How to size a solar power system: energy audit, panels, batteries and payback

Start with a load list and an energy audit

List every load, its power in watts and the hours it runs, then multiply to get kilowatt-hours (kWh) per day. Add the totals and compare them with your electricity bills over twelve months, because summer air conditioning can double the winter figure. A bill shows the total; a load list shows what the total is made of and which loads must keep running when the grid fails.

Two numbers matter and are often confused. Energy (kWh per day) sets how many panels and how much battery you need. Peak power (kW, the loads running at once, including motor starting) sets the size of the inverter and the wiring. A villa can use a modest amount of energy but have a high peak when two air conditioners start together.

On-grid, hybrid or off-grid

This choice sets everything that follows. On-grid reduces the energy you buy and has no battery; it stops when the grid fails. Hybrid adds a battery so chosen circuits keep running through an outage. Off-grid has no grid at all and is sized for days of autonomy, as in solar-powered surveillance units. The comparison is set out on our solar PV page.

Panels: from sun hours, with derating

Peak sun hours are the hours per day at the standard strength of 1,000 W/m². Riyadh gets strong sun, roughly 5 to 6 peak sun hours on average and fewer in winter. Panel output is then reduced by temperature (cells run much hotter than the air), dust, wiring losses, inverter efficiency and mismatch. We usually take a system efficiency of 70 to 80% for planning, then check it against the specific site.

The panel size in kWp is the daily energy you want solar to supply divided by the peak sun hours and the system efficiency.

Inverter and battery

• Inverter: sized to the peak load it must carry and to the array. For the backed-up circuit, include motor starting currents.
• Battery: the energy you need at night or in an outage, divided by the usable depth of discharge and by the inverter efficiency. A lithium iron phosphate battery typically allows deep use; lead-acid allows much less, so the nominal size needs to be larger.
• Autonomy: how long the critical loads must run on battery alone. For a villa this is hours; for an off-grid point it is days.
• Batteries age faster in heat, so the room matters as much as the capacity.

Worked example

A villa that wants to cut its daytime consumption and keep lights, network and a few circuits running through an outage. Every figure is an assumption for the example.

The same villa with a different roof, shading or air-conditioning habits will land elsewhere. The point of the table is that every line can be questioned.

Roof, shading and orientation

Look at usable area, not total area. Water tanks, parapets, satellite dishes, stairwell blocks and neighbouring buildings cast shade that cuts output on a whole string, not just one module. In Saudi Arabia a south-facing tilt gives the best yearly yield, but flat roofs and low tilts reduce wind load and suit dusty sites. Row spacing, roof strength, waterproofing and safe access for cleaning are part of the design.

Maintenance and cleaning in dust

Dust is a recurring loss. Output drops between cleanings and a dust storm can cut it in a day. The yield estimate and the maintenance plan should state how often panels are cleaned, who does it, and where the water comes from. Inverter and battery health, connections and mounting should be inspected on a schedule. See maintenance.

Monitoring

Monitoring should show production, consumption, battery state and alarms, and compare actual yield with the estimate. A system that quietly produces 20% less for a year is a common outcome without it.

Planning for expansion

Leave space on the roof, capacity in the distribution board and a battery that can be added to, and choose an inverter that supports more modules or batteries. Expansion later is far cheaper if it was planned at the start.

Generator and solar hybrid

For sites that cannot accept an outage, solar and battery can carry the load most of the time, with a standby generator for long outages and poor-sun days. The generator transfer, the inverter settings and the fuel plan must be designed together. See generator interfaces and power continuity. Critical electronics still need a UPS to ride through any transfer, and emergency lighting follows its own rules; see central battery systems.

What a feasibility study should contain

• The load list and 12 months of consumption data
• Roof or ground survey with shading analysis
• System type and which loads are backed up
• Array, inverter and battery sizing with every assumption stated
• Heat and dust losses, and the cleaning plan
• Single-line diagram and the utility approval route for grid-connected systems
• An energy and cost comparison with a stated method, including replacement of batteries and inverters over the system's life

A word on payback

Payback depends on your tariff, your consumption pattern, the system cost, the share of solar used on site and the maintenance. We do not give payback figures in advance. A feasibility study should show the method and its assumptions so you can test them against your own bills.

Frequently asked questions