What Size Solar and Battery System Does a Sri Lankan Home Actually Need?

The most common question we hear is not "how much does solar cost?" — it is "what size do I actually need?" Oversize and you waste money; undersize and the battery runs flat at 9 pm. Here is the plain-language version of how we size systems.

Three numbers, not one

Your monthly CEB units are the single best sizing input — but they size three different things, and the trick is to keep them apart: the solar array (kWp of panels, sized to your energy use), the inverter (kW of power, sized to the loads running at once), and the battery (kWh of storage, sized to your evening and backup needs). A single "6 kW system" figure blurs all three, so we size them separately below.

Inverter kW: the width of the pipe

The inverter's kW rating decides how much power can flow at once — how many appliances can run simultaneously. As a rough guide to the inverter class: a home using around 200–300 units a month is usually well served by a 5–6 kW single-phase inverter; 300–500 units points to 8–10 kW single-phase; and larger or three-phase homes above 500 units are usually 10–15 kW three-phase territory. A 6 kW hybrid inverter comfortably runs a typical home's lights, fridge, TV, fans and a couple of air conditioners. The inverter is often deliberately larger than the panel array, to give power headroom for air conditioners, pumps, EV charging and future expansion — so a "6 kW system" does not mean 6 kW of panels. If your home has three-phase supply, you need a three-phase inverter so all three phases stay balanced and backed up.

Battery kWh: the size of the tank

The battery decides how long you can run after sunset or during a power cut. A 10 kWh battery typically carries a modest home through the evening; 16–20 kWh covers a family home into the night; 30 kWh and above is for homes that want to run air conditioning through extended outages. LiFePO4 chemistry matters here: EAST LiFePO4 rated for 6,000+ cycles means the battery is built for daily use, not just emergencies.

Panels (kWp): sized to your energy, not your inverter

Panels are sized to how much energy you use, not to the inverter rating. As a planning figure, each 1 kWp of panels generates roughly 115–120 units a month in Sri Lanka (about 4 units per kWp per day) before roof and shading losses. So a 200–300 unit home needs only about 2–4 kWp to offset most of its use, a 300–500 unit home about 4–6 kWp, and a 500-unit-plus home roughly 6–9 kWp. Panels do two jobs before dark — run the home and refill the battery — so our kits pair, for example, around 5 kWp (about 8–10 panels) with 16 kWh of storage. Roof direction and shading adjust this, which is what a site visit confirms.

The expandability rule

The smartest sizing strategy we know: start conservative on battery, generous on inverter. EAST systems stack — you can add a matching battery module later with nothing to replace. Buying headroom in the inverter today costs little; replacing an undersized one later costs a lot.

Shortcuts: our pre-sized kits

We have packaged the most common answers as fixed-price kits — from the Backup Starter Kit (6 kW, 10 kWh) to the 3ph Large Kit (15 kW, 41 kWh, 35 panels). Every kit includes installation, and eligible EAST packages carry warranty coverage of up to 10 years.

Not sure which bracket you fall into? Request a free site assessment — we size it from your actual bills and roof, and give you a fixed proposal.