A solar quotation tends to contain several numbers that look alike: a 5 kWp system, a 5 kW inverter, a 10 kWh battery, 6,750 kWh of generation per year. If you do not know what each one measures, comparing two quotations goes wrong from the first line.
There are really only two ideas here — power and energy. Everything else just tells you which part of the system is being measured.
kW is power, kWh is energy
A kilowatt (kW) tells you how hard a device is pushing or drawing electricity at a given moment — like a car's speed. A kilowatt-hour (kWh) tells you how much electricity was used or produced over a period of time — like the distance the car has traveled.
The link between them is simple: power × hours = energy. An appliance drawing 1 kW for one hour uses 1 kWh. Run it for 8 hours and it uses 8 kWh.
One kWh equals one unit on your Thai electricity bill. The units you see each month are kWh, and the rate per unit is the price per kWh.
kWp: the size of the panel array
The p stands for peak — the panel's maximum output when tested under standard laboratory conditions. A current-generation panel is rated at about 620–630 watts peak, so a 5 kWp system uses roughly 8 panels (8 × 625 W = 5,000 W).
The key point is that kWp is an installed rating, not what you get at every moment. On a real roof that is hot and a little dusty, peak output on a clear day is usually around 75–85% of kWp, and less in the early morning, late afternoon and on overcast days.
The figure to plan finances with is therefore not kWp but energy produced per year, in kWh. For Southern Thailand we use about 1,350 kWh per kWp per year, with the rainy season included.
| Array size | Approx. panels | Energy per year (Southern Thailand) | Average per day | Average per month |
|---|---|---|---|---|
| 5 kWp | 8 panels | ~6,750 kWh | ~18.5 units | ~560 units |
| 10 kWp | 16 panels | ~13,500 kWh | ~37 units | ~1,125 units |
| 15 kWp | 24 panels | ~20,250 kWh | ~55 units | ~1,690 units |
| 20 kWp | 32 panels | ~27,000 kWh | ~74 units | ~2,250 units |
Monthly figures are full-year averages. Sunny months come in higher and monsoon months lower, so don't judge whether a system is performing from a single month.
An inverter's kW doesn't have to match the panels' kWp
The inverter converts direct current from the panels into the alternating current your building uses. Its kW rating is the maximum power it can deliver — not an amount of energy.
In practice, many systems are designed with slightly more panel capacity than inverter capacity, because panels almost never reach their full kWp on a real roof. This pairing lets the inverter run close to full output for longer each day. It is not a downgrade.
If a quotation just says 5 kW system without stating whether that refers to the panels or the inverter, ask. A 5 kWp array and a 5 kW inverter are different things, and how much energy you get depends mainly on the panel capacity.
Inverter size also has to suit your supply. A single-phase PEA meter handles about 10 kW in practice; anything larger needs a three-phase supply.
A battery's kWh is capacity, not power
Batteries carry two numbers that need to be read together.
- Capacity (kWh) is how much energy it can store — a 10 kWh battery holds roughly 10 units.
- Output power (kW) is how hard it can deliver at any moment, which decides how many appliances can run at once.
The capacity on the label is often the total (nominal) capacity. Usable capacity is lower, because the battery management system keeps a reserve that is never drawn down, to extend battery life. When comparing quotations, ask for the usable figure.
For example, if a home uses about 8 units between evening and morning, a battery with 5 kWh usable will only cover part of it. The rest is still bought from the grid. That is not a fault — it is the result of the size chosen.
How your electricity bill connects to all this
Your bill shows the units you buy from the utility each month. If a home uses 700 units a month and installs a 5 kWp system averaging about 560 units a month, the bill does not drop to 140 units. Some daytime output is not used in time and flows out, while evening and night use is still bought as before.
The units that actually disappear from your bill are the ones the system produces and the building uses at that moment. Our article on why you may still have an electricity bill after going solar explains this in detail.
Check a quotation against these numbers
- Total array size in kWp should equal watts per panel × number of panels. Multiply it yourself; if it doesn't match, ask.
- Panel brand, model and wattage should be stated clearly — not just a grade or a technology name.
- Inverter size in kW, brand, model, and whether it is single-phase or three-phase to match your meter.
- For a battery: total capacity, usable capacity and output power.
- Expected energy per year in kWh and the sunshine figure behind it — for Southern Thailand, around 1,350 kWh per kWp per year.
- Warranties listed separately for panels, inverter, battery and installation work, since each has a different term.
To compare two quotations, divide each total price by its array size in kWp to get a price per kilowatt, then check whether the brands, inverter and inclusions are the same. Comparing total prices for systems with different array sizes leads to the wrong conclusion.
Frequently asked questions
Does a 5 kW system produce 5 units a day?
No. 5 kW is power, not energy. A 5 kWp array in Southern Thailand produces an average of about 18.5 units a day across the year — more on sunny days, less on rainy ones.
Why does my app show a 5 kWp system peaking at only about 4 kW?
That is normal. kWp is measured under standard laboratory conditions. On a real roof — hotter, a little dusty, with the sun not square to the panels all day — peak output on a clear day is usually around 75–85% of kWp. Only if it is well below that on sunny days is it worth investigating.
Is it a problem if the panels have more capacity than the inverter?
No, it is a common design, because panels almost never reach their full kWp. What you should ask for is a quotation that states panel capacity and inverter capacity separately.
Will a 10 kWh battery run the air-con all night?
It depends on how many kW the air-con draws and for how many hours, plus anything else running at the same time, and on the battery's usable capacity, which is less than the label. Multiply the power of everything running by the hours, then compare that with the usable capacity.
The Thai solar tax deduction has a 10 kW limit. Which number does that refer to?
For individuals, the condition is total installed generating capacity of no more than 10 kilowatts. Before signing, make sure the size on the quotation matches the permit documents, and if you are unsure, check your own case with your accountant or the Revenue Department.
Related services and areas
Keep reading
- 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.
- Is Solar With a Battery Worth It in Areas With Frequent Power Cuts?A battery doesn't always save you more on electricity. What it really buys you is peace of mind during power cuts — and you have to judge whether that is worth the price.
- Single-Phase vs Three-Phase Power: Do You Need a New Meter for Solar?Your home's power supply sets how big a solar system you can install — and it's worth knowing early whether you need a new meter, which is a separate cost.
- Solar Payback in 2–3 Years? How to Work Out a Realistic Figure for Your HomePayback depends less on system size than on how much of the power you use yourself during the day. Here is every step of the calculation, and how much the result moves when each input changes.