One of the most frequent questions we receive is: "What size solar system do I need?" The answer is not based on the size of your house or the number of bedrooms. It is based on your load profile: what you run, when you run it, how long it runs, and what must stay on during outages.
A proper design answers three different questions:
- Power question (kW): What is the highest load the system may need to support at one time?
- Energy question (kWh): How much energy is used over a day or night period?
- Reliability question: Which loads are essential, and for how long must they stay online during grid outages?
Step 1: The Energy Audit
Before choosing panels, batteries, or an inverter, start with a load audit. This is where many estimates go wrong because people use memory instead of data. A reliable audit combines appliance lists, typical runtime patterns, and (where possible) utility bills or measured logs.
We normally group loads into operating priorities, not just appliance types:
- Essential loads: lights, fridge/freezer, router, alarms, CCTV, selected sockets, office equipment, and water-pressure pumps (where needed).
- Shiftable loads: washing machines, ironing, some pumping tasks, and other loads that can run during daytime solar hours.
- High-demand / managed loads: geysers, cookers, large AC units, welders, boreholes, or other equipment that may require separate planning or dedicated circuits.
The goal is not to make the list longer. It is to make it accurate enough to support engineering decisions. Nameplate ratings are useful, but actual usage behavior matters even more. A fridge rated at one value does not run continuously; a pump may have a short but very high start-up demand.
What data improves sizing accuracy fastest?
- Recent utility bills (to estimate seasonal trends and base consumption)
- Appliance list with estimated runtime per day
- Backup expectations (for example, "essentials for 8 hours" versus "whole house for 2 hours")
- Planned additions (extra fridge, office equipment, borehole pump, EV charging, etc.)
Step 2: Calculating Peak Load (kW)
Peak load is the highest total power drawn at a single moment. It determines inverter sizing and is one of the most common causes of undersized systems. A battery can have enough energy for the night and still fail to support the house if the inverter cannot handle the momentary load.
Peak load is about coincidence, not just totals. If your microwave (1.5kW), kettle (2.0kW), and pump start (high surge) all run together, the inverter must tolerate that combined demand and any start-up surge. That is why simply adding panel wattage or battery size does not tell you the full story.
In practice, we review:
- Continuous power demand: what loads can realistically run together?
- Motor/compressor startup behavior: fridges, pumps, and AC units can demand much more at startup than at steady state.
- Circuit strategy: whether some loads should be excluded from the backup DB or scheduled outside peak windows.
Your inverter is sized for both normal operation and practical headroom. For many homes, a 5kW or 8kW class inverter is common, but the correct size depends on your actual usage pattern and whether heavy loads are included in the backup circuit.
Step 3: Calculating Daily Consumption (kWh)
Energy consumption is power over time. Ten bulbs at 10W each running for 5 hours use 500Wh (0.5kWh). When you total the energy used by all appliances across the day, you get the daily kWh demand that informs solar generation and battery capacity.
This is the step that determines how long the system can sustain loads and how much solar energy must be harvested during the day. For example:
- Battery storage sizing: If your essential night-time usage is 6kWh, the battery bank must provide that usable energy with a safety margin.
- PV array sizing: The solar array must cover daytime loads while also charging batteries, subject to weather, roof space, and system efficiency losses.
- Backup strategy: A system designed for "essentials only" will look very different from one designed for "near-whole-home backup."
Daily kWh should also be viewed across seasons or operating patterns. Offices, schools, retail sites, and homes all have different weekday/weekend profiles. A good design considers the load pattern you actually live with, not a single perfect day.
The Iselle Difference
Engineering-led sizing is where raw numbers become a dependable design. We use billing history, appliance mapping, project goals, and site constraints to create a system recommendation that is technically defensible and commercially realistic.
What gets added beyond basic calculations
- Site constraints: roof area, shading, orientation, cable routes, and board condition can all affect the final system design.
- Safety margins: headroom is applied to avoid nuisance trips and performance collapse under real operating conditions.
- Circuit separation planning: essential vs non-essential load mapping improves backup reliability without oversizing the whole system.
- Upgrade thinking: where practical, designs are structured to allow future expansion instead of forcing a full replacement later.
Common sizing mistakes to avoid
- Choosing a "package size" before defining essential loads and backup duration
- Ignoring startup surge loads for motors and compressors
- Assuming all appliances run at the same time all day
- Designing around ideal sunlight only, without system losses and seasonal variation
For a quick preliminary range, start with the pricing calculator. For an engineering-backed recommendation, pair that estimate with a technical survey and the engineering-led sizing glossary note.