Gulf Island Off-Grid System - Part 3

As we have almost two years of off-grid living under our belts, we're ready (and have saved up enough) to install an off-grid solar system for our guest house and wood shop.

We don't have many significant loads in our cabin (Part 1, Part 2), and our Internet Radio Repeater system is even smaller, so this represents the largest system we've worked with yet to date.

Load Analysis

Here are the major pieces of equipment that we're needing to power in the wood shop:

• Planer
• Drill press
• Table saw
• Miter/Cross-cut saw
• Table router
• Sawdust collector

The largest loads are the table saw (2 HP at 220V) and the planer (2.5 HP at 220V). Fortunately, they are rarely run at the same time.

While we're looking at doing most of our wood working during sunny days, so we're hoping that we don't have to dip into the battery (or run the genset) too often, but the surge current required to handle motor startup is a significant factor in our system design. Plus, with our weather, one often needs to do repairs during the winter.

The guest house loads are pretty conventional. Based on our cabin usage statistics, we're averaging 2 KWh per day, and when guests are over, 4 KWh per day.

Solar Configuration

We don't have very many good solar locations near the shop, with the best nearby location being around around 300 feet away. We're building a ground-mounted system for the panels, and we'll sadly have to take down a dozen trees to ensure we get good winter sun exposure.

Once the mount is built, we'll be putting up 24 Sharp ND-H235Q2 panels, which we were planning to arrange into two strings, each powering a SMA Sunny Boy 2500HF-US inverter.

For worst case voltage, we took the Voc (37.2v), multiplied it by 12, then by 1.2 for overvoltage. That gives us 535.7 volts, which is well below the maximum rated voltage of 600 volts. So, we're good there.

For max power, we took the rated power Pmax (235w) and multiplied it by 12. That gives us 2820 watts. The max power rating for the 2500hf-us is 2750 watts, so we're a little high there.

However, we have _really_ poor sun conditions, and we've never seen anything close to the rated panel power from our existing panels (we're lucky if we get 60% of the rated power on the best, most sunny day given our location), so this is close enough that we're still good. During very cold weather, we could get a higher theoretical current rating, but that is also the time when we get the weakest sun due to the low angle the sun is in the horizon.

For max amperage, we took the maximum power amperage Imp (7.81a), and with only a single string, that is well below the 15 amp rating.

For minimum voltage, we took the maximum power voltage Vpm (30.1) and multiply that by 12. That gives us 361.2 volts, which is well above the 220 v startup voltage rating of the inverter.

Unfortunately, after talking with SMA about our proposed configuration, they told us that due to "compatibility problems" with their own Sunny Island off-grid inverter, they no longer support using the 2000/2500/3000HF-US inverters for off-grid applications(!).

So, there went that plan.

We're currently re-assessing exactly what SMA inverter we will use instead of the 2500HF-US — we do want to stick with two inverters, so that in the case of an inverter failure, we haven't lost of our solar power, just half.

Inverter Configuration

We're going to run Tek cable from the solar array down to a 10 by 10 foot shipping container that we will install on a concrete foundation near the shop. We're going with an external building for resiliency purposes, and to keep the batteries separate from the guest home. Using a container also allows us to get the system assembled where the electricians live, which is substantially less expensive then bringing the electricians over to the island where we live.

Inside the shipping container, we'll install plywood on two of the three walls, and mount two Sunny Island 6048 inverters in a split-phase configuration. We're also installing a Sunny Smartformer, with a transfer switch so that if one of the two 6048 fails, we can resume operations using the surviving inverter.

DC wiring will run through a raceway to the battery bank located on the opposite wall from the inverters. This is illustrated in the below diagram:

Attachment not found.

From left to right we have:

• 200 Amp Load Centre
• Electrical box for transfer switch and power quality monitoring
• Smartformer
• Sunny Islands A & B

Everything will be connected via RS-485 and connected to a Sunny WebBox to allow for local and remote monitoring.

Battery Configuration

We'd like to be able to run our woodshop for 2 hours taking the batteries from a 100% SOC to 75% SOC, and we'd like to be able to have four days of load at the guest house taking the batteries to 50% SOC. Given the different discharge levels, these work out to roughly the same numbers of watt-hours, assuming that running a 2 1/2 HP motor consumes around 2 KWh per hour.

Plugging through the numbers, we get a battery size of 32 KWh, or around 666 amp-hours. That's a beast of a battery, but not too unmanageable, and SMA recommends that we consider increasing the battery size to 1000 amp-hours, given our plans to expand to 10 KW of solar next year.

We've been very happy with the Deka Unigy II batteries we have installed in they power system for our cabin, but these are pretty expensive, so we've been exploring other AGM and gel battery options, and we haven't finalized that part of the system.

Remaining Odds and Ends

The remaining bits and pieces include Tek cable back underground from the container to a load panel in the guest house, and a distribution panel in the shop. We could really confuse people by installing a power meter at the service entrance. ^_^

We're very impressed with SMA's products and the pre-sales support they've provided us. Their products are pricey, but in our opinion, it is well worth it.

So, given this description, any thoughts on how this system looks? Are our calculations in order, and look reasonable?

We're looking forward to your advice and wisdom, as usual, and we'll be posting photos as everything starts to get built and installed.

Peace,

David & Laura