Solar power calculations

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Ultimate Geek
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#270108 23-Apr-2020 16:42

I'm trying to work out some figures for solar power.

I've been quoted $11,500 for a 4kW solar setup with a 2.4kwH battery. There is an option for a second battery but that can be added later.

They've estimated monthly savings of around $50 but that's without the savings from the battery.

The 2.4kwH sounds quite small compared to say Tesla's powerwall 2 at 13.2kWh but then you get what you pay for and 2.4kwh may be enough for now and it can be expanded anyway.

I'm trying to work out though, if the battery is fully charged from the 4kw cells every day, how much do I save from night time use with the battery?

Is it as simple as saying it's a 2.4kwh capacity so can deliver 2.4kwh which at say 22c = 2.4*.22 = 53 cents a day?

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#2468778 23-Apr-2020 16:47

I would seriously only consider solar if someone is there most of the day to use it as it is generated. You do not save money storing it or selling it.

boosacnoodle

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#2468796 23-Apr-2020 17:12

If you are signed up Flick Electric on solar you sell back to the grid at wholesale which can make you some good money when wholesale is high and go on their Fixie product so that you have the best of both worlds (sell high, buy low). Unfortunately this has changed in recent months and shouldn't be a major factor you consider when costing but still a factor nonetheless.

About the Powerwall. These tend to be in the order of $12k+ installed. About batteries generally. These tend to not make sense due to most people receiving a night rate that is orders of magnitude cheaper than their day rate. Storing 2.4kWh is tiny and likely to make little difference in the grand scheme. About your 53 cents a day calculation it's actually less due to the losses involved with storing then using the power again so you can clearly see that the benefit is minimal.

Your solar quote also seems to be quite high, but this is likely the doing of the battery inclusion. I would ask for a quote without the battery. Also if you're using one of the larger outfits beware that they use subcontractors and in many cases are just the to clip the ticket (cough Harrisons cough). Harrisons also pad the price if you pay using "interest-free" - they have a separate cash price.

JeremyNzl

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#2468804 23-Apr-2020 17:22

Have a read of this link, If max capacity of your battery is 2.4kw, you don't have 2.4kw useable.

To get a better understanding have a read of this link.

https://www.solarchoice.net.au/blog/depth-of-discharge-for-solar-battery-storage

snnet

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#2468811 23-Apr-2020 17:41

Also keep in mind what actually happens with a typical solar installation -

It does not send the solar/battery capacity thru the entire house. They typically have 20A or 32A of capacity depending on what you have, so you'd only have a few circuits running thru the system.

So many of my customers (electrician) assume the solar will just be used no matter what they turn on - stove, heat pump, etc. - not the case. On one job a client had me move things around in the switchboard to change what was powered. He is rural and the installers put some power points beside beds on the system, nothing more. Obviously it'd be handy to have the water pump, fridge/freezer, some lights, maybe the garage door/gate / electric fence if possible.......

Mark

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#2468820 23-Apr-2020 18:03

My little setup on the a cottage has a ~5kWh battery bank, but as it's lead acid I'd not run it down past 50% so it's really only a ~2.5kWh.

My main house I rent a system from Vector, it is 3kw of panels and 10kw of battery (usable number, it's lithium based which you can run down 80% so probably really a 12Kw bank) .. costing me $85 a month, had it in for 6 years now.

In the past 12months the house has used 9816kWh, of which 3344kWh was from solar/battery and 1157kWH was exported to the grid (we are quite a heavy power use house), it's not worth exporting power to be honest, I'd rather have more storage.

k1w1k1d

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#2468842 23-Apr-2020 18:57

Hi Mark, probably got this wrong, but does your system actually pay?

$85 x 12months = $1020 per year.

$1020/3344kWh = 30.5c per kWh self generated and used. What is your kWh rate?

You exported 1157kwh back to the grid. Don't know what Vector pay you, but at 10c per kWh that would only be $115.70?

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Mark

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#2468852 23-Apr-2020 19:07

No it's not economical for me :-) I live rural, and power outages are common so for me it's more a reliability thing for the house, most power glitches we barely notice, extended outages we can get through with power management. Where I am no power means no water, no light, no internet, no opening garage doors, friedges and freezers offline etc. Solar setup covers lighting, water pump, gas hot water, internet, fridges/feezers and selected power points (Vector and Aztech did a very nice tidy install).

Monthly bills for the house in winter are still hideous (too many kids and a south african wife who is cold in anything less than 27degrees!), summertime I can get monthly bills of $10 (plus the solar rental). Renting has still be cheaper than buying the setup so far .. and any issues are solved by a team of people.

k1w1k1d

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#2468854 23-Apr-2020 19:13

Makes more sense with your dodgy rural power supply, so you use it as a backup rather than to save money.

Have you ever considered a stand by generator?

Mark

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#2468858 23-Apr-2020 19:25

k1w1k1d:

Makes more sense with your dodgy rural power supply, so you use it as a backup rather than to save money.

Have you ever considered a stand by generator?

For my next house I might get a generator as well, current setup it's not supported (as it's a rented solution.

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#2468862 23-Apr-2020 19:39

JeremyNzl:

Have a read of this link, If max capacity of your battery is 2.4kw, you don't have 2.4kw useable.

To get a better understanding have a read of this link.

https://www.solarchoice.net.au/blog/depth-of-discharge-for-solar-battery-storage

Interesting - thanks.

So let me run through an example to get my numbers right. I don't have the details of the battery but the sales person did mention it was about $3k for another one. I found this one online so will use this as an example: https://mytools.co.nz/products/pylontech-2-4kwh-battery-us2000-2400-us-series

Cost $3822

2.5kwh @ 80% DOD = 1.92kwh usable

6000 cycle life so 4500 x 1.92kwh = total life of 11520kwh

$3822/8640 = 33.2 cents per kwh - plus the cost of the panels as well.

I'm better off staying on the grid! The battery will need to get down to around $2,300 to make it comparable to grid prices.

Edited to add in total cost:

Total cost of installation $11,500

Battery pack has life of 16 years (6000 cycles / 365 days)

IRR say 5% (i.e. cost to borrow $11,500 at say 5%) = monthly mortgage repayments of $87 x12 = $1044 p.a.

They say I will save $51 a month ($612) in power plus the battery savings. Battery savings is 1.92kwh x 22c = 42c per day = $153.30 p.a.

Total savings $612 +153 = $765. A bit short of the $1044 it will cost.

However that doesn't take into account that the system is good for at least 25 years (or longer) but the battery is only good for 16 years. If I amortise the cost over 25 years it works out at $803 p.a. so only $38 p.a. difference however the battery will be long gone at 16 years. Battery technology will probably be better (and cheaper) in 16 years plus power prices will increase resulting in bigger savings. Still doesn't look like there's much in it for the effort.

KrazyKid

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#2468919 23-Apr-2020 22:34

Last time I looked at solar down here in Dunedin I think I calculated a 12 year payback without a battery. Seemed marginal to me. Brought a cheap electric car (Leaf).
Figured that since 100% power in the south Island was green and as cheap as the solar, a car was a better choice to save money and the environment.
Maybe solar will be cheaper again in 5 years.

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Jase2985

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#2468997 24-Apr-2020 08:10

my mate after a years worth of data has worked out about an 11 year payback on his setup. No batteries but has a high base load so a lot of self consumption.

linw

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#2469000 24-Apr-2020 08:22

So, with 11 yr payback, by the time you break even, it would be renewal time. Makes no sense to me.

Makes more sense in hotter climes when you need cooling when the panels are producing.

kotuku4

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#2469001 24-Apr-2020 08:24

The price quoted seems ok. The battery is wimpy. Do you have electric water heating? If yes consider a solar diverter.

Solar probably won't work out financially, on buy back. Branz, consumer, ECCA come to the same conclusion. There is a great tool of the EECA website to calculate. https://www.energywise.govt.nz/tools/solar-tool/

I ran the numbers for my situation, and had a 10 year payback.

I have a 7 year old system, with some higher buy back rates earlier on, I now consider it paid off. Still looking at battery options and electric car, but they still seem way off to me. I'm happy that I invested in my own power company (Solar)

tdgeek

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#2469004 24-Apr-2020 08:26

I love solar. But its not worth it for many. You cant generate it when you need it. While we have a large house our roof profile is intricate so cant really get much over 3.5kW and not cheap the way it would need to be, but if you can do whatever you can to use it, it could be ok, that was my rationale. Timeshifting. Delay washing and dishes to sunnier days etc if you can. The Powerwall 2 is too expensive but very cool (Ihave the app for my mates install to look at. But 13kW at 16c night rate is a LONG payback. Te otgher way to look at it is if the payback is say 13 years, its guaranteed for 25 so you get the last 12 years as savings with no capital cost. But you do need to weigh up the opportuntity cost when you buy an install. The sunshine hours might be 2200 per annum, but in the Summer months you dont use much power, and the credit is low. In Winter you wont generate much and you will be paying the grid for the vast majority of Winter power.

I recommend Solar HW. Ours was 8k back in 2011, including 300L cylinder. Its great and a real money saver, but you do need to manage it. If left to the standard solar HW with grid boost its not optimal.

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