.
I’ve been writing about UK solar long enough to have a running grievance with the EPC. Not with the idea of it — with the maths underneath it. For years I’ve heard from readers whose homes run on sunshine for half the year and whose certificate still calls them a D. The Home Energy Model (HEM) is the government’s answer to that complaint, and it genuinely changes what your panels are worth on paper.
Here’s my plain-English read on what shifts, when it shifts, who gains most, and what I’d do differently about assessments, batteries and selling a house because of it.
What your EPC actually measures today
Every EPC lodged on an existing home in England and Wales is currently produced under RdSAP 10 — the reduced-data version of the government’s Standard Assessment Procedure. It boils your home down to a cost figure: estimated energy spend per square metre of floor area, converted into a score from 1 to 100+ and slapped into a band.
| EPC band | SAP score | Where most UK homes sit |
|---|---|---|
| A | 92+ | Rare — typically new builds with heat pumps and solar |
| B | 81–91 | Modern builds, or deep-retrofitted older homes |
| C | 69–80 | The target band for most green finance products |
| D | 55–68 | The UK’s most common band |
| E | 39–54 | Older stock, minimal upgrades |
| F / G | 1–38 | Hard-to-heat homes, often solid wall |
Now the bit that winds solar owners up. Under RdSAP, your panels are credited mainly for annual generation offset against a standardised household’s demand — not your actual kettle habits, your EV, your shift patterns. The calculation runs in twelve monthly steps. Export income from the Smart Export Guarantee doesn’t enter the rating at all. A battery gets only a blunt self-use adjustment rather than a proper simulation of what it does across the day. And smart tariffs don’t exist as far as the maths is concerned.
So the certificate describes a home that doesn’t quite exist: yours, averaged away.
What the Home Energy Model replaces
HEM is DESNZ’s new calculation engine, built to succeed SAP over time — for EPCs on existing homes and for the Future Homes Standard that governs new builds, which is why it also matters if you buy new rather than retrofit. I’ve covered that side separately in our guide to solar panels on new builds. HEM was first consulted on through the Future Homes Standard work, and a dedicated consultation on using it for existing-home EPCs ran from January to 18 March 2026. The technical documents behind it are publicly available on DESNZ’s Home Energy Model programme page if you fancy the unfiltered version.
The headline change is resolution. Where RdSAP works in monthly averages, HEM simulates the home in half-hourly steps across a full reference weather year — including appliance demand, hot water, heating controls, generation, storage and smart switching. It’s the difference between a photograph and a film.
Timetable, honestlyThe switchover has slipped, and much of what’s still online is out of date. Earlier plans pointed to October 2026; in March 2026 the government confirmed HEM-based EPCs are now expected in the second half of 2027. Until then, every EPC lodged is still an RdSAP 10 product. The Future Homes Standard comes into force in March 2027, but SAP 10.3 is the compliance route at launch, with HEM following later. Scotland and Northern Ireland run their own EPC regimes and haven’t committed to following England and Wales. Nothing below invalidates a certificate you already hold — EPCs last ten years.
Twelve buckets vs 17,520 half-hours
A year contains 17,520 half-hours. RdSAP looks at twelve of everything. That single design choice is why solar gets short-changed today: a monthly average can’t see that your panels peak at 1pm while your demand peaks at 7am and 6pm. It can’t see summer surplus or winter shortfall within a day. It certainly can’t see a battery soaking up midday generation and handing it back over your evening curry.
Why does resolution favour solar? Because the value of a panel in a home is mostly about coincidence: how much generation lands in the same half-hour as something switching on. HEM can see that coincidence. It can see an immersion heater diverting surplus at noon, an EV charging off the roof at 1pm, a heat pump pre-heating before the evening peak. RdSAP smears all of it into a monthly blur and credits you for an average that never actually happened.
The battery blind spot gets fixed
This is the change I care about most. Under today’s RdSAP 10, a battery only earns a blunt self-use adjustment, because a monthly calculation simply can’t represent storing your own sunshine and using it at 8pm. HEM simulates charge and discharge across the day, so a battery finally shows up as what it is: a device that moves free electricity from the hours you don’t need it to the hours you do.
In practice that makes solar-plus-battery the biggest gainer from the whole reform. If you’ve been sitting on the fence about retrofitting storage to an existing array, this is one more nudge — we’ve crunched the retrofit numbers in our guide to adding a battery to existing solar panels, and the broader market picture is in our UK battery storage hub.
Smart tariffs sit in the same bucket. HEM is built to model time-of-use pricing and smart controls, which is how many battery homes actually make their money. Whether EPC assessments will run on your real tariff or a standardised one is still being settled in the methodology work, and I won’t pretend otherwise. Either way, the capability is a step change from a model that pretends everyone pays one flat rate forever.
One letter becomes four metrics
There’s a second change that matters even more to solar owners than the half-hourly maths, and it’s the one most articles skip. The government’s proposals for reformed EPCs don’t just recalculate the letter — they replace the single A–G rating with four headline metrics: Fabric Performance, Heating System, Smart Readiness and Energy Cost. A legacy A–G rating is planned to run alongside them until the end of 2029, so nothing that relies on today’s bands falls off a cliff overnight.
Why does that matter for panels and batteries? Because they do nothing for your walls or your boiler. Under one blended letter, solar could only ever nudge an overall score dominated by fabric and heating. Under four metrics, you’d logically expect solar and storage to show up mainly in Energy Cost and Smart Readiness instead of being averaged away. I say “logically expect” on purpose: exactly what feeds each metric is still being settled through the consultation, so treat this as direction of travel, not a guarantee.
Do solar panels improve your EPC rating? Where HEM helps — and where it doesn’t
| Feature of your system | RdSAP 10 today | HEM as consulted and planned |
|---|---|---|
| Solar self-consumption | Annual generation offset against standardised demand | Matched against half-hourly demand profiles |
| Battery storage | Blunt self-use factor adjustment only | Charge/discharge simulated across the day |
| Smart tariffs & controls | Not modelled | Time-of-use and smart switching modelled; tariff source still being finalised |
| SEG export income | Excluded from the rating | Not expected to count; Energy Cost metric detail still being finalised |
| Solar + heat pump interaction | Crude combined allowance | Integrated half-hourly modelling of generation against heat pump runtime |
| Orientation, tilt, shading | Already modelled | Retained, within a finer overall simulation |
For scale, a word of caution against expecting miracles. Under the current maths, solar can give a meaningful bump — enough to tip a home over a band boundary if it’s already close — but rarely a whole band on its own. HEM doesn’t turn panels into a band-jumping machine either. Fabric and heating still dominate the score. What it does is stop the maths ignoring half of what your system actually achieves.
And where it won’t help: an oversized array that exports most of its output won’t score proportionally better, because the rating rewards matched use, not export volume. Uncertified, undocumented kit won’t earn its keep in an assessment either — assessors need evidence, which is why the paperwork section below matters. And none of this touches your SEG cash — that lives entirely outside the certificate, which is why it’s still worth checking you’re on a competitive exporter via our SEG rate comparison.
Sellers, landlords and band boundaries
Band boundaries are where EPCs bite in the real world: green mortgage pricing, estate agent brochures, and the perennial argument about whether landlords will be pushed towards upgrades again. For now, the certificate you hold or commission today is the one that counts for sales, lettings and grant eligibility, and the minimum for private rentals is still EPC E. The bigger political question hasn’t gone away — our piece on whether landlords are about to be forced to fit solar under EPC reforms tracks it, and it’s worth a read before you assume a scrapped mandate stays scrapped.
If you’re selling and you’ve added panels or a battery since your last assessment, commissioning a fresh lodgement after the works is often the cheapest marketing money you’ll spend. A certificate that predates your system is a certificate that undersells the house. We’ve written up the full selling side, including buyer questions and paperwork handover, in our guide to selling a house with solar panels. And if you’re wondering whether the panels add value beyond the certificate, that’s a separate question with a separate answer — we’ve dug into whether solar panels increase house value too.
One timing nuance worth planning around: once HEM-based assessments arrive, a solar-plus-battery home assessed under the new maths should present better than the same home assessed under the old. But with the launch now pencilled in for the second half of 2027, that’s a long wait. If you’re selling or remortgaging in the next year, an assessment under today’s rules is still the practical route — you can always commission another later if the new method treats you better.
Paperwork worth keeping
HEM-leaning assessments are likely to lean harder on evidence than assumption. Whether you’re assessed next month or next decade, keep these somewhere findable:
- Your MCS certificate — proof the install meets the standard schemes and exporters expect. If you’re unsure what the badge actually guarantees, our explainer on MCS-certified installers covers it.
- Commissioning sheets and inverter/battery datasheets — including usable battery capacity in kWh, not just the marketing number.
- Smart control and tariff evidence — tariff name, controller settings, diverter or immersion schedules.
- A year of monitoring history — generation, export and self-consumption traces. Our rundown of solar monitoring systems explains what good data looks like.
Don’t buy hardware for the certificateThe money in solar lives in your bills and your export payments. The EPC is a shadow those savings cast, not the savings themselves. If upfront cost is the barrier, check what’s currently claimable in grants and VAT relief before you commit — the 0% VAT rate on domestic solar and battery installation runs to 31 March 2027, after which it reverts to 5%, and our UK solar grants guide tracks the live schemes.
My bottom lineThe certificate was never the reason to go solar. But for two decades it’s been the paper that decides how buyers, lenders and landlords judge your home. Soon, at least, it’ll be doing the sums properly — and homes with panels, storage and a bit of smart thinking will finally be described accurately.
FAQs
Will my existing EPC change when the Home Energy Model arrives?
No. Your certificate stays valid for its full ten-year life with the rating it was lodged with. HEM-based EPCs aren’t expected until the second half of 2027, and the new methodology will only apply to assessments carried out after that switch. If you’ve added solar, a battery or a heat pump since your last assessment, you can pay for a fresh assessment now under the current RdSAP 10 rules — and again later if you think HEM will treat you more kindly.
Do solar panels still improve my EPC rating under the Home Energy Model?
Yes, and generally more fairly than today. Half-hourly simulation credits the generation you actually use in the home, and gives far more weight to storage and smart controls than the current method. Solar-only homes should see a modest uplift; solar-plus-battery homes the largest. It won’t turn a G into a B on its own — fabric and heating still dominate the score.
Will my SEG export income count towards my EPC rating?
Not under today’s rules, and I wouldn’t count on it under the new ones. The current EPC cost metric doesn’t include export income, and I haven’t seen anything in the published Home Energy Model material that adds it, though the exact Energy Cost metric is still being finalised. Your export payments are real money regardless — they just live in your bank account rather than your certificate. Compare current SEG tariffs to make sure you’re on a decent one.
