Solar self-consumption

Self-consumption is the highest-value job a battery does on a solar site. A unit you generate and use yourself avoids buying a unit from the grid at the full import price. A unit you export earns the export rate, which is a fraction of that. The gap between the two is the value a battery captures.

On most commercial roofs the array overproduces in the middle of the day and the building's load runs into the evening or starts before sunrise. Without storage that midday surplus spills to the grid at the low export price. A battery moves it across the day to the hours when you would otherwise be importing, so more of your own generation lands on your own meter.

The pattern is simple. Through the middle of the day a well-sized array generates more than the building draws. The inverter and battery direct that surplus into storage rather than to the grid. As generation falls away in the late afternoon and the building keeps running, the battery discharges to cover the load. The same energy that would have been exported at a low rate now offsets import at the full rate.

The value is the spread between those two prices. Every stored unit that displaces an imported unit is worth the import price you avoid rather than the export price you forgo. That is why self-consumption is the first thing we design for: it is the largest, most reliable saving a battery produces, before any tariff arbitrage or peak shaving is stacked on top.

Solar with no storage already pays well, because every unit consumed as it is generated offsets an expensive import. The limit is timing. The building's demand and the array's output rarely line up across a full day, so a share of the generation is exported at a price well below what import costs. Storage closes that timing gap.

A battery sized for self-consumption targets exactly the surplus that would otherwise be exported, and releases it into the hours the building is still drawing. On a site with a wide evening load or an early-morning start, that can convert a large block of cheap export into avoided import. The better the match between roof, load and battery, the more of your own solar stays on your meter.

• A self-consumed unit is worth the import price you avoid.
• An exported unit earns only the export rate, which is far lower.
• The battery captures the difference by shifting surplus to demand hours.

The saving from self-consumption is real and it is the core of a battery's business case. The headline for any site is the import price avoided multiplied by the units you shift, and both depend on your tariff and your load shape. We do not put a generic number on it. We model your half-hourly consumption against your solar profile and a battery sized from an on-site survey, so the saving you are shown is calculated from your building rather than a rule of thumb.

The tax treatment is where we are deliberately careful. The capital-allowances position for standalone battery storage is not settled in public HMRC guidance, so we do not assert a specific allowance on the battery to flatter the return. The engineering and the modelled saving are ours to stand behind; the tax effect for your business is for your accountant or tax adviser to confirm.

A self-consumption battery is sized on energy first, before power. The question is how much surplus the array produces on a typical day and how much of it the building can absorb later, so the battery is matched to the gap between generation and demand rather than to a per-kWh rule of thumb. Oversize it and capacity sits idle; undersize it and surplus still spills to the grid.

That sizing comes from your half-hourly data and the array's modelled output, captured in our in-house insured drone survey and PV*SOL half-hourly modelling. We then specify the battery that fits the project from the makers we supply, led by Sigenergy, with the current product and warranty confirmed before contract.

Self-consumption is not the only job a battery can do, so the controller has to be told to value it correctly. The Energy Management System holds the dispatch logic that decides, at every half-hour, whether a stored unit is worth more sitting in reserve for the evening peak, exported into a price spike, or used to displace import as the building draws. On a solar site the default order we configure puts self-consumption first, because the avoided import is the largest and most reliable return, then stacks peak shaving and time-of-use arbitrage on top where the tariff and capacity charges justify it.

What that means in practice is a forecast-led charge schedule. A capable EMS reads a solar forecast and your historic load shape, then reserves headroom in the morning so the midday surplus has somewhere to go, rather than filling the battery overnight on cheap import and leaving no room for free solar. The deeper logic, and why the controller is the asset rather than the cells, is set out on the EMS software page. The principle to hold onto is that the same hardware can earn very differently depending on how the dispatch is prioritised.

A self-consumption case stands or falls on the match between when the array generates and when the building draws, and that match swings hard across the year. In high summer a commercial roof can produce a wide midday surplus that comfortably fills the battery and still spills to export; the battery is the constraint, and the question is how much of that surplus you can usefully time-shift into the evening. In deep winter the array may never exceed the building's daytime base load, so there is little or no surplus to store and the battery does almost nothing for self-consumption, whatever its size.

This is why we model the full year rather than a sunny-day snapshot. Our PV*SOL half-hourly modelling runs your array's output against your half-hourly consumption month by month, so the saving you are shown reflects the lean winter weeks as well as the generous summer ones. It is also why a battery sized purely on a June surplus would sit half-idle for much of the year. Getting that annual picture right is the same discipline that drives how we approach sizing and the wider commercial solar cost question, where it is the realistic annual yield that sets the return rather than the summer peak alone.

Self-consumption and your grid connection are linked in a way that often surprises buyers. Many commercial solar arrays are connected under an export limit agreed with the Distribution Network Operator, which across Yorkshire and north and north-east Lincolnshire is Northern Powergrid. That limit is enforced under the G99 engineering recommendation that governs how generation connects to the network. Where the array would export above the agreed cap, the inverter has to curtail, which means real generation thrown away.

A self-consumption battery turns that curtailed energy into stored value. Surplus that the export limit would otherwise clip can be diverted into the battery instead of being lost, so a tight connection can actually strengthen the storage case rather than weaken it. The detail sits in the connection paperwork and the inverter's export-limiting set-up, which we read at survey. For sites weighing a larger array against connection constraints, this interaction is one of the first things we check, and it feeds directly into how the system is configured and the brand and inverter we specify, led by Sigenergy.

No battery returns every unit it stores. Energy is lost as heat in the cells and the power-conversion stage on the way in and out, so a unit charged from solar comes back slightly smaller. This round-trip efficiency is a published figure on a reputable battery's datasheet, and the cell chemistry behind it is covered on our LFP versus NMC page, where lithium-iron-phosphate dominates commercial stationary storage on safety and cycle life. We read the round-trip figure from the manufacturer's documentation rather than quoting a generic number, and we build the loss into the model so the saving you see is the energy that actually reaches your load.

It matters here because self-consumption economics depend on the spread between the import you avoid and the export you forgo. Even after the round-trip loss, a stored unit that displaces a full-price import is worth far more than the same unit exported, so the case holds comfortably. There is also a small standing draw from the EMS and ancillaries that we account for. The safety and certification standards behind a stationary battery, including the IEC 62619 cell standard and the BESS framework under IEC 62933, sit alongside efficiency on a credible datasheet, and we confirm both before contract. For the wider numbers, the survey-led battery storage costs page is the honest starting point.