SOLA2540 Chap.1 System Design and Load Assessment
System Design and Load Assessment
Stand-alone PV design starts by defining the service and tracing the energy path. Unlike a grid-connected system, a stand-alone installation cannot rely on the network to absorb surplus or cover a shortfall.
The load assessment must therefore distinguish daily energy, credible maximum demand and start-up surge, identify critical and shiftable loads, separate seasonal cases, and state assumptions about future growth and acceptable outage.
Those quantities control different decisions: daily watt-hours feed array and storage work, simultaneous apparent power feeds inverter and cable checks, and surge feeds short-duration equipment capability. This chapter uses original scenarios to build an auditable load table and diagnose assumptions that can corrupt every later component size.
What this chapter covers
- 01
Grid-connected and stand-alone systems use different design philosophies because only the grid-connected case has network backup
- 02
The stand-alone energy path links array, controller, battery, direct DC loads, inverter and AC loads
- 03
Daily design energy is the sum of appliance count, power and operating time
- 04
AC and DC energy are calculated separately before AC demand is referred to the DC bus through inverter efficiency
- 05
Maximum demand uses credible coincidence, not an automatic sum of every nameplate
- 06
Surge demand is a short apparent-power event that may control inverter and battery selection
- 07
Seasonal variation, standby consumption, future growth and demand management belong in explicit scenarios
- 08
Critical and shiftable loads connect the numerical load table to the user's required level of service
Worked example · free
Build a load table for an original remote workshop
- +1Communications energy is 110 × 24 = 2,640 Wh/day and lighting energy is 4 × 12 × 7 = 336 Wh/day.
- +1Refrigeration energy is 450 × 6 = 2,700 Wh/day and pumping energy is 1,100 × 0.75 = 825 Wh/day.
- +1Total AC energy is 2,640 + 336 + 2,700 + 825 = 6,501 Wh/day before referring it through inverter efficiency.
- +1If pumping is scheduled with lights off, a credible pump-period running total is 1,100 + 450 + 110 = 1,660 W before power-factor and surge adjustments.
Key terms
- Stand-alone system
- A power system not connected to a distributor network. It must meet the agreed service with local generation, storage, controls and any specified backup.
- Daily design energy
- The watt-hours consumed by the planned loads over a design day, calculated from appliance power, number and operating duration for a stated seasonal case.
- Maximum demand
- The credible simultaneous power of the highest-power load and other loads likely to operate at the same time under the stated management plan.
- Surge demand
- A short-duration apparent-power requirement produced by starting or another transient event. It can exceed the ordinary running demand by a large margin.
- Demand management
- Scheduling or controlling loads to lower maximum demand or shift energy use toward periods of strong renewable generation without losing the required service.
- Critical load
- A service that must remain available under the agreed reliability target and therefore cannot be shed as freely as a deferrable or shiftable load.
System Design and Load Assessment FAQ
Why are daily energy and maximum demand different?
Daily energy accumulates appliance use over time and is measured in watt-hours. Maximum demand is a simultaneous power condition measured in watts or volt-amperes. A small motor can use little energy yet create the controlling surge, while a low-power communications load can dominate daily energy because it runs continuously.
Arrays and storage need the energy result, but inverters, batteries, conductors and protection also need demand and surge checks.
Should every appliance be added to maximum demand?
Only if every appliance can credibly operate at the same time. Build a scenario from the highest-power load plus the loads likely to coincide, then state any scheduling, interlock or user practice that prevents another load from joining it. A demand-management rule is valid only if the user can follow it and critical service remains available. Keep a separate adverse case when coincidence is uncertain.
How should seasonal loads be handled?
Create separate seasonal load cases rather than hiding heating, cooling or operating changes inside one annual average. Recalculate appliance hours and criticality for each relevant season, then compare those loads with the matching seasonal solar resource. The controlling design period can be caused by high demand, low resource or their combination, so the load and resource calendars must use compatible time periods.
What usually goes wrong in a load table?
Common errors include confusing rated power with energy, ignoring standby consumption, using traffic or active time instead of energised time, assuming every load coincides, treating motor surge as ordinary daily energy, and mixing AC energy with DC-bus energy before applying inverter efficiency.
The repair is to label units and boundaries, show each duration assumption, and maintain separate energy, running-demand and surge sections.
Assessment move
Build one original load table and keep it alive through the course. Start with counts, nameplate or measured power, operating hours, supply type, power factor, start duty, criticality and season. Calculate daily AC and DC energy separately, then refer the AC energy to the DC bus using the appropriate weighted inverter efficiency.
Draw a twenty-four-hour sketch to identify credible coincidence rather than summing all appliances. Add a start-up case for the largest motor and any load that may start with it. Next, change one assumption: move the pump into solar hours, add winter lighting, or introduce future growth. Explain which total changes and which component decision should respond.
For written answers, lead with the service requirement, show the calculation, and close with the missing compatibility check. Practise spotting category errors: watt-hours cannot directly size an inverter, running watts cannot replace apparent power, and a surge factor should not inflate the entire daily energy total.
Before using the result downstream, ask whether the demand-management plan preserves critical loads and whether the user has accepted the implied reliability.
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