How to Size Your Solar System Malaysia 2026 | kW Calculator by Bill

The most common question we get at Trexon is: "How many solar panels do I need?" The honest answer depends on your electricity bill, roof space, and energy goals. This guide gives you two reliable methods to size your solar system correctly — and explains the costly mistakes to avoid.

Method 1: Bill-Based Sizing (Quick and Reliable)

Your TNB electricity bill is the single best indicator of your energy consumption. On the RP4 tariff a given bill converts to kWh in a predictable way, which makes bill-based sizing reliable.

The table below uses Trexon catalogue sizes for grid-connected Solar ATAP systems. Estimate on the RP4 domestic tariff (from 1 Jul 2025), before AFA, the 1.6% RE Fund and service tax; 3.5 sun-hours a day; about 40% of solar used at home and the rest exported and credited at the 27.03 sen Energy Charge (capped at what you import) for 10 years under Solar ATAP; no tariff increases. Your bill and roof will change the figures. Savings and payback are for the monthly bill shown in each row, not the catalogue headline figure for that system size. Prices are Solis; Huawei adds RM3,000 and Sungrow adds RM5,000.

Note: sizes above 7.44 kWp need a three-phase supply. Single phase is capped at 5 kWac, which is up to 7.44 kWp.

Why this works: A 1 kWp solar system generates about 105 kWh a month (3.5 sun-hours a day x 30 days). Solar used at home avoids RM 0.4443/kWh (RM 0.5443 above 1,500 kWh a month), while exported power earns the Energy Charge of RM 0.2703/kWh. A quick starting size is your monthly kWh divided by 105; most homes choose a size at or a little below that.

Quick Formula

System size (kWp) ≈ Monthly kWh ÷ 105

• 739 kWh (RM 300 bill) → 739 ÷ 105 = 7.0 kWp to offset every kWh; our sizing for this bill is 5.58 kWp, because exported power earns less than power you use yourself
• 998 kWh (RM 500 bill) → 998 ÷ 105 = 9.5 kWp to offset every kWh; our sizing for this bill is 7.44 kWp
• 1,614 kWh (RM 1,000 bill) → 1,614 ÷ 105 = 15.4 kWp to offset every kWh; our sizing for this bill is 11.78 kWp

Use our solar savings calculator to enter your exact bill and get a personalised estimate including ROI projections.

Method 2: Roof Space Method

If you want to maximise your roof's potential (rather than just offsetting your current bill), start from available roof area instead. This approach is popular for new homeowners expecting electricity usage to increase.

Malaysia Solar Panel Space Requirements

A Trexon catalogue system uses 620W panels. Allowing about 2.8 sqm per panel (including spacing for access and to avoid self-shading):

• 1 kWp of solar requires approximately 4.5 sqm of usable roof space
• A 6.20 kWp system (10 panels) needs roughly 28 sqm of usable roof
• A 9.92 kWp system (16 panels) needs roughly 45 sqm of usable roof

Roof Space Calculator

Measure your available roof area (avoid areas with shade from trees, water tanks, or neighbouring buildings). Then:

Maximum system size (kWp) ≈ Usable roof area (sqm) ÷ 4.5

For example: a 50 sqm rooftop → 50 ÷ 4.5 ≈ 11 kWp maximum. You would then pick the nearest catalogue size at or below that (9.92 or 11.16 kWp) depending on your bill and budget. Browse our 6 kW system packages and 8 kW system packages to compare options.

The Recommended Approach: Combine Both Methods

The most robust sizing strategy uses both methods together:

1. Calculate the bill-based size (what you need)
2. Calculate the roof-based maximum (what fits)
3. Choose the smaller of the two — or add battery storage if you want to capture excess generation

Example: Your bill (RM 700) suggests 9.92 kWp, but the roof fits 8.06 kWp at most. In this case, install 8.06 kWp: it generates about 846 kWh a month, around 81% of the output of the full-size system. This is a perfectly common and economical outcome.

Common Solar Sizing Mistakes in Malaysia

These are the errors our survey team sees most often — each one costs homeowners money:

Mistake 1: Oversizing the System

Installing a larger system than you actually need wastes capital. Under Solar ATAP, excess electricity exported to the grid earns only the Energy Charge, RM 0.2703/kWh — far less than the RM 0.4443/kWh you save by self-consuming — and export credit that exceeds your bill is forfeited. An oversized system sends too much electricity to TNB at a discount instead of offsetting your own bill at full value.

Rule: Size for your daytime use. Size to offset, not to export.

Mistake 2: Undersizing the System

Conversely, a system that is too small leaves savings on the table. If your bill is RM 500 and you install only 5.58 kWp, you save about RM 230 a month (payback about 7.1 years), compared with about RM 292 a month (payback about 6.0 years) from 7.44 kWp. The incremental cost of adding more kWp during initial installation is lower than upgrading later.

Mistake 3: Ignoring Future Load Growth

Planning to buy an EV? Adding air conditioning to a new room? Installing a heat pump water heater? Factor these in when sizing. A system sized for a current RM 400 bill (6.20 kWp) may be undersized within 2 years if you add an EV charger. Panel quality also affects long-term output — understand how panel degradation affects your system's lifetime yield when deciding on system size.

Mistake 4: Not Accounting for Roof Shading

Shading from a single obstacle can reduce total system output by 20–40% if microinverters or power optimisers are not used. Always conduct a proper shading analysis before finalising system size.

Mistake 5: Using Total Roof Area Instead of Usable Area

Not all roof space is usable. Deduct area for: water tanks, A/C compressors, skylights, access hatches, shaded zones, and required setbacks from roof edges (typically 0.5m). Many homeowners overestimate usable area by 30–40%.

Professional Survey: What Happens During a Trexon Site Visit

Beyond the calculator, every Trexon installation begins with a comprehensive site assessment:

• Roof structural check: Load-bearing capacity for the weight of the panels and mounting frame (check the panel weight on the datasheet)
• Shading analysis: Using irradiance data and on-site measurement
• TNB meter inspection: Confirming bi-directional meter compatibility for Solar ATAP
• Wiring audit: Ensuring existing DB box and earthing meet safety standards
• Inverter location planning: Ventilation, cable routing, and monitoring access

The RM100 site visit typically takes 45–60 minutes, and the full RM100 is credited when you proceed. Book the visit online or view our full pricing and package options.

Battery Storage: When Does It Make Sense?

If your roof is already maxed out but you want to capture more value from your solar system, battery storage is the next step. Batteries store excess daytime generation for use at night, which raises the share of your solar that you use at home.

Battery systems add significant cost to a system; ask for a current quotation. They make strongest financial sense for households with:

• Evening-heavy electricity usage (air conditioning running until midnight)
• Planned EV ownership (charging overnight from stored solar)
• Frequent power outages (battery provides backup power)

Ready to find the right system size for your home? Use our solar calculator or browse our complete package pricing to get started today.