How Many kW of Home Solar Do You Need? Sized From Your Real Bill and Usage
The “right” size is not read off your bill total — it depends on how much you use in the daytime, because solar only produces by day. This page gives a 3 / 5 / 10 kW table (who each fits, output per day/month/year on a yield of 1,300–1,500 units/kWp/year, and the roof area needed), a method to size it yourself from your bill, and the caps that can decide the number for you (sell-back only ≤5 kW · the home tax deduction only for on-grid ≤10 kWp).
Most Thai homes land at 3–10 kW: start at 3 kW for a ~1,500–2,500 THB/month bill with some daytime use · 5 kW for 2,500–4,500 THB with someone home by day · 10 kW for 5,000 THB and up, big houses with several ACs or work-from-home. But the truly correct number comes from one rule: size the system to your DAYTIME load, not the whole monthly bill, because units produced beyond your daytime need are only bought back at 2.20 THB (homes ≤5 kW only) or given away for free outside the program — a 5 kW system makes about 583 units/month (19.2 units/day) and needs roughly 30–40 m² of roof. The full table and the do-it-yourself method are below; “how many years to pay back” lives on the payback page and “which system type” on the on-grid vs off-grid vs hybrid page.
How Many kW of Solar Does a Home Need?
| System size | Fits a home that… | Output / day | Output / month | Output / yr | Roof area | House supply |
|---|---|---|---|---|---|---|
| 3 kW | small house/townhouse, light daytime use (bill ~1,500–2,500 THB) | ~11.5 units | ~350 units | ~4,200 units | ~18–24 m² | single-phase |
| 5 kW | home occupied by day, 1–2 ACs (bill ~2,500–4,500 THB) — the popular size | ~19.2 units | ~583 units | ~7,000 units | ~30–40 m² | single-phase (some 3-phase) |
| 10 kW | big house, several ACs / an EV / work-from-home (bill ~5,000 THB+) | ~38.4 units | ~1,167 units | ~14,000 units | ~60–80 m² | usually 3-phase |
3 / 5 / 10 kW sizing table (yield 1,400 units/kWp/yr · roof 6–8 m²/kWp approx.)
How Do You Size Solar From Your Electricity Bill?
1) Your daytime share of usage
On-grid solar produces only ~09:00–16:00; units beyond your daytime need are bought back at just 2.20 THB (homes ≤5 kW) or given away — read your behaviour: is anyone home by day, when do the ACs/fridge/pump/laundry run? The heavier the daytime use, the bigger you can size profitably; a home empty by day should start small and shift loads into daylight first.
2) Convert the bill to units at the real rate
Some bills already print the number of units — use that; otherwise units/month ≈ bill ÷ per-unit rate, where a heavy-use home pays its last units around 4.9 THB (top tier 4.4 + Ft 0.1623 + 7% VAT) and averages about 4.0 THB across the bill — use the 4.0–4.9 THB/unit band as your divisor (full rates on the Thailand tariff page).
3) The roof, the supply, and the caps
Even if the numbers say “go big”, three things still cap it: sunlit roof area (~6–8 m²/kWp) · the house supply (above ~5 kW usually needs 3-phase) · and the caps — sell-back is homes ≤5 kW only (sell-back program) and the home tax deduction is on-grid ≤10 kWp only (home tax deduction).
How to Size Home Solar Yourself (5 Steps + Example)
Use your bill plus how you actually use power. Five steps:
Get units/month from the bill
Read the “units” line on the bill, or estimate = bill ÷ about 4.0–4.9 THB/unit — a 3,000 THB bill ≈ 678 units/month (~22.6 units/day)
Estimate your daytime share
Work out what share of the day (09:00–16:00) your usage falls in — a home occupied by day with AC is often 40–60%. At 50%: 22.6 × 50% ≈ 11.3 units/day for solar to cover
Divide by output per kWp per day
kW needed ≈ daytime units/day ÷ 3.8 (= 1,400 units/kWp/yr ÷ 365) → 11.3 ÷ 3.8 ≈ 2.9 kW, so pick a ~3 kW package
Check the roof and the supply
Is there enough sunlit roof (~6–8 m²/kWp; e.g. 5 kW ≈ 30–40 m²), can it face south/west, any midday shade, and is the house single- or 3-phase (above ~5 kW usually 3-phase) — a grid-tied system must be filed for approval either way (home grid-parallel filing).
Adjust for caps, budget and the future
To sell back at 2.20 THB you must be ≤5 kW · for the home tax deduction, on-grid ≤10 kWp · if an EV or an extension is coming soon, leave headroom or pick an inverter you can expand — or let our solar calculator run the numbers.
Who Do 3 kW / 5 kW / 10 kW Each Fit?
By behaviour and daytime load, not just the bill total:
3 kW — small house/townhouse, light by day
For singles/couples out during the day but with a fridge, water pump and one evening AC; ~350 units/month shaves the expensive daytime tier, needs little roof (~18–24 m²), single-phase is fine — if nobody is home by day, shift loads (timed pump/laundry) into daylight before sizing up.
5 kW — someone home by day (the popular pick)
For families with someone home by day (elderly, small children, a homemaker, occasional WFH) running 1–2 ACs; ~583 units/month covers a typical home's daytime load well and stays within the ≤5 kW sell-back cap — the sweet spot between value and roof area.
10 kW — big house / EV / work-from-home
For big houses with 3+ ACs, a pool/large pump, daytime EV charging or full-time WFH; ~1,167 units/month, needs a wide roof (~60–80 m²) and usually 3-phase; above 5 kW you cannot sell back and above 10 kWp you fall outside the home tax deduction, so design for maximum daytime self-use.
Why “Bigger Is Better” Is Not Always True
About the Author
Compiled by the CapSolar team under Frank Lee (Founder). CapSolar is a commercial & industrial solar EPC/PPA provider in Thailand that sizes systems from real load profiles as routine work and publishes every yield assumption so the numbers can be re-checked.
FAQ
Related Articles
Solar Payback: How Many Years & Worth It
The payback formula + 3-scenario table, after you pick a size.
On-Grid vs Off-Grid vs Hybrid
Pick the system type before the size.
Condo / Townhouse / Rental: Can You Install?
If you don't own a detached-house roof.
Thai Solar Yield per kWp
Where 1,300–1,500 units/kWp/yr comes from.
Want the Right Size for Your Own Home?
CapSolar sizes it from your 12 months of bills, your load profile and your real roof — free, no obligation.