How to size commercial solar PV using a business load profile

Why identical bills can call for different solar systems

Two businesses can purchase the same amount of electricity each month. One operates mainly during daylight hours; the other carries out most of its work in the evening. With identical solar generation, their direct use will differ. A solar array produces electricity when sunlight is available, while demand follows the operating schedule.

Matching annual generation to annual consumption does not eliminate grid purchases. Without storage, a daytime surplus cannot supply a night-time shortfall. Summer output does not physically replace winter generation either, even when annual totals match. Any payment arrangement for exported electricity must be assessed separately.

First define the objective: reduce purchases during working hours, prepare for expansion or combine solar with other energy sources. A grid-connected solar system for a business may be the starting point for the first objective. Select its capacity by comparing feasible configurations, rather than fitting the largest possible number of modules on the roof.

Which data to prepare

Request hourly or more detailed meter records. A full year is preferable for an initial assessment because it captures seasons, weekends and shutdowns. If only a shorter period is available, identify its limits and check whether it represents normal production. This is a practical data-quality approach, not a universal regulatory requirement.

• Metering: date, time, interval duration, imported and exported energy, units and measurement point.
• Business operations: shifts, weekends, seasonal processes, maintenance, outages and planned new loads.
• The site: address, roof or land plans, orientation, shading, rooftop equipment and usable space.
• Connection: a single-line diagram, incoming supply parameters, current documentation and the agreed operating arrangement.
• Economics: bills, applicable price components, budget and criteria for comparing options.

Check missing readings, duplicate timestamps and time-zone changes. A zero reading during an outage does not necessarily mean that production had no demand. If the site already has solar, a generator or storage, grid imports are not the same as total site consumption. Establish a consistent balance across all sources. Where documentation is incomplete, start with a solar site assessment.

Module capacity, inverter output and site demand

Keep three quantities separate in a proposal: the combined nameplate capacity of the PV modules, the rated output of the inverters and the business's load. Module nameplate power is measured under standard test conditions. It does not mean that the array will produce that power throughout the day. The notation kWp helps distinguish rated peak capacity from actual output.

For example, a hypothetical array of 100 modules rated at 600 W each has a combined nameplate capacity of 60 kW. This is equipment arithmetic, not a specification for a proposed product or a generation forecast. Inverter output, allowable voltage and current, string configuration and equipment limits require separate checks.

See the separate guide to matching solar panels and an inverter for the electrical checks.

Energy is measured in kWh. A load averaging 40 kW over 15 minutes consumes 10 kWh during that interval. Our handbook explains the distinction between power and energy in more detail. PV sizing must not compare module power directly with a monthly total expressed in kilowatt-hours.

How to obtain a usable generation profile

Model each configuration using the location, orientation, tilt, equipment characteristics and losses. The result must include a time series that can be matched to demand, not just an annual total. Roof areas facing different directions need their individual generation profiles to be represented.

If more than one layout is feasible, compare south-facing and east-west solar layouts on the same basis and against the business load profile.

Ask which weather period, time zone, data interval and losses the model uses. A typical-weather-year calculation is a model, not an exact description of future days. Local shading from buildings, trees and structures must be checked against the conditions at the site.

Compare feasible capacities using consistent assumptions. Adding modules may require a different inverter, layout or electrical arrangement. Increasing nameplate capacity by a given percentage therefore does not always produce the same percentage increase in useful energy. The model should identify inverter output limits, export restrictions and system availability.

Worked example: how much generation is used directly

Illustrative example with assumed inputs. The table represents one day without batteries or other sources. To make the arithmetic transparent, demand and available generation are assumed constant within each four-hour interval. These figures are neither customer measurements nor a forecast for a particular PV capacity. Actual sizing requires finer intervals and a complete assessment period.

For each interval, direct use is the smaller of consumption and available generation measured at the same AC boundary. Add the interval results. Here, direct use totals 500 kWh, although the system could have produced 560 kWh. The remaining 60 kWh requires permitted export, another use or generation curtailment.

This produces two different indicators: the direct self-consumption ratio is 500 ÷ 560 × 100 = 89.3%, while the share of site demand covered is 500 ÷ 960 × 100 = 52.1%. Grid purchases for the load remain 460 kWh under the stated assumptions. A high first percentage does not imply a high degree of self-sufficiency.

Long aggregation intervals can overstate the match because demand and generation may fluctuate differently within an hour. That is why the constant-power assumption is explicit here. Do not multiply one illustrative day by 365 to forecast annual savings.

Comparing smaller and larger configurations

Request the same results for every option: available generation, direct use, remaining grid purchases, potential exports and curtailed energy. Review weekends, summer conditions and periods of reduced production separately. A system sized for a busy working day may produce substantial surplus during shutdowns.

A larger array may cover more consumption, but assess the increase in direct use separately from the increase in generation. If extra modules mainly add midday surplus, the financial outcome depends on how that energy can be used and under which conditions. Compare incremental investment with incremental benefit, rather than selecting the largest generation figure.

Check whether some operations can move to sunny hours without disrupting production. If storage is relevant, assess solar PV combined with energy storage as a separate scenario, including losses, charging power and cost. A battery does not automatically resolve every mismatch between the two profiles.

Checking a contractor's calculation

Request an explanation of the calculation columns, not just a final presentation. The underlying file can include time, consumption, available generation, direct use, imports, exports and curtailment. Without a battery, check two balances: consumption equals direct use plus imports; available generation equals direct use plus exports and curtailment. Equipment auxiliary consumption should be included once at the agreed boundary.

Do not mix actual output after curtailment with potential generation before it. If the system could have produced 560 kWh, used 500 kWh and curtailed the rest, actual generation is 500 kWh. Its use as a share of actual generation would then be 100%. That does not remove the 60 kWh of unused potential. Indicator definitions must stay consistent across options.

Check several rows manually: a working-day midday interval, an evening and a weekend. Verify units, formulas and time alignment, then reconcile monthly consumption with meter records. Explain differences such as another measurement boundary or an incomplete period. Self-consumption percentages cannot be compared directly if one proposal includes only working days and another covers the whole year.

What the sizing assessment should deliver

A useful assessment explains why a configuration was selected. It identifies input data, module and inverter parameters, the time-based energy balance, site limitations and changes in future demand. Any assumptions replacing unavailable data should be visible to the customer.

Before agreeing, check three practical questions. Does the calculation include changes to the operating schedule? Is the layout feasible given roof condition, access and shading? Does the electrical arrangement and operating mode suit the specific connection? The maximum area available for modules cannot answer these questions.

If the business is considering both roof and land, compare them using the solar site selection matrix.

Value avoided purchases using the applicable components of the electricity contract, and calculate potential sales separately. A financial assessment also needs the project budget, operating costs and agreed scenarios. The illustrative day's arithmetic is not a payback promise.

Prepare meter profiles, the connection diagram, site plans and a description of planned changes. These form the basis of solar PV system design and allow proposals to be compared on consistent terms. Capacity selection can then follow the business's needs and a verified energy balance.