Quick answer
Assessment of a building's electricity consumption patterns to properly size a solar system and optimize self-consumption ratio.
Answer: Solar load analysis examines how much electricity a site uses and when it uses it. Annual energy informs an initial production target, while interval demand helps assess self-consumption, tariff exposure and storage operation. Record the data’s period, units and measurement boundary; a yearly total alone cannot determine how much PV generation coincides with demand.
Choose data for the decision
The REopt load-data guide from the National Laboratory of the Rockies distinguishes simulated building profiles, utility bill totals and measured interval data. It recommends reviewing a full year of bills and explains that timing matters where electricity prices vary over the day.
| Data | Useful for | Main limitation |
|---|---|---|
| Annual or monthly kWh | Energy baseline and seasonal comparison | Does not reveal simultaneous PV use |
| Interval kWh or average kW | Matching demand to production and tariff periods | Needs consistent timestamps and interval lengths |
| Billing demand | Reviewing demand charges | Tariff rules determine the billed quantity |
| Circuit measurements | Identifying individual equipment use | May cover only part of the building |
| Simulated profile | Exploring scenarios when measurements are unavailable | An estimate, not measured customer behaviour |
Use actual data when available and label synthetic profiles clearly. A simulation scaled to the right annual total can still have the wrong daily shape.
Prepare a traceable baseline
Check billing dates for overlaps and gaps. For interval files, confirm units, time zone, daylight-saving treatment, missing readings and whether timestamps mark the start or end of intervals. Record unusual operating periods before adjusting them; retain the original data alongside any normalised scenario.
If a 15-minute interval contains 2 kWh, its average power is 2 ÷ 0.25 = 8 kW. That is not necessarily its instantaneous peak. Demand charges need the actual tariff’s measurement interval and billing rules.
Initial sizing from annual energy
DC capacity (kWp) = annual usage (kWh) × target offset fraction ÷ net specific yield (kWh/kWp)
For hypothetical usage of 10,000 kWh/year, an 80% target and net specific yield of 1,400 kWh/kWp/year, initial capacity is approximately 5.71 kWp. Use 0.8 for the offset fraction. Net specific yield already includes the production model’s losses; do not apply those losses again.
This calculation is an energy-based starting point. Roof geometry, equipment limits, connection conditions, tariff economics and customer objectives still affect the final design.
Calculate direct self-consumption by interval
For a simplified system without storage, match PV and load energy over identical intervals. Directly consumed PV energy in each interval is the smaller of the two quantities, assuming matching boundaries and no additional flow restrictions.
In a hypothetical two-interval example:
| Interval | PV generation | Load | Direct PV use |
|---|---|---|---|
| First | 4 kWh | 1 kWh | 1 kWh |
| Second | 1 kWh | 4 kWh | 1 kWh |
| Total | 5 kWh | 5 kWh | 2 kWh |
Direct self-consumption is 2 ÷ 5 = 40%, even though total generation equals total load. A daily-total shortcut would miss the mismatch. Battery flows, curtailment and losses require additional modelling, and coarse intervals can conceal shorter mismatches.
Future loads and electrical design
Model planned EV charging, heating or production changes as separate scenarios with explicit usage and timing assumptions. Do not assign one annual consumption figure to every EV or heat pump. Keep the measured baseline visible so reviewers can distinguish current use from projected growth.
Energy analysis also does not approve a service panel, backfeed connection or conductor size. Those need a qualified electrical review against the applicable jurisdiction’s rules. A universal oversizing percentage is not a substitute for connection approval or a financial comparison.
For detailed timing, see load-profile analysis. For production inputs, see specific yield. The system-size calculator offers a simplified initial estimate; review its assumptions before use. When evaluating generation and financial modelling, confirm supported data formats and modelling behaviour with a representative project rather than assuming automatic interval imports.
Reviewed 30 September 2026. The examples are hypothetical calculations, not customer case studies or verified savings. The linked REopt guidance supports the data distinctions; project tariffs, electrical compliance and software capabilities require separate verification.
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