Half-hourly data and self-consumption — the number that decides your solar payback

Ask most people what makes commercial solar pay and they'll say "how much electricity the panels generate." That's the headline number on every quote, and it's almost the wrong number to focus on.

The figure that actually decides your payback is self-consumption: the share of what you generate that you use on site, in real time, instead of exporting to the grid. Get that right and the economics are strong. Get it wrong and a big, impressive array underperforms its own brochure. This guide explains why, and how to nail it down before you sign anything.

Why self-consumption is the whole game

Every unit of solar electricity has two possible values, and they are wildly different:

• A unit you use on site is worth what you'd otherwise have paid your supplier — typically 25–35p/kWh for a UK business in 2026 once you include all the non-commodity bits of the bill.
• A unit you export earns the Smart Export Guarantee — currently around 8.5–15p/kWh depending on tariff (more on real rates in our grants and funding guide).

So a self-consumed unit is worth roughly two to four times an exported one. The same solar panel, in the same location, generating the same kilowatt-hour, can be worth 30p or 10p depending entirely on whether you happen to be using power at the moment it's produced.

That's why two businesses with identical arrays can have payback periods years apart. The one whose demand lines up with the midday solar peak self-consumes 70–80% and pays the system off fast. The one that's empty during the day exports most of it and waits much longer.

What "half-hourly data" actually is

If your business is on a larger supply (broadly, anything with a half-hourly or "HH" meter — common above roughly 100 kVA, and increasingly standard as the rollout continues), your meter already records your consumption in 48 half-hour slots per day. Your supplier holds this data and must give it to you on request.

That dataset is gold for a solar decision. It tells you not just how much electricity you use, but when — hour by hour, day by day, across the year. And "when" is exactly what determines self-consumption.

If you're on a smaller, non-half-hourly supply, you won't have this granularity. You can still estimate self-consumption from your daily operating pattern, but be more conservative, and be more sceptical of any installer who quotes you precise self-consumption figures off a single annual kWh total — they're guessing.

How to get your half-hourly data

1. Ask your supplier for "12 months of half-hourly consumption data" for your meter (MPAN). Most provide it as a CSV by email or through an online portal. It's your data; they shouldn't charge meaningfully for it.
2. Or ask your broker if you bought energy through one — they can often pull it faster.
3. Or use a data service that connects to your meter directly.

Twelve months matters: it captures the full seasonal swing in both your demand and the solar resource. A summer snapshot flatters the system; a winter one buries it.

Reading the data: line up two shapes

Modelling self-consumption is, at heart, overlaying two daily profiles:

• Your demand curve — when your building actually draws power.
• The solar generation curve — a predictable bell shape, low at the edges of the day, peaking around midday, much taller in summer than winter.

Self-consumption is the area where those two curves overlap. Some patterns overlap beautifully; some barely touch.

Great fit — high self-consumption:

• Manufacturing, workshops, warehouses running daytime shifts
• Offices with weekday 8–6 occupancy and air conditioning (AC load peaks with the sun)
• Cold stores, data rooms, anything with a steady daytime baseload
• Farms with daytime irrigation, ventilation or refrigeration

Poor fit — low self-consumption:

• Premises that are mostly empty in daylight and busy in the evening
• Pure overnight operations
• Highly seasonal sites that close over summer when generation peaks

Most real businesses sit somewhere in between, which is precisely why you model it from data rather than guess.

Sizing the array to your demand, not your roof

Here's where the data changes the design. The instinct is to fill the roof. But if your demand can only absorb, say, 60 kW at midday, a 150 kW array spends the sunniest hours dumping two-thirds of its output to the grid at the low export rate.

A well-sized commercial array is matched to your daytime baseload — the floor of demand that's reliably present whenever the sun is up. Generation up to that floor is almost entirely self-consumed and earns the full retail-offset value. Generation above it spills to export and earns far less.

Sometimes the right answer genuinely is to fill the roof — if export economics stack up, or you're planning to add load (an EV charging fleet, a heat pump, a second shift). But that should be a deliberate decision grounded in your profile, not a default.

Where batteries fit — and where they don't

If your demand curve is shifted away from midday — you generate a surplus at noon but need power at 7am or 9pm — a battery can store the midday spill and release it when you need it, converting low-value export into high-value self-consumption.

That only pencils when the gap between your generation and demand is large and predictable enough to justify the capital. For a daytime-shift business that already self-consumes 75%, a battery adds cost for little gain. We walk through the cases where storage does and doesn't pay in our battery storage guide.

The red flag this guide exists to catch

If an installer quotes you a payback period without ever asking for your half-hourly data — or your daily operating pattern at minimum — be cautious. They're modelling on a generic self-consumption assumption (often a flattering 50–70%) that may have nothing to do with your building. A serious installer will ask for the data, or tell you honestly that without it the numbers carry a wide error bar.

This is the single most useful question you can ask to separate the consultative installers from the box-shifters: "What self-consumption rate are you assuming, and what's it based on?"

Sanity-check, in one line each

1. Self-consumed power is worth 2–4× exported power.
2. Self-consumption depends on when you use power, not just how much.
3. Your half-hourly data is the evidence — request 12 months from your supplier.
4. Size the array to your daytime baseload, not the roof area.
5. Batteries help only when there's a big, predictable generation-vs-demand gap.
6. Any quote built on an unexplained self-consumption figure is a guess.

The bottom line

Commercial solar is not really an electricity-generation investment — it's an electricity-substitution investment. The value lives in the units you use yourself. Before you compare quotes, get your half-hourly data, understand your daytime demand shape, and insist that every quote is modelled against it. It's the difference between a system that pays back in single-digit years and one that quietly disappoints.

To see how self-consumption assumptions move the numbers, run the calculator and compare scenarios. To pressure-test a quote, read reading a commercial solar quote. For a monthly dose of this kind of plain-English guidance, subscribe to the Brief.

General information, not financial advice.