Before You Request an Off-Grid Solar Quote: 12 Inputs That Define the System

An overseas buyer asks three suppliers for a “5 kW off-grid solar system.” The replies look comparable: panels, inverter, battery and a total price. They may describe three different systems.

One supplier assumes five hours of useful sun. Another assumes eight. One sizes the battery for overnight use. Another counts only nominal battery energy. One allows a refrigerator compressor to start while a pump is running. Another does not model surge loads at all.

Before asking which supplier is cheaper, make sure every supplier is quoting the same load, operating conditions and service objective.

IEC TS 62257-200:2026 describes off-grid system selection and design as a process that starts with user needs, functional requirements and suitable architecture. IEEE 1526-2020 likewise describes complete stand-alone photovoltaic system tests with a defined load. In practical sourcing terms, the load and service requirement must be frozen before a system can be compared or verified.

1. Define the system boundary

State what the proposed package must include: PV modules, mounting, inverter or inverter-charger, charge controller, battery, battery-management system, protection, combiner, monitoring, cables, distribution equipment, generator interface, installation support, commissioning and documentation.

Also identify what is supplied locally. A low quotation may exclude mounting, breakers, battery cables, communications devices or site commissioning.

RFQ input: included equipment, locally supplied equipment, required accessories, spare parts and exclusions.

2. Record the site and destination

Provide the country, nearest city or coordinates, altitude, temperature range, humidity, dust, salt exposure, indoor or outdoor installation, available equipment space and any noise limits.

Solar resource and temperature influence energy production. Altitude and temperature may also affect inverter output, cooling and equipment selection. Destination-country requirements cannot be inferred from a generic “export model.”

RFQ input: location, environmental conditions, installation area, destination standards and responsible installer.

3. Build an appliance-level load schedule

List each load rather than giving only a monthly utility bill or a desired inverter size. For every appliance, record quantity, rated watts, expected operating hours, operating pattern and whether the load is AC or DC.

If possible, use measured power rather than only the nameplate. Separate weekday, weekend and seasonal use. Identify loads that may be added later.

RFQ input: appliance, quantity, running power, hours per day, energy per day and measurement source.

4. Identify starting and surge loads

Motors, pumps, compressors and some power tools can require substantially more power during startup than during steady operation. The important question is not simply whether the inverter has a “surge rating,” but whether the stated duration and conditions match the loads that may start together.

Ask for the basis of the surge calculation and the inverter overload curve—not only a peak number.

RFQ input: starting method, estimated or measured starting power, duration, simultaneous-start scenario and priority.

5. Separate critical, flexible and prohibited loads

Not every load requires the same availability. A refrigerator or communications equipment may be critical. Water heating or discretionary air-conditioning may be flexible. High-energy resistance heating may be intentionally excluded.

This classification lets the designer propose load shedding or separate circuits instead of oversizing the entire system.

RFQ input: critical loads, flexible loads, loads that may be interrupted and loads excluded from backup.

6. Describe when energy is used

Daily energy in kilowatt-hours is not enough. Two sites can consume the same energy but have different evening peaks, daytime PV use and overnight demand.

Provide at least a simple hourly profile. Identify the maximum simultaneous load and the longest high-load period.

RFQ input: hourly or time-block load profile, daytime share, evening peak and overnight base load.

7. Define autonomy and acceptable shortage

“Two days of battery” is ambiguous. Clarify whether autonomy means all loads or critical loads, whether PV generation is assumed during the period and what minimum battery state of charge is permitted.

IEEE 1562-2021 highlights load calculation, solar radiation and the critical nature of the load when selecting acceptable availability. Its detailed scope is PV-only charging with lead-acid batteries, so buyers should not copy its sizing method blindly into lithium or hybrid projects. The procurement lesson remains valuable: autonomy and availability are explicit design inputs.

RFQ input: required hours or days, loads covered, weather assumption, minimum state of charge and acceptable service interruption.

8. Define every charging source

State whether the system is PV-only, PV plus generator, PV plus unreliable grid or a hybrid with another source. For a generator, specify voltage, phase, frequency, rated power, start method, fuel constraints and whether automatic start is required.

For a weak grid, describe actual availability and voltage or frequency quality. Do not let “grid input” become an assumed unlimited charging source.

RFQ input: source ratings, availability, operating rules, automatic-start requirement and charging priority.

9. Describe the PV installation constraints

Provide roof or ground-mount area, orientation, tilt, shading, structural limitations, cable routes and distance to the inverter. Ask the designer to state the solar-data source and monthly production assumptions.

IEC 62548-1:2023 covers PV array design requirements including DC wiring, protection, switching and earthing. Its scope does not replace the separate design of storage, loads or distribution.

RFQ input: usable area, geometry, shading evidence, array location, cable distance and mounting responsibility.

10. Set the battery operating requirements

Specify required usable energy and power—not only nominal kilowatt-hours. Identify chemistry preference, location, temperature conditions, expansion expectations, communications, monitoring, warranty objective and service access.

Request charge and discharge limits, usable state-of-charge window, temperature derating, parallel limits and inverter compatibility. IEC 61427-1:2013 provides requirements and test methods for secondary batteries used in photovoltaic off-grid applications; request evidence that relates to the proposed battery model and application.

RFQ input: usable energy, continuous and peak power, operating window, environment, expansion plan and model-specific evidence.

11. Freeze inverter and output requirements

Define output voltage, phase, frequency, continuous power, overload requirement, transfer behavior, generator or grid input, PV input window, communications and remote monitoring.

Ask how the factory controls documented properties, firmware, test equipment and final records. IEC TS 63157:2019 provides a quality-assurance framework for photovoltaic power conversion equipment and ongoing consistency beyond the existence of a certificate.

RFQ input: electrical output, overload profile, charging sources, PV limits, communications, firmware and final-test evidence.

12. Define the acceptance package before ordering

Agree what must be submitted with the quotation, before production, before shipment and during commissioning. A controlled package may include a requirement-response matrix, single-line diagram, bill of materials, model-specific datasheets, calculation summary, protection schedule, compatibility confirmation, manuals, nameplates, test records and warranty terms.

IEEE 1526-2020 notes that stand-alone PV performance verification applies to a complete system with a defined load. Your acceptance criteria should identify the configuration and load being accepted.

RFQ input: document register, responsible party, submission date, approval gate and change-control rule.

A quotation should expose assumptions

Ask every supplier to return a completed requirement-response matrix, stated load and solar-resource basis, monthly energy balance, array/battery/inverter selection logic, model numbers, assumptions, exclusions and an evidence list for the next verification gate.

A professional quotation does not hide uncertainty. It shows where a measured input exists, where an assumption was used and what must be confirmed before the design is frozen.

The buyer-side decision

Do not compare total price until the requirement boundary is aligned. First compare included scope, load profile, autonomy assumptions, solar and loss assumptions, usable battery energy, inverter surge basis, documentation scope, exclusions and commercial responsibilities.

The goal is not to design the system through email. It is to make the requirement clear enough that qualified designers and suppliers solve the same problem—and provide evidence behind the proposed solution.

Educational information only. Final sizing, safety, compliance and installation decisions require qualified professionals and applicable local requirements.

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