The single most important document for sizing your solar system is your TNB electricity bill. Yet most homeowners hand it to their installer without understanding what it says. This guide teaches you to read your bill like a solar engineer — and verify that your installer's recommendation actually makes sense for your usage.
The Key Numbers on Your TNB Bill
A standard TNB residential bill (Tariff A) contains three numbers that drive your solar system sizing decision:
1. Monthly kWh Consumption
Find this under the heading "Jumlah Unit Digunakan" or "Total Units Consumed". It appears in your Current Month column. This is your most critical number — the total kilowatt-hours of electricity your household consumed in that billing cycle.
Rule of Thumb: A residential solar system is typically sized to generate about as much as your monthly kWh consumption (the formula below does this). Anything it generates beyond what you use at home is exported at the Energy Charge only, so a low bill may not justify more than the smallest package.
2. Energy, Capacity and Network Charges
Since 1 July 2025, TNB bills domestic customers under the RP4 Domestic General tariff. Each kWh carries three charges, and the Energy Charge steps up only above 1,500 kWh a month:
| Charge | Rate (sen/kWh) | What This Means |
|---|---|---|
| Energy Charge (up to 1,500 kWh/month) | 27.03 | Applies to every kWh up to 1,500 |
| Energy Charge (above 1,500 kWh/month) | 37.03 | Applies to all kWh once usage is above 1,500 |
| Capacity Charge | 4.55 | Every kWh |
| Network Charge | 12.85 | Every kWh |
Why this matters for solar: Every unit of solar electricity you self-consume replaces a unit from TNB, avoiding about 44.43 sen/kWh (27.03 + 4.55 + 12.85). The Energy Efficiency Incentive deducted for households using up to 1,000 kWh a month, and the AFA (monthly fuel adjustment), also appear on your bill, so check them when you compare quotes.
3. Retail Charge
TNB charges a Retail Charge of RM10 a month, which is waived for households using up to 800 kWh a month until 31 December 2026 (normally up to 600 kWh). It is a fixed charge that remains regardless of how much solar you generate.
Step-by-Step: Using Your TNB Bill to Size Your Solar System
Step 1: Find Your Average Monthly Consumption
Pull out your last 12 TNB bills (or download them from the TNB website or myTNB app). Add up all 12 months' kWh consumption and divide by 12. This gives your average monthly kWh.
Using just one month can be misleading — bills spike in March–May and October–November during hot spells when air conditioning usage peaks.
Step 2: Calculate Daily kWh
Divide your average monthly kWh by 30. This is your daily energy consumption.
- Monthly average 600 kWh ÷ 30 = 20 kWh/day
- Monthly average 400 kWh ÷ 30 = 13.3 kWh/day
- Monthly average 900 kWh ÷ 30 = 30 kWh/day
Step 3: Apply the Solar Sizing Formula
Trexon's estimates assume about 3.5 sun-hours a day of usable output — about 105 kWh per kWp per month after inverter, cable and temperature losses. Output varies by state and season.
Formula: System Size (kWp) = Daily Consumption (kWh) ÷ 3.5
Then pick the nearest catalogue size, and remember that under Solar ATAP exported power earns only the Energy Charge and unused credit is forfeited at the end of each billing period, so oversizing reduces your return.
- 20 kWh/day ÷ 3.5 = 5.71 kWp → 5.58 or 6.20 kWp
- 13.3 kWh/day ÷ 3.5 = 3.80 kWp → our smallest package, 5.58 kWp
- 30 kWh/day ÷ 3.5 = 8.57 kWp → 8.06 kWp (three phase; the single-phase maximum is 7.44 kWp)
Step 4: Apply the Solar ATAP Cap
Under Solar ATAP, domestic system capacity is capped at 5 kW for single-phase and 15 kW for three-phase connections per §8 of GP/ST/No. 60/2025. Systems exceeding these limits require a Connection Confirmation Check (CCC). Confirm your phase type with TNB before sizing.
Example: Reading a Real TNB Bill for Solar Sizing
Let's say your bill shows:
- Month: January 2026, 31-day billing period
- Total kWh consumed: 587 kWh
- Bill amount before Solar ATAP: about RM 208 (RP4 Domestic General estimate, before the RE Fund)
- Tariff breakdown: 587 kWh × (27.03 + 4.55 + 12.85 sen) = RM 260.80, less the Energy Efficiency Incentive of 9.0 sen × 587 kWh = RM 52.83
Sizing calculation:
- Daily consumption: 587 ÷ 31 = 18.9 kWh/day
- System size needed: 18.9 ÷ 3.5 = 5.4 kWp → recommend the 5.58 kWp system (from RM 15,999)
- Why not larger?: Extra panels mostly add exports, credited at only RM 0.2703/kWh, and unused credit is forfeited each billing period.
Special Cases: What to Look For on Commercial TNB Bills
If you are sizing solar for a shop, office, or light industrial space (a TNB non-domestic tariff, formerly Tariff B, C, D or E), your bill includes an additional field: Maximum Demand (MD) in kW. This is the peak power your premises consumed at any 30-minute interval during the month. TNB charges for this demand capacity regardless of whether you used it consistently.
Solar alone does not reliably reduce your MD charge: one cloudy half-hour during your peak can set the month's MD. Reducing MD needs a site-specific study of your half-hourly consumption data, often with battery storage.
Three Situations Where Standard Sizing Rules Break Down
1. Night-Shift Households
If you work night shifts and most of your power usage occurs between 8pm and 7am, solar's daytime generation won't match your consumption pattern. Solar ATAP's export credit (RM 0.2703/kWh) partially compensates, but ROI will be lower than a family home with daytime consumption. In this case, consider adding battery storage or sizing the system smaller.
2. EV Charger Owners
If you own or plan to own an EV, add 150–300 kWh to your monthly consumption estimate to account for charging. A Tesla Model 3 consuming 200km/month at 15 kWh/100km uses 30 kWh/month. A BYD Atto 3 driven 1,500km/month uses approximately 225 kWh/month in charging. Add these to your TNB bill baseline before sizing. See our guide on solar and EV charger combos.
3. Planned Upgrades
If you plan to add an air conditioner, pool, or EV within the next 3 years, size your solar system for your future consumption, not just today's bill. The marginal cost of adding 1–2 extra panels now versus installing a second system later is significant.
Use our Solar Savings Calculator to input your TNB bill figures and get an instant system size recommendation and ROI projection for your specific address and consumption pattern.
