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On grid or off-grid? Some advice for a Solar Choice customer

We were recently in touch with a customer who was deliberating between whether to have his new-build home connected to the grid, or to go off-grid with a solar-plus-storage system. Located in regional Queensland, the customer’s home had no existing grid connection was over half a kilometre away from the nearest distribution lines. With that distance, a new connection would have been expensive, potentially making a solar+storage system the more financially viable option.

Below is a slightly edited version of the email that we sent to the customer (who, by the way, is opting for an off-grid system in the end). Our aim, as always, was to help enlighten them on their range of options to help them make a more well-informed decision.

We’ve put together some notes for your situation to get you started on deciding whether or not installing an off-grid system might be the right option for your place. We hope it helps you make a decision!

How much will grid connection cost you? That’s the biggest question. If it’s less than $10,000, probably makes sense to go with grid connection. If it’s over $10k, the case starts to tilt in favour of getting an off-grid system installed. If it’s $20,000, you’d almost definitely be better off with the off-grid system.

I’m assuming that your home will consume about 15kWh (units) of energy per day. It’s possible, however, that you might end up using more like 20kWh/day with two people.

You’ll want an 8-10kW solar system. Let’s assume a 10kW solar system to be conservative, and assume the cost will be around $10,000-$12,000 for the solar alone.

You’d also need about 30-35 usable kilowatt-hours of battery capacity. This would give you about 2.5-3 days of ‘energy autonomy’ – the ability to run on just the sun and batteries with a bit of ‘fat’ to get you through patches of inclement weather. If you end up using 20kWh per day, you’ll have more like 2-2.5 days of energy autonomy.

Keep in mind that not all battery chemistries are the same – you can only use 40-50% of the capacity of lead acid battery, while you can use 90% of the capacity of a lithium-ion battery (e.g. Tesla Powerwall or LG Chem RESU) or 100% of the capacity of Aquion’s ‘salt-water’ batteries or RedFlow’s flow batteries. (Learn more about ‘depth of discharge’ – DoD – here.)

This translates into about 35kWh of lithium-ion battery capacity or about 60kWh of lead acid battery capacity. Lithium-ion batteries are more robust and have longer life spans (typical warranty period of 10 years). Lead acid should be a bit cheaper than lithium-ion, but will not last as long (typical warranty period 3-5 years, depending on brand, maker, usage case and type of battery).

Here’s what you should look for: Either a) 8-10kW solar plus 60kWh of lead acid batteries for about $25,000-$30,000 (with the expecation that you’ll replace the batteries in 5 years), or 8-10kW solar plus 35kWh of lithium-ion batteries for $40,000-47,000 (with the expectation that you’ll replace the batteries in 10 years).

Alternatively, you could opt for 10kW of solar and 32kWh of Aquion (saltwater) or Redflow (‘flow battery’) – these are both relatively unconventional battery types that show a lot of promise and can be fully discharged without damaging the batteries. The Aquion battery has a warranty of 5 years (plus another 3 years of prorated discount on replacement), so you’d want the total system price to come in at about $25-30,000 (unfortunately they are much more expensive than this at the moment). As for RedFlow, they have a 10 year warranty, so you should feel okay paying about what you would pay for lithium-ion ($40-$47,000).

All of this should be taken as general guidance. The next step would be to get some quotes & second opinions from installers!

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