How Many Solar Panels Do I Need in Hawaii?

Grab your last HECO bill. You can answer this question in about 60 seconds.

The number of panels you need comes down to three things: how much electricity you use, how much sun your roof gets, and what wattage panels you choose. Hawaii makes the math easy because our sunshine barely changes from month to month — no dramatic seasonal swings like the mainland.

The 60-Second Formula

1. Take your monthly HECO bill and divide by $0.40 (the approximate Oahu residential rate per kWh).^[1] That gives you your monthly kWh usage.
2. Divide that kWh number by 130. That is your system size in kilowatts. The 130 factor accounts for Hawaii's peak sun hours and real-world system efficiency.^[2]
3. Divide the system size (in watts) by your panel wattage. For the REC 460W panels we typically install,^[3] a 7,700W system needs 17 panels. For Hyundai 440W panels, the same system needs 18.

So a $400/month HECO bill works out to roughly 1,000 kWh, which needs about a 7.7 kW system, which means 17 REC panels on your roof.

The Quick Reference Table

If you do not feel like doing math, find your bill amount below:

Monthly Bill	Monthly kWh	System Size	REC 460W Panels	Hyundai 440W Panels
$200	~500	~4 kW	9	10
$300	~750	~6 kW	13	14
$400	~1,000	~8 kW	17	18
$500	~1,250	~10 kW	22	23
$600	~1,500	~12 kW	26	28
$700	~1,750	~14 kW	30	32

Based on Oahu HECO residential rates (~$0.40/kWh), 5.2 peak sun hours, 85% system efficiency, and 95% bill offset target.

Why Your Number Might Be Different

The table above assumes a south-facing roof with full sun exposure. In practice, most roofs are not that perfect, and a few real-world factors push the panel count up or down.

Which Way Your Roof Faces

South-facing roofs are the gold standard. East and west-facing roofs produce roughly 10 to 15% less energy per panel,^[2] which means you may need one to three extra panels to hit the same output. We install on east and west roof faces all the time — the production loss is modest and the math still works. North-facing roofs are a different story. We generally skip those entirely in Hawaii because the sun angle makes them unproductive.

Shade

A big monkeypod tree that shades half your roof at 2pm changes everything. Trees, neighboring buildings, chimneys, satellite dishes — anything that casts a shadow on your panels reduces output. The good news with Enphase microinverters is that shading on one panel does not drag down the rest of the array. The bad news is that shaded panels still produce less on their own. We map every shade source during the free site survey so the design accounts for reality, not just the Google Earth view.

How Much Roof You Have to Work With

Each 460W panel takes up about 21 square feet.^[3] A 10 kW system — 22 panels — needs roughly 460 square feet of usable roof area. That sounds like a lot until you subtract the area lost to vents, skylights, plumbing stacks, and fire code setbacks from the edges. Homes with complex rooflines that break the surface into small sections lose more usable area than a simple gable roof. Sometimes the roof dictates the system size more than the electric bill does.

What You Are Planning Next

This is the one most people do not think about until it is too late. If you are considering an EV, a pool heater, or central air conditioning in the next few years, factor that electricity into your sizing now. Adding panels later is not just more expensive per panel — it requires a brand new HECO interconnection application^[4] and all the permitting that comes with it. We have seen homeowners buy an EV six months after their solar install and end up with a system that only covers 60% of their new usage. Size for where your electricity needs are going, not just where they are today.

Get the Exact Number for Your Roof

The formula and table above get you in the right ballpark, but your actual panel count depends on your specific roof, shading, orientation, and electrical usage patterns. Our free solar calculator uses your actual HECO bill to estimate system size, panel count, battery recommendation, and savings projection. Takes about 60 seconds. For a complete system design with a roof layout and shade analysis from your actual property, request a free in-home estimate.