How half-hourly demand data changes commercial solar sizing
A working technical guide to sizing commercial solar against half-hourly demand profiles rather than headline annual consumption. Why annual consumption misleads, how to obtain half-hourly data, what the analysis looks like, and how the optimal size decision lands.
Why annual consumption is misleading
A commercial site consuming 800 MWh per year could be: a 24/7 logistics operation with flat overnight load (suggests 1.2–1.5 MWp solar with significant self-consumption), a daytime-heavy manufacturer (suggests 700 kWp at very high self-consumption), a school with summer shutdown (suggests 350 kWp because excess summer generation has nowhere to go), or a hotel with seasonal variation (suggests battery-storage-paired sizing). Same annual consumption, four different optimal answers.
The unhelpful default is sizing solar to "60% of annual consumption" or some similar rule-of-thumb. This works on average but is wrong on most specific sites. The right approach: model self-consumption percentage at a range of system sizes against half-hourly demand, and pick the size that maximises NPV.
Step 1: Obtain the half-hourly file
For sites on a half-hourly settled tariff (most commercial sites above ~100 kW peak demand), the supplier holds the data and is required to provide it on request. Standard request:
"To [supplier customer service]: please provide our half-hourly consumption data (HH data, sometimes called AMR data) for the past 12 months for our supply at MPAN [your MPAN]. CSV format with date, half-hour-period, and kWh consumption columns. This is for energy efficiency and renewable feasibility analysis."
Suppliers typically respond within 5 working days. No charge — they're required to provide this on request.
If your site is on a monthly-settled tariff (rare for above ~100 kW but possible), you have two workarounds: (a) install a temporary smart meter or current-transformer logger for 4–6 weeks across a representative season; (b) use industry-standard load profile estimates calibrated to monthly consumption (workable for offices and retail but inadequate for industrial or seasonal businesses).
Step 2: Profile the demand
A useful first pass on the data: load it into a spreadsheet or analysis tool and compute four summary statistics:
- Mean demand (kW) — annual consumption ÷ 8,760 hours
- Maximum demand (kW) — highest single half-hour reading
- Minimum demand (kW) — lowest single half-hour reading (typically overnight or weekend)
- Standard deviation of demand — measure of demand variability
Three useful ratios from these:
- Min/mean ratio — if above 0.5, demand is fairly continuous; below 0.2, demand is highly variable
- Max/mean ratio — if below 2, demand is fairly flat; above 3, demand has substantial peaks
- Daytime/24h ratio — fraction of consumption during 8am–6pm. High (above 0.6) = daytime-heavy, suits solar without storage. Low (below 0.5) = continuous or evening-heavy, suits battery-paired sizing.
Step 3: Model self-consumption at multiple solar sizes
For each candidate solar size:
Generate a synthetic solar profile
For each half-hour interval across the year, compute solar generation as: kWp × yield(month, hour, latitude). Standard libraries like NASA POWER, PVGIS, or MCS Solar Yield provide hourly irradiance data; convert to half-hourly via interpolation.
Compute self-consumption per interval
For each half-hour: self-consumed kWh = min(solar generation, demand). Export = solar generation − self-consumed.
Sum across the year
Total self-consumed kWh / total solar generation = self-consumption percentage. Total export kWh × export tariff = export revenue.
Compute economic value at each size
Avoided cost = self-consumed kWh × tariff. Total annual benefit = avoided cost + export revenue. Use this in the standard 25-year cash-flow model.
Plot value vs size
Self-consumption percentage will fall as size increases (more export). Annual benefit will rise but at diminishing returns. The optimal size is where marginal £/kWp of avoided cost equals marginal £/kWp of capex.
Step 4: Land the size decision
The optimum typically sits where the marginal kWp delivers ~70% self-consumption — at higher self-consumption you're leaving capacity on the table; at lower, you're over-discounting capex against export value.
Two patterns to look for: (a) sites where overnight load is above 30% of summer-midday peak — these are typically under-sized by rule-of-thumb; (b) sites with strong summer/winter seasonality — these are typically over-sized in summer-light models. Half-hourly data is the only reliable way to identify which case applies.
Worked example: Yorkshire food production site
A site we analysed in 2026 demonstrates the impact:
| Size | Self-consumption | Avoided cost | SEG export rev | Project IRR |
|---|---|---|---|---|
| 500 kWp | 94% | £94k | £2k | 15.2% |
| 700 kWp (rule-of-thumb) | 86% | £120k | £6k | 12.8% |
| 900 kWp | 78% | £140k | £13k | 14.4% |
| 1,100 kWp (optimum) | 71% | £156k | £21k | 16.9% |
| 1,300 kWp | 62% | £161k | £33k | 15.4% |
The optimum landed at 1,100 kWp — 36% larger than the installer's rule-of-thumb 700 kWp proposal — with a project IRR of 16.9% versus the installer-proposed 12.8%. The continuous overnight refrigeration load (70–110 kW) created enough demand that even at 1,100 kWp, summer-midday generation didn't overwhelm the site's ability to consume.
Half-hourly sizing FAQs
How do I get half-hourly demand data from my supplier?
What if I'm on a non-half-hourly tariff?
How does half-hourly data change the optimal solar size?
What's a typical commercial demand profile shape?
Can a calculator do this analysis or do I need full advisory?
Need this analysis on your specific site?
Our advisory engagement processes your half-hourly file and computes optimal size against your actual demand profile. That analysis is what determines whether your project IRR runs 12% or 17%.
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