How many kW does your roof need?

How to size a rooftop solar system from your roof area and your monthly electricity bill.

Guide · 09/10/2026 · 2 min read

Cover graphic for “How many kW does your roof need?”: the article title on a navy background with a solar-panel grid motif and the Asia Energy name

The first question almost everyone asks: how many kW does my roof actually need? Undersize it and you still buy power from the grid; oversize it and you have money sitting in capacity you never use. This article shows how to size a rooftop solar system using two things you already have: your electricity bill and your roof area.

Start from the bill, not from the roof

Many people start by measuring the roof. Working the other way round is more reliable: first find out how much electricity you really use, then work out how many panels you need to cover it.

Take your most recent bill and divide it by the average electricity price to get your monthly kWh. This site calculates on an average of 2,700 VND per kWh. A bill of 1.2 million VND works out at roughly 440 kWh a month. That is the figure to cover.

From kWh to the kW you need

In Vietnam one kWp of panels produces roughly 3.6 – 4.2 kWh a day averaged across the year — less in the rainy season, more in the dry season. Multiplied by 30 days, each kWp gives about 108 – 126 kWh a month.

Divide 440 kWh by that and you get roughly 3.5 – 4 kWp. In the bundle table on the home page that lands on the 5 kW bundle — and a 5 kW system covers most of the bill rather than all of it, because at night you still draw from the grid unless you have storage.

Check it against your roof area

Only once the capacity is settled should you turn to the roof. Each Mono 630W panel takes roughly 3.6 – 4.2 m² once you allow walkways for installation and later maintenance. The 5 kW bundle needs 8 panels, so about 29 – 34 m² of sun-facing roof.

What usually eats into that is shade: chimneys, water tanks, tall trees, the neighbouring building. A single panel partly in shadow drags down the whole string it is wired into. On a real survey the usable area is almost always smaller than the area on the drawing.

Five sizes for quick reference

3 kW — 5 panels, about 18 – 21 m². A two or three person household.
5 kW — 8 panels, about 29 – 34 m². A four or five person household.
6 kW — 10 panels, about 36 – 42 m². A home running air conditioning often.
8 kW — 13 panels, about 47 – 55 m². A large house or small office.
12 kW — 20 panels, about 72 – 84 m². The most frequently chosen size.

When you use the power decides whether you need storage

Two homes both using 440 kWh behave very differently if one uses it during the day and the other in the evening. A daytime user — an office, a small workshop, a home with someone in it — consumes the power as it is generated and does well without storage. A household that mostly uses power after dark will waste most of what the panels make during the day, and that is when storage starts to make sense.

These numbers are estimates

Area per panel, output per kWp and the average electricity price are all industry norms, not commitments. Roof pitch, orientation, shading and how sunny your region is each move the figures. The only reliable way is to have an engineer measure the roof and calculate on real numbers.

Need advice for your own roof?

Asia Energy surveys your roof free of charge and returns a plan with estimated output, all-in cost and payback time.

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