Solar self-consumption estimates

Estimating how much PV is used on site from time-matched generation, demand, storage and flexible loads.

Solar self-consumption is the part of PV generation used within the property rather than exported. Self-sufficiency is the part of the property’s electricity demand supplied by its own PV and associated storage.

They answer different questions:

self-consumption = on-site use of PV energy ÷ total PV generation

self-sufficiency = on-site use of PV energy ÷ total site electricity demand

A small array can have high self-consumption because the home absorbs nearly all it produces, yet provide low self-sufficiency because it covers little of total demand.

Annual totals are not enough

PV energy can only supply a load when both occur at the same time, unless storage moves energy between intervals. A household using the same annual kWh as its array generates will still import at night and export on bright days.

The best estimate uses interval generation and demand data. Half-hourly data can be useful for domestic appraisal, while finer data better represents short appliance cycles and rapidly changing solar. The model should cover a full year and preserve seasonal patterns.

If measured demand is unavailable, state the occupancy profile, annual consumption and assumptions used. A generic “typical household” percentage is not a property estimate.

Start with direct self-consumption

For each interval, direct PV use is the smaller of PV generation and site demand before discretionary storage charging. Summing those intervals gives direct annual use.

Important demand-profile inputs include:

background loads
weekday and weekend occupancy
cooking and appliance timing
electric hot water
heat-pump and direct-electric heating
EV arrival, departure and energy need
seasonal or holiday absence

Changing total annual demand without changing its time profile can misstate the benefit. A heat pump adds large winter demand when PV is weakest. An EV adds flexible demand only while it is at home and connected.

Modelling a battery

Storage can charge from PV surplus and discharge later, but it cannot move all surplus without loss or limit. The model needs:

usable energy allocated to solar shifting
maximum charge and discharge power
charge and discharge efficiency or round-trip losses
minimum and maximum state of charge
backup reserve
standby consumption
control priority and tariff schedule
temperature or state-dependent limits where material

Do not allocate the same battery capacity simultaneously to backup, tariff arbitrage and PV capture. If the controller grid-charges before a sunny day, it may leave less space for solar.

The current MCS battery-storage standard refers to MGD 003 for domestic PV self-consumption estimates. Where the proposed use falls outside that method, a suitable proprietary model can be used, but its inputs and allocation assumptions should be disclosed.

EV charging

An EV can absorb substantial daytime surplus when it is plugged in. The estimate should use the vehicle’s actual location and energy requirement, not its full battery capacity every day.

Solar-aware charging also has power constraints. The chargepoint and vehicle have a minimum supported current, so a small or fluctuating surplus may cause import, pausing or a mixed solar-and-grid session depending on settings. Dynamic load control and a home battery can change the result further.

Model departure reserve first. Maximising PV use is not successful if the car lacks the energy required for travel.

Heat pumps and hot water

A heat pump increases electricity demand, but the strongest space-heating demand normally occurs when solar production is low. Annual PV generation should not be subtracted from annual heat-pump consumption as though they coincide.

Thermal storage and weather-compensated operation may move some heating or hot-water demand into brighter periods. Any assumed preheating must respect comfort, cylinder temperature, heat-pump efficiency and control limits.

An immersion diverter can use surplus after other priorities. Its model should stop when the cylinder thermostat is satisfied and account for heat that would otherwise have been provided by another source.

Financial value is a second calculation

One self-consumed kWh avoids an import only if it displaces energy the household would otherwise have bought. Exported energy may earn a tariff, so the marginal value is generally:

avoided import value - export value forgone - conversion or cycling cost

Import and export prices change. Keep them in a dated financial model rather than embedding a current pence-per-kWh claim in the technical article. Include tariff windows, standing controls and battery wear only where the contract and assumptions support them.

Measuring after installation

Useful monitoring separates:

PV generation
grid import and export
battery charge and discharge
EV or other controlled load where relevant

With a simple no-storage system, PV generation minus export is a practical measure of PV used on site. With grid-charged storage or battery export, net meter totals can mix energy sources. Use interval flows or inverter accounting that can attribute the battery energy correctly.

Compare actual operation with the assumed occupancy and control strategy before blaming the hardware. A self-consumption estimate is particularly sensitive to behaviour changes.

Estimate inputs and assumptions

The estimate records:

annual PV generation method
demand data period and interval
occupancy and flexible-load assumptions
direct PV use before storage
battery usable allocation, power and losses
EV availability and required driving energy
heat-pump and hot-water seasonal treatment
predicted export, self-consumption and self-sufficiency
dated tariff assumptions in a separate financial case
sensitivity to different behaviour or system sizes