Before talking about system size, you need to know which supply your home is on, because it determines the size you can install, the inverter price, and whether the meter has to be changed.
How to tell whether your home is single- or three-phase
- Look at the meter outside: two incoming wires means single-phase; four means three-phase.
- Look at the breaker box inside: a main breaker with a wide lever spanning three slots side by side is three-phase.
- Look at your electricity bill, which states the meter size.
- If you're not sure, send us photos of the meter and the breaker box and we'll take a look.
What the difference means in practice
| Item | Single-phase | Three-phase |
|---|---|---|
| Found in | Typical houses and townhouses | Large houses, shops and factories |
| Suitable solar size | Up to about 5 kW | 10 kW and up |
| Inverter price | Cheaper | About 15% more expensive |
| Load distribution | Everything on one line | Split across three lines, with less voltage drop under heavy loads |
A three-phase supply doesn't make electricity cheaper in itself — you're billed on units used either way. What you get is the ability to run heavy loads without the voltage dropping.
When the meter has to be changed
If your home is on single-phase and you want a system of 10 kW or more, you need to apply to the utility to switch to three-phase — a cost that is separate from the solar installation.
The Provincial Electricity Authority (PEA) published new meter installation fees in the Royal Gazette, effective from February 23, 2026. They start at THB 3,300 and go up by meter type and size — much lower than the previous 2019 rates.
Besides the meter, there may be costs to upgrade the incoming service cable and the breaker box to handle three-phase, depending on the condition of what's already there. Ask your installer to itemize this separately in the quote.
If you don't want to change the meter
- Install a system no bigger than a single-phase supply can take — which is enough for most homes anyway.
- Consider whether a smaller system suits your daytime use, because a system larger than your daytime use will send surplus power out at a low price.
- If it's a business that's about to expand, switching to three-phase once now may cost less than upgrading bit by bit later.
Frequently asked questions
Does switching to three-phase really make electricity cheaper?
No. Electricity is billed on the number of units you use, not the number of phases. What you gain is the ability to run heavy loads without the voltage dropping, and to install a larger solar system. The price per unit stays the same.
What's the largest solar system a single-phase home can take?
In practice, about 5 kW, which is enough for a home with a bill of roughly THB 3,000 to 5,000 a month. Only if your bill is much higher than that, so you need a bigger system, is it worth considering three-phase.
Who do I pay the meter change fee to?
Directly to the utility, not through the installer. If a quote includes this cost, ask whether it's what the utility actually charges, with the utility's receipt, or a flat fee.
Related services and areas
Keep reading
- PEA Meter Replacement Fees Are Much Lower in 2026 — Check the Current RateThe new 2026 PEA meter fees start at THB 3,300, well below the THB 8,500–9,000 set under the 2019 rules. Here's how to check which figure your quote uses.
- Monthly Bill of THB 3,000, 5,000 or 10,000 — What Size Solar System Do You Need?The best-value size is not the largest your roof can hold — it is the size that matches the electricity you use during the day.
- How Long Does Solar Installation Take, From Signing to Switching On?The installation itself takes only a few days, but the paperwork with the utility takes months. Many people assume the system can be used as soon as it's installed.