Solar PV and Battery Storage in HEM — Technical Guide

The Home Energy Model (HEM) models solar PV generation and battery storage at half-hourly resolution — a transformative improvement over SAP's monthly approach. Using the methodology defined in HEM-TP-18, HEM calculates PV output at each of 17,520 timesteps per year based on actual solar irradiance data, panel orientation, tilt angle, shading, and inverter efficiency. It then determines how much of that generation is consumed on site, how much is stored in batteries, and how much is exported to the grid. This is the first time a UK regulatory energy model has been able to represent the real-world behaviour of on-site generation and storage systems.

HEM-TP-18: PV Generation and Self-Consumption

HEM-TP-18 is the technical paper that defines how the Home Energy Model handles on-site photovoltaic generation and electricity self-consumption. Published as part of the HEM technical documentation on GOV.UK, this module sits within the electricity balance step of the core calculation loop. At each half-hourly timestep, after space heating, cooling, and hot water demands have been determined, HEM-TP-18 calculates how much electricity the PV array generates and how that generation interacts with household demand and any battery storage present.

The module draws on solar irradiance data already computed by the external conditions module (HEM-TP-03) and the solar gains module (HEM-TP-08), which calculates direct and diffuse irradiance on surfaces of any orientation and tilt using BS EN ISO 52010-1:2017. HEM-TP-18 then applies panel-specific and inverter-specific parameters to convert irradiance into usable AC electricity.

Half-Hourly PV Generation Modelling

At the heart of HEM's PV modelling is a half-hourly generation calculation that accounts for the actual solar conditions at each timestep. This contrasts sharply with SAP, which estimates an annual PV yield and distributes it across months using fixed proportions.

Irradiance Calculation

For each 30-minute period, HEM determines the total solar irradiance incident on the plane of the PV array. This calculation considers:

• Direct (beam) irradiance — sunlight arriving directly from the sun, dependent on the solar altitude, azimuth, and the angle of incidence on the panel surface
• Diffuse irradiance — sunlight scattered by the atmosphere and clouds, arriving from the whole sky hemisphere
• Ground-reflected irradiance — sunlight reflected from surrounding surfaces onto the panel, calculated using a ground reflectance (albedo) factor

The irradiance data comes from the hourly weather file (in CIBSE TRY or EPW format) and is interpolated to half-hourly resolution. The sun position — altitude and azimuth — is calculated geometrically for the site latitude, longitude, and the exact date and time of each timestep.

Panel Orientation and Tilt Angle

The orientation (azimuth) and tilt angle of the PV array directly affect how much irradiance reaches the panel surface. HEM models this by computing the angle of incidence between the sun's position and the panel normal at each timestep:

• Orientation (azimuth) — measured as degrees from due south (0° = south, 90° = west, −90° = east). South-facing arrays in the UK receive the greatest total annual irradiance, but east–west split arrays can improve self-consumption by spreading generation across morning and afternoon periods.
• Tilt angle — measured from horizontal. In the UK, a tilt of approximately 30–40° maximises annual yield for south-facing panels. Lower tilts favour summer generation; steeper tilts favour winter performance.
• Multiple arrays — HEM can model separate PV arrays with different orientations and tilts on the same dwelling (e.g. panels on both east-facing and west-facing roof slopes), calculating generation independently for each array and summing the results.

Shading Effects

Shading can significantly reduce PV output. HEM accounts for shading through overshading factors that reduce the irradiance reaching the panel surface at each timestep. The shading model considers:

• Horizon shading — obstructions on the distant horizon (hills, treelines) that block direct beam irradiance at low solar altitudes, particularly relevant during winter months
• Nearby obstructions — adjacent buildings, chimneys, dormers, or trees that cast shadows on the array. Because these shadows move throughout the day as the sun's position changes, the half-hourly resolution captures the time-varying nature of shading far more accurately than a fixed annual factor.
• Self-shading — for arrays arranged in multiple rows (less common on domestic rooftops), HEM can account for one row shading the row behind at low sun angles

The shading input is specified as a series of factors that reduce incident irradiance. Where detailed shading analysis has not been performed, HEM allows simplified overshading categories (heavy, more than average, average, modest), but detailed site-specific shading data will always produce more accurate results.

Inverter Specifications and Efficiency

The inverter converts DC electricity from the PV panels into AC electricity for household use or grid export. Inverter efficiency is not constant — it varies with the load on the inverter relative to its rated capacity. HEM models this relationship explicitly.

Inverter Efficiency Modelling

At each timestep, HEM applies the inverter efficiency to the DC output from the panels to determine the usable AC electricity. The key parameters are:

• Rated power (kW) — the maximum AC output of the inverter. If DC generation exceeds rated power, the inverter clips the output to its rated capacity.
• Efficiency at rated output — typically 95–98% for modern inverters. HEM can accept either a single rated efficiency or a load-dependent efficiency curve.
• Part-load efficiency — inverters are less efficient at very low loads (early morning, late afternoon, cloudy conditions). A load-dependent efficiency curve captures this, while a single rated value may overstate output during low-generation periods.
• Standby consumption — the electricity consumed by the inverter when the array is not generating (overnight). This is subtracted from the overall electricity balance.

String Inverters vs Micro-Inverters

HEM accommodates both common inverter configurations. With a string inverter, a single unit handles the entire array (or a string of panels), and shading on any one panel can reduce output from the entire string. With micro-inverters, each panel has its own inverter, meaning shading on one panel does not affect the others. This distinction matters for the accuracy of generation estimates, particularly on partially shaded roofs.

Self-Consumption Calculation

Self-consumption — the proportion of PV-generated electricity that is used directly within the dwelling rather than exported to the grid — is one of the most important metrics that HEM can calculate and SAP cannot. At each half-hourly timestep, HEM compares the PV generation with the total household electricity demand at that moment:

• If generation exceeds demand, the surplus is either directed to battery storage (if present and not fully charged) or exported to the grid
• If demand exceeds generation, the shortfall is met by discharging battery storage (if available) or importing from the grid
• If generation matches demand, self-consumption is 100% for that timestep and no grid interaction occurs

The household electricity demand at each timestep includes all electrical loads modelled by HEM: the heat pump compressor and controls, MVHR fan energy, lighting, cooking, appliances, and any electric immersion heater top-up for hot water. Because heat pump electricity consumption is typically the largest single electrical load in a Future Homes Standard dwelling, the timing interaction between PV generation and heat pump demand is critical.

Interaction with Heat Pump Electricity Consumption

The interaction between PV generation and heat pump demand is one of the most consequential modelling improvements in HEM. In winter, when PV output is low but heating demand is high, the heat pump draws heavily from the grid. In spring and autumn, moderate PV generation may partially offset the heat pump's consumption during daytime heating periods. In summer, space heating demand drops to near zero, but hot water heating via the heat pump (or an immersion diverter) can absorb some surplus PV generation.

HEM resolves all of these interactions at half-hourly intervals. The heat pump module (HEM-TP-12) calculates the electricity consumed by the heat pump at each timestep based on the heating load and the coefficient of performance (COP) at that moment. The PV module then determines whether that consumption can be met by on-site generation, battery discharge, or grid import.

Battery Storage Modelling

HEM is the first UK regulatory energy model to include battery storage modelling. SAP has no capacity to represent battery systems at all — a significant limitation given that batteries are increasingly common in new-build homes and critical for maximising the value of on-site PV generation.

Charge and Discharge Behaviour

At each half-hourly timestep, HEM runs a charge/discharge algorithm for the battery:

1. Surplus PV available? — if PV generation exceeds household demand and the battery is not fully charged, the surplus is directed to the battery at a rate up to the maximum charge power
2. Demand exceeds PV? — if household demand exceeds PV generation and the battery has stored energy above its minimum state of charge, the battery discharges to meet the shortfall at a rate up to the maximum discharge power
3. Efficiency losses applied — charging and discharging incur round-trip efficiency losses (typically 85–95% for lithium-ion systems), which HEM applies to each charge/discharge cycle
4. State of charge updated — the battery's state of charge is carried forward to the next timestep, creating a continuous simulation of battery behaviour throughout the year

Battery Specification Parameters

HEM requires the following data for each battery system:

• Usable capacity (kWh) — the total energy storage available between maximum and minimum state of charge
• Maximum charge power (kW) — the maximum rate at which the battery can accept energy
• Maximum discharge power (kW) — the maximum rate at which the battery can release energy
• Round-trip efficiency (%) — the proportion of energy recovered after a full charge/discharge cycle, accounting for conversion losses
• Minimum state of charge (%) — the lowest level to which the battery will discharge, protecting battery longevity

Where specific battery product data is not available, HEM applies default values that are conservative — understating the battery's contribution to self-consumption. As with all HEM inputs, providing accurate manufacturer data from the Product Characteristics Database (PCDB) or equivalent source will produce more representative results.

Grid Import/Export Balance

After resolving PV generation, self-consumption, and battery charge/discharge at each timestep, HEM calculates the net grid interaction:

• Grid import — electricity drawn from the national grid when household demand exceeds the combined output of PV generation and battery discharge
• Grid export — surplus electricity sent to the grid when PV generation exceeds household demand and the battery is fully charged (or no battery is present)

The annual totals for grid import and export are key outputs of the HEM calculation. For FHS compliance, the Target Emission Rate (TER) and Target Primary Energy Rate (TPER) both account for the carbon and primary energy implications of grid electricity at each timestep using forward-looking emission factors (based on the projected 2025–2029 grid mix). Exported electricity receives a credit that partially offsets the carbon and primary energy of imported electricity.

This granular import/export accounting is impossible with SAP's monthly resolution. SAP can only estimate a lump annual PV yield without knowing when that electricity was generated or consumed.

Time-of-Use Tariff Implications

HEM's half-hourly resolution opens up a significant new capability: meaningful time-of-use tariff analysis. Because the model knows exactly when electricity is imported and exported, it can distinguish between:

• Cheap off-peak imports — electricity drawn overnight or during low-demand periods when unit rates are lowest (e.g. under Octopus Go, Intelligent Octopus, or Economy 7 tariffs)
• Expensive peak imports — electricity drawn during late afternoon and evening demand peaks when unit rates are highest
• Export value timing — the value of exported electricity varies depending on when it reaches the grid, with potential for higher payments during peak periods under emerging agile export tariffs

While the FHS compliance wrapper currently applies standardised assumptions rather than real tariff data, the half-hourly output from HEM provides the foundation for future policy that could credit smart battery operation, demand shifting, and tariff optimisation. This is directly relevant to the government's broader smart energy strategy and the rollout of smart meters.

SAP's monthly resolution makes time-of-use analysis meaningless — it cannot distinguish between electricity consumed at 2am and electricity consumed at 6pm.

SAP vs HEM — Solar PV Modelling Comparison

The table below summarises the fundamental differences in how SAP and HEM handle solar PV and battery storage:

For the full technical comparison across all HEM modules, see the HEM Technical Reference. For a non-technical overview of the differences, see SAP vs HEM.

FHS Mandatory Solar Requirement

Solar PV is not merely a compliance option under the Future Homes Standard — it is becoming a functional requirement. In June 2025, the government announced that solar PV would be mandatory for new-build homes under the FHS, moving beyond the consultation position where solar was strongly encouraged but not formally required.

40% Floor Area Coverage Target

The solar requirement targets PV coverage equivalent to approximately 40% of the ground floor area of the dwelling. For a typical three-bedroom semi-detached house with a ground floor area of around 45 m², this equates to roughly 18 m² of PV panels — approximately 3.0–3.6 kWp depending on panel efficiency.

The 40% target is not a rigid rule applied identically to every dwelling. Adjustments are expected for:

• Flats and apartments — shared roof space means individual coverage targets may be lower, with communal PV arrays shared between units
• Terraced properties — limited roof area relative to floor area may require the target to be assessed on a per-plot basis
• North-facing roofs — where the primary roof slope faces north, east–west split arrays or reduced coverage with higher-efficiency panels may be accepted

Exemptions and Exceptional Circumstances

The government has indicated that exemptions from the solar requirement will apply only in genuinely exceptional circumstances:

• Severe shading — where site-specific shading analysis demonstrates that PV generation would be so low as to render installation impractical
• Heritage constraints — conservation areas or listed building adjacency where planning conditions prohibit visible PV installation
• Structural limitations — where the roof structure cannot support the additional load of PV panels without disproportionate structural reinforcement

Even where the full 40% target cannot be met, homes must be “solar ready” — with suitable roof orientation, structural capacity, and electrical infrastructure to allow PV installation in the future.

Product Data Requirements

HEM's accuracy depends on the quality of the product data provided for PV panels, inverters, and battery systems. The model requires significantly more detailed specifications than SAP:

Panel Specifications

• Rated power output (Wp) — the peak watt rating under Standard Test Conditions (STC: 1000 W/m² irradiance, 25°C cell temperature)
• Panel area (m²) — used to determine the total array area and coverage relative to the FHS target
• Temperature coefficient (%/°C) — PV panel output decreases as cell temperature rises above 25°C. HEM uses this coefficient to adjust generation at each timestep based on ambient temperature and irradiance-driven cell heating.
• Nominal Operating Cell Temperature (NOCT) — used in conjunction with ambient temperature and irradiance to estimate the actual cell temperature at each timestep

Inverter Specifications

• Rated AC output power (kW)
• Efficiency at rated output (%) or load-dependent efficiency curve
• Standby power consumption (W)
• Inverter type (string, micro-inverter, or hybrid with battery integration)

Battery Specifications

• Usable storage capacity (kWh)
• Maximum charge and discharge power (kW)
• Round-trip efficiency (%)
• Minimum state of charge (%)

The Product Characteristics Database (PCDB), currently being revised for HEM, will be the primary source for verified product data. Manufacturers will submit test data for their products, and assessors will select from the database during the assessment process. Where a product is not yet in the PCDB, HEM applies conservative default values that are likely to penalise the assessment result — creating a strong incentive for manufacturers to register their products promptly.

Practical Implications for Assessors and Designers

The shift from SAP's simple annual yield calculation to HEM's half-hourly PV and battery modelling has significant practical consequences:

System Design Optimisation

Because HEM rewards genuine self-consumption and battery performance, system designers can now optimise PV and battery configurations with confidence that the compliance model will reflect real-world benefits. Key design decisions that HEM properly values include:

• East–west split arrays — spreading generation across morning and evening periods to improve self-consumption, even though total annual yield is slightly lower than a south-facing array
• Right-sized battery capacity — matching battery size to the gap between daytime generation and evening demand, rather than simply installing the largest available system
• Inverter sizing — selecting an inverter rating that balances peak generation capture against efficiency at typical operating loads
• Heat pump scheduling — where controls permit, running the heat pump preferentially during high-generation periods to absorb PV output directly rather than exporting it

Data Collection Requirements

SAP assessors transitioning to HEM will need to collect considerably more data for PV and battery systems. Under SAP, a PV system requires little more than the array size, orientation, and tilt. Under HEM, assessors must also specify inverter make and model (or manual specifications), battery system parameters (if present), detailed shading data, and panel specifications including temperature coefficients.

The government's consultation response acknowledged that HEM assessments take significantly longer than SAP — approximately 1 hour 40 minutes for a standard house type versus around 20 minutes in SAP. The additional PV and battery data contributes to this increase. Setting up a project data room from the design stage, with all product specifications readily available, is strongly recommended.

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