Battery ROI

D_Eng

Hi all,

Having an argument with myself on battery payback periods, so I said I'd ask the experts.

I have 6kWp hybrid system with 2x ~5kWh batteries, and an EV.

For solar energy, there are 3 main ways the energy can be used; 1. Direct self-use, 2. Export, 3. Battery storage (not the EV). I'm with Electric Ireland (Nightboost EV), so I'm going to talk in terms of their rates.

Scenario 1, Self-use. Each unit "earns" the equivalent of day rate - 1x €0.35

Scenario 2, Export. Each unit earns the export rate - 1x €0.21

All fairly straight forward.

Scenario 3, Battery storage — This is where my argument begins.

Initially, I classed this as effectively self-use, because I'd use up the battery (almost) everyday before the night rate began, and therefore Each unit stored "earns" the equivalent of day rate - 1x €0.35, same as scenario 1.

However, recently it occurred to me, if I didn't have the batteries, I'd have exported this and earned the export rate - 1x €0.21.

So, should each unit (of solar energy) stored in my battery actually "earn" 1x (0.35-0.21) i.e. 0.14, the difference between day & export rate?

I do charge my batteries at night as well, but that energy isn't included in these calculations.

10-10-20

Yes, the feed in tariff (FiT) means that the ROI on a battery is lengthened as the value of a solar kWh unit is reduced by the FiT rate offered. A battery comes into it's own when the imported unit tariff is split across the day, such as the peak rate between 5pm to 7pm on smart meters for example.

Additionally you can extrapolate from this and think about variable rates on import and export, a cheap import unit rate offered during the day may be stored within the battery and exported back to the grid for a higher FiT unit price later in the evening, etc.

curioustony

https://www.boards.ie/discussion/2058371754/battery-roi

@D_Eng, Stick your figures into 'eco power optimiser'. It will do the calcs, and compare costs for a year across whatever suppliers you add to the app

I think you are forgetting about the fact that not all days or all parts of every day, will be covered by solar. The battery balances this when you would otherwise be paying full whack

kabakuyu

You are paying too much for your day rate,I pay 25c Inc vat with flogas.night rate is 12.5c Inc of vat.

10-10-20

Or have a play with you kWh data on this site:

https://garydoessolar.com/utilities/dailymodellingutility/

D_Eng

Thanks for the info @curioustony & @10-10-20

I'm not confused about the benefits of batteries, I'm glad I have them. Just thinking about getting a few more, but doing the sums first. I've pretty much maxed out the "nightboost" charging window (with the EV as well). Could charge on the normal "night" rate, but the margin is even smaller.

With the market price of batteries falling, and the relatively low rate per solar kWh stored, I think I'll hold off for another while.

D_Eng

https://www.boards.ie/discussion/comment/122679259#Comment_122679259

Good for you. I'm not looking for supplier recommendations. Thanks.

10-10-20

https://www.boards.ie/discussion/comment/122679303#Comment_122679303

In that case do play with that site that I linked, it will show you the benefits of sizing according to your questions. 👍️

kabakuyu

https://www.boards.ie/discussion/comment/122679321#Comment_122679321

It wasn't a recommendation, i was merely pointing out you are on an expensive tarriff.

Unfortunately alot of people pay too much for their electricity either through inertia or ignorance.

slave1

As you are looking at Payback then your rate of electricity is too high so the FloGas point is very relevant.

I see lots of folk generating huge savings because the are paying through the nose for their electricity, that's false economy.

To adjust your approach you should take into account the cost of night charging (and maximise this, you can charge at a higher rate to charge them up quicker) with an efficiency factor of say, 90%. Then if you are also charging during the day your cost is the higher of either cost of import or opportunity missed from not exporting. Your savings should be through using the battery during your highest tariff period

Summer2020

FIT rates won’t be here forever , other countries already reducing them . A battery makes sense

D_Eng

https://www.boards.ie/discussion/comment/122679767#Comment_122679767

Fair point, so really the payback should be benchmarked against the best tariff I could get at that time. Well aware there are lower rates available. I'm in contract with EI for now. Looking at EnergyPal with my smart meter data, it's the 2nd best for my current use case. But I suppose that is self-fulfilling in a way.

I appreciate the sounding board, long time lurker, but first time conversing on this.

bullit_dodger

I think FIT will always be there, but perhaps not the (generous) rates that we can get today. I mean €0.24/kWh with Energia currently is pretty good. In terms of the battery ROI which is the original question, as slave mentions the rate your being charged for your import/export is key.

A simplistic way to look at the whole ROI is create a "baseline floor" (your mimimum) and then your ceiling assuming perfect stategy, and your savings/earning will naturally have to be in between

Assume you got in 10Kwh battery. Taking the view of filling that up everynight, 365 days a year and then exporting it at FIT rates, and account for 15% losses, then you can work out what you can generate per day. Lets go night rate = €0.12, FIT rate = €0.21 and day rate = €0.30 (makie up numbers not from any specific supplier)

So that's cost to fill battery = 10 (kwh) x €0.12 (price per kwh) = €1.20
Exporting 85% of that out (to account for losses) = 8.5 x €0.21 (FIT rate) = €1.78

Profit per day = €0.58…..or 365 days = €211/year

So your "baseline floor" if you forget solar, and just use the battery to import/export units will save/earn you €211 a year. This is the minimum that it will save you. However we know that your battery will save you from having to import more expensive "day units".

So if you take filling the battery at night, and you were able to use every single unit at day rate (stopping you from having to buy in day rates at €0.30/kWh…..you'll save

Cost to fill battery = 10 (kwh) x €0.12 (price per kwh) = €1.20
Not importing 85% of that at day rate (to account for losses) = 8.5 x €0.30 (day rate) = €2.55
Savings per day = €2.55 - €1.20 = €1.35/day = €492/year

So your returns on a battery will be between this €211 and €492 figures. Roughly speaking since you won't get a "perfect" consumption your reality will be in about the €300-350 / year ballpark.

slave1

….and if the difference between your cheapest rate and export FIT is not that different then I'd forget about exporting for minimal benefit, especially when losses and battery degradation taken into account, mine is 8c before losses so just not worth it

Exiled Rebel

https://www.boards.ie/discussion/comment/122680435#Comment_122680435

What also needs to be factored in is the pound of flesh Revenue will no doubt want at some point in the future for all those exporting over tax free amount - the €211 in your example is almost half the allowance a single bill payer is allowed. With approx. 1000 PV installations per week they will no doubt come calling for this handy passive income stream before too long.

As things stand with FIT it's hard to make the case for an off the shelf 10kWh battery if you're only saving 300-350 / year.

bullit_dodger

https://www.boards.ie/discussion/comment/122680435#Comment_122680435

Agreed - but the math above was really just to help create a lower bound of the savings your battery could generate, to ballpark the savings. Like you mate, I don't force export either as my night rate is €0.18 and my FIT is €0.24, so at about €0.05 cent / kWh, it's hardly worth it.

(Aside: I know I really should look around since my Energia contract expired)