Tuesday, February 16, 2016

Our House Build - PV Solar - Why Not Yet

PV solar is and always has been on the cards for our house.

We are not doing it at first however.

The main reason for this is cost. Our funds are limited and this is the exact reason we aren't putting any PV solar in at first. It's the same reason we're not wiring or lighting the shed at first.

Paying for the system over a 30 year mortgage doesn't make sense, since it'll probably need to be updated/replaced in 10-15 years also.

There are other reasons which I'll try and explain here. Like my A/C post, I'll try and be as accurate as I can based on what I know - but there's no guarantee that I know what I'm talking about.

One of these reasons is that because we don't know our new house, we don't know how to anticipate our energy costs. Currently we use around 22kWh per day. We don't currently heat our home with electricity though. This may make a small, or large difference to this consumption. Our current home is quite dark, the light filled living areas of the new house may reduce our electric lighting requirements. The kids often leave lights on in our current home - the main culprits are planned to be sensor driven lights and so this should reduce some consumption.

One thing man people overlook with solar is the way you pay and are paid for electricity. You don't pay for anything you generate when you use it immediately. If you don't use it, you're paid around 7 cents per kWh. If your panels aren't generating power (it's dark, overcast etc) then you buy it at around 25 cents per kWh. I'd originally assumed that your meter just totalled the difference at the end of the month, and that you paid 25c/kWh you used. The problem with that assumption vs the actual way it works means that the TIME you use your electricity matters, a lot.

We have a 60kW solar array at work, and while it halves the energy costs of the building, the energy it produces in excess of the requirements only pays for a quarter of what the building uses while the panels aren't generating power.

Sizing the system larger than the amount of power you need during sun-shine times is pointless; without a feed in tariff to make it possible to make serious money out of your excessive solar production, your over-the-top-for-your-needs solar array won't pay for itself for a very long time. Possibly not before it starts to fail or becomes due for replacement.

Thinking about the way most families use energy, the evening when solar drops off (in summer) or is non existant (in winter) is when most energy is used. The house heats up (or cools down) in preperation for your arrival home, you switch lights on, you run your oven, you use your televisions, charge your phones, plug in laptops/computers etc. I'd wager at least 60% of our current 22kWh is in this non-solar time.

Battery or other storage (flywheel etc) systems are the solution. Some remote regions use central community storage solutions to limit the region's exposure to power supply cuts however that's a very distant and unlikely reality in our area.

Tesla's powerwall system is an interesting technology but still expensive for us. At 6.5kWh of storage, we'd need two to cover our ~13kWh evening usage. At $12,000 approx per unit, that's $24,000 - and that is a whopping 96,000kWh worth of electricity at 25c/kWh - Assuming 13kWh/evening that's just over 20 years before it pays for itself. Given that in ten years time the batteries are likely to be due for replacement, it's hardly a worthwhile investment. Further, in ten years time, once the tech becomes mainstream (as PV solar has) the same capacity is likely to be available for a quarter of the cost and hence a much shorter pay-back.

It's not all bad news, what it does tell me is that I don't actually need a 7kW array. 7kW should generate 22kWh on average; but because of when we use our electricity it would be a waste. I believe it's possible to get information from powercor about when you use your electricity via your smart meter. Once we know that information about the new property, we can work out our actual requirement for PV solar to cover our during-the-day use - including for pre-arrival air conditioning use in summer, and size the array to suit.

Interesting as a side note on the powerwall (or other battery based) unit - if you buy one, they use 7kW from your day-time PV to charge enough to provide 6.5kW of battery based energy. If you've got a poorly sized solar array with excess power consumption you may be able to add a powerwall without any extra PV solar requirement. Further, if it depletes, it'll probably not do so until you're into your off-peak power time. This means your remaining grid based power will be at a cheaper rate, and you can even use that off peak priced grid power to charge the powerwall enough to run your HVAC to condition the house for the morning before the solar panels see the sun for the new day.

Most of the energy companies currently advertising finance options on PV solar are cashing in on the opportunity to make bonus money from punters unaware of how PV solar works; assuming the only payment they will have is for the solar equipment - while they'll still actually be paying for grid fed electricity for their evening use - and they're also charging the systems at way over the going rate (have a look at what's included and the pricing and check for yourself!)

We'll save for whatever system we end up using once we work out exactly what we need.

Hopefully by that time micro-inverter systems are affordable; they're great in that if one panel or inverter fails, only that panel stops producing power. People we know have had an inverter fail in a single large inverter system, and not noticed until they get a power bill with no solar shown!

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