The first choice in design is: consider paying somebody else to do the design. There are vendors who will do design and installation. Designing a PV system consists of selecting panel and inverter models, determining the number of panels, specifying the location, creating formal schematics, and taking into account costs and aesthetics.

Turnkey Solutions (pay somebody else to do everything)

There are several vendors on the island who offered to provide me with a turnkey system; they will do all the stuff described here. But they are expensive; I would have to pay a 30% premium for their services (almost 50% after tax credits).

The turnkey vendors justified this expense with their general smartness, ability to use a calculator at lightning speed, or promises of future support (however grid-tie PV systems are very reliable). I did not like the prospect of working with the vendor who wanted a large (70%) payment for being generous enough to accept my order. The rest of this site describes the alternative to throwing money at (to paraphrase the Harry Potter books) ‘Those Who Must Not Be Named’.

Designing the PV system (capacity, location, what to buy)

I needed PV panels, an inverter, an AC disconnect switch, mounting hardware, and assorted electrical wire, conduit, and boxes. Normally the system connects into the service panel (breaker box), but I had no spare slots in the breaker box so I used an AC disconnect switch with a built in fuse and connected the system at the main breaker located by the electric meter.

Choosing the panel and inverter models

If the rest of this section sounds too hard then buy a kit consisting of inverter, PV panels, and mounting hardware. Just pick one from the table that has the “PV Watts (STC)” around the size needed (Usage/Sunlight Estimate), and an “Output Vac” of 240.

The following paragraphs explain why I picked various brands for the major components. There are several choices of equipment vendors, and probably several clever people who will express strong opinions on which is best. As far as I can ascertain properly designed PV installations (without batteries) ‘just work’. My criterion was to select equipment that looked like it would minimize installation cost (minimum number of parts), and maximize reliability (no moving parts such as a fan).  Only time will tell if I made good choices.

For PV panels I chose the most powerful (most watts) available in order to minimize the total number of panels (and so installation cost), these were the Sanyo HIP-200 which have a power of 200W. For some manufactures the selling point is efficiency (watts per unit area), from my point of view it usually doesn’t matter if a panel is a few percent bigger (less efficient), unless roof space is absolutely critical. Sanyo marketing talks about the high temperature performance of the panel, it’s a good story but I don't know if it makes a noticeable difference. The major vendor by number of panels is I believe Sharp.

There are lots of inverter vendors (SMA, Xantrex, Outback, Fronius) all with similar electrical specifications. For the inverter I chose the Xantrex GT3.0 because it has an attached DC disconnect switch (one less thing for the installer to do); and a large heat sink with no fan (more reliable if adequate heat sink) .

PV panels are typically warranted for 20 or 25 years, inverters are typically warranted for 5 or 10 years.

Determine the number of panels

My goal is to have no net movement of my electricity meter over a year. I need to decide how many panels I need to meet this goal. [Update: in retrospect I learned of HELCO’s minimum bill of $20.18, this includes a connection fee of $10, and $10 worth of electricity or about 30kWh a month (even if I don't use it). So since I will always be buying about 1kWh a day from HELCO that would be a better design target than my original 0kWh/day. It is of course a small difference]

To determine the total number of PV panels, I needed to determine my current kWh/day usage and the effective kWh/day generated by each panel. The average kWh/day usage is shown in the bar graph on my electricity bill; the actual average daily numbers are in the fifth column to the right of the graph.

The effective kWh/day depends on the amount of sunlight at my house which I estimate is 4.4h/panel/day (see Sunlight Estimate for why 4.4).  So a single 200W panel will, I estimate, generate 880Wh/day (0.88kWh/day) of electricity at the grid connection. Conveniently the HELCO bill lists electricity usage in kWh/day, which divided by the (just determined) estimated power a panel generates gives the number of panels required. [Update: based on actual inverter data shown in Generating Power, a better Sunlight Estimate at my house is 4.1h/panel/day].

My actual usage is 8.0kWh/day, so I need 9.1 panels (8.0/0.88 = 9.1). I rounded 9.1 up to 10 panels. This rounding up means that my PV system will generate almost 10% more energy than I have said I need. Obviously the accuracy of this number of panels depends on the accuracy of the Sunlight Estimate, so I hope it is good. Psychologically humans are much happier with a positive error than a negative one (although in this case both have a cost) so I like the idea of the 10% positive margin (I even call it a margin rather than an error!). 

I can’t just connect any number of panels to an inverter: there are only certain allowable combinations. The reason lies in electrical engineering. As a consequence I may not be able to implement exactly the number of panels I previously determined that I needed (which is in part why I rounded 9.1 up to 10, rather than down to 9). Luckily the inverter manufactures provide configuration tools on the web. I used the Xantrex tool , in part it looks like this: :

The Xantrex configuration tool showed that my 10 PV panels would be configured as 2 strings of 5 panels. Please see the full Xantrex web page for details.

Location

The panels should face south and have a slope of about 20 degrees (the actual optimal angle varies by month but the average for this latitude is about 20, as shown here). A south facing roof is ideal. My panels are located adjacent to my solar hot water panels. I had to check the size of the remaining roof space and the total size of my panels. No objects such as trees can shade the panels, this is important. My roof is oriented 8 degrees west of south, so the peak energy will be reduces to 99% of a south facing roof (cosine of 8 degrees : 100 * cos(8) = 99.02% . FYI: 10 deg = 98.5%, 20 deg = 94%, 30 deg = 86.6%)

I talked with two electrical contractors who had experience with Net Metering PV installations (D R Electric, Powers Electrical); off grid PV experience is not sufficient because of the HELCO requirements. The contractors needed to know the size of the inverter and panels I had chosen when he looked at my house (have copies of the data sheets handy). He had suggestions about the inverter location as there are all kinds of electrical code regulations involved (also excessive heat can kill an inverter, don’t locate it in direct sun or in a hot attic). The contractor also looked at the orientation, size, and construction materials of my roof; and stud locations for mounting the inverter.

PV Panel Mounting Hardware

A row of panels mounts on a pair of rails attached by clamps, the rails then attach to the roof using feet; don’t even consider just using a few screws from Ace Hardware (handy as it is) to mount PV panels. I needed to decide the layout of my panels (how many rows of how many panels) based on the space on my roof. Then select the racks and clamps, from the catalogue of mounting hardware based on PV panel type and number of panels in a row.  The Unirac documents suggest that in a tropical environment the rails run up (rather than across) the roof, I assume this is for airflow to help cool the panels.

The rails, feet, and clamps are shown below on the left (note how the bolt heads slide into the rail). The black part is the clamp, it fits over a lip on the Sanyo PV panel as shown on the right. One clamp attaches two adjacent panels to one rail. The design of the clamps is different for each PV manufacturer, the H style clamps shown here are for Sanyo panels.

This is a sketch of my roof plan. The MC connectors are on the outside edge to facilitate connection, and this is also the edge of the panel that has the hole for the grounding wire and lug. The mounting rails are longer than needed so I have the option to add more panels in the future.

Boxes, conduit, wire, and screws

The I bought the electrical boxes (Equipment Purchase) and the contractors provided the remaining hardware.

Electrical Engineering (the blessing of a professional)

In order to have my configuration blessed as legal and meeting code I needed a Hawaii Professional Electrical Engineer (even if I had bought a kit which includes a schematic). Hawaii Professional Electrical Engineers are hard to find as they are usually employed by others, I worked with Art Russell he was great. We traded a few emails to scope the job and his price was reasonable.

I sent Art the information about the equipment I had decided on, and some photos of my house annotated with text and arrows to show PV equipment locations and a shaded gray rectangle for the inverter location (I used MS Paint).  These are the photos I sent to Art:

The engineer then generated all the electrical details specifying wire sizes, panel voltages, switch, fuses, and conduit and so on; and created a schematic. Art added his stamp the schematic; this is required by Hawaii County for a building permit.

Be certain all the information provided to the engineer is complete and final. If he ends up doing extra work because of any change for whatever reason there will probably be extra billing. Rework is frustrating for all concerned, try to avoid it.

This is a partial detail from the schematic:



The schematic shows two strings of 5 panels, connected to a Combiner Box, then the Inverter, the AC Disconnect, and finally the main breaker.

Tax Credits (get the blessing of tax professional)

There are some great tax credits from the State and the Federal ‘Residential energy efficient property credit.’

I designed the system with a partially populated PV array, so that I can expand the system at a future date and take advantage of additional tax credits.

A ‘How To’ for Grid-Tie PV on the Big Island of Hawaii

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