How to Size a 3–5kW Off-Grid Solar System
Do not select an off-grid kit by inverter power alone. Use this five-step method to check daily energy, peak power, battery capacity, PV input and compatibility.
BUYER GUIDES
A 3kW or 5kW inverter rating tells you the maximum power class of the system. It does not tell you how many hours the system will run, whether it can start a pump or refrigerator, or whether the solar array can refill the battery each day. Good sizing starts with the load and works backward to the inverter, battery and PV array.
Quick answer
Use running watts to size continuous inverter power, surge watts to check motor starting, watt-hours to size the battery, and local solar yield to size the PV array. Then verify that every component is electrically compatible.
Step 1 — Build a Real Load List
List every appliance that must operate during an outage or in off-grid mode. Record quantity, running power, expected hours per day and whether the appliance contains a motor or compressor. A small shop, for example, may need lights, fans, a router, point-of-sale equipment, a refrigerator and phone charging, while an air conditioner, kettle or water heater may be optional rather than essential.
Daily energy is calculated as: appliance watts × quantity × operating hours. Add the daily watt-hours for all priority loads and divide by 1,000 to obtain kilowatt-hours per day. If a device cycles on and off, use a realistic duty cycle or measured energy data instead of multiplying its nameplate power by 24 hours.
Step 2 — Check Continuous Power and Surge Power
The inverter must support the combined loads that may operate at the same time. It must also tolerate short starting surges from refrigerators, freezers, pumps, air conditioners and some tools. A load list showing only daily energy can therefore select the wrong inverter. Record which motor loads can start together and ask for their starting current or surge power.
A 3kW system can be a practical starting point for priority circuits in a home, office or small shop. A 5kW system gives more headroom for larger simultaneous loads, but it still does not make every appliance automatically acceptable. High-power heating loads and motor starts must be checked against the actual inverter datasheet.
Step 3 — Size Battery Energy for the Required Backup Time
Battery capacity is measured in kilowatt-hours. Usable energy is lower than the nominal label because the design must consider the permitted depth of discharge, conversion losses, operating temperature, battery age and any reserve that the project requires. Use the usable-energy value or warranty conditions in the approved datasheet rather than assuming that every nominal kilowatt-hour is available.
A simple planning formula is: required nominal battery energy ≈ critical-load energy during the backup period ÷ (allowed usable fraction × conversion efficiency). For example, if priority loads require 5.2kWh overnight and the planning assumptions are 90% usable capacity and 90% conversion efficiency, the calculated nominal capacity is about 6.4kWh. This is an illustration only; final assumptions must come from the selected battery and inverter.
Step 4 — Size the PV Array from Local Solar Yield
Solar panel capacity must replace the energy used by the loads and cover system losses within the available sunlight. A preliminary calculation is: PV array kW ≈ daily energy kWh ÷ (effective peak-sun-hours × overall derating factor). The Global Solar Atlas can provide a location-based starting point, while tools such as NREL PVWatts explain how irradiance, temperature, orientation and losses affect output estimates.
Do not use annual average sunlight alone for a reliability-critical system. Review the weaker solar months, shading, roof direction, module temperature, dust and the required recovery time after a cloudy day. The final array must also stay within the inverter’s MPPT voltage range, maximum open-circuit voltage, current limit and maximum PV input power.
Step 5 — Verify Component and Site Compatibility
· AC output voltage and frequency match the destination and connected loads.
· Battery nominal voltage, charge/discharge current and BMS communication match the inverter.
· PV string operating voltage and cold-condition open-circuit voltage stay within inverter limits.
· DC and AC protection, cable size, isolators, earthing and surge protection follow the approved design and local rules.
· Installation clearances, ventilation, temperature, moisture and ingress protection suit the site.
· Mounting, connectors, accessories, manuals and commissioning settings are included in the final bill of materials.
3kW or 5kW: A Practical Starting Point
These configurations are useful discussion points, not universal promises. The same 5kWh battery can last many hours on lights and communication loads but a much shorter time on cooling, pumping or heating loads.
Information to Send for an Accurate Recommendation
· Country and installation city.
· AC voltage and frequency.
· Appliance list with watts, quantity and hours per day.
· Which appliances can run at the same time.
· Motor or compressor starting information.
· Required backup time or days of autonomy.
· Roof or ground-mount space, shading and photographs.
· Grid availability, generator use and target budget.
· Sample or project quantity and required delivery date.
Send the load list and project location to sales@topfei-energy.com. TOPFEI can help narrow the configuration, identify open technical points and coordinate a model-specific quotation.
Technical references
Global Solar Atlas — World Bank Group solar resource service: https://globalsolaratlas.info/
NREL PVWatts Calculator: https://pvwatts.nrel.gov/
IEA PVPS Task 18 — Li-Ion Battery Systems in Off-Grid Applications (2025): https://iea-pvps.org/wp-content/uploads/2025/10/IEA-PVPS_T18-04-2025_REPORT_Li-Ion-Systems.pdf
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