How much do you save installing a 10KWh battery?

[matthurry1995]

Hi guys - I am in the UK thinking about installing a 10KWh battery to my 4KW solar panel set up.

Could anyone who has done similar (we are East Midlands) break down exactly how much money you think you save per month, what you paid, and whether or not you'd do it again?

Thanks!

[Ampster]

I am in California with rates as high as $0.55 per kWh and I have a a 42 kWh battery and a 5kW hybrid inverter AC coupled to 7 kW of Enphase inverters. My payback is over ten years for the hybrid and batteries but less that five years for the GT inverters. I also have a hedge against further erosion of Net Energy Metering benefits and a source to charge my two EVs. I also have frequent power outages and the benefit of having backup is not quantifiable.
Amortizing the cost of power expected from the GT solar my cost per kWh is $0.08. The additional cost to store that energy in my batteries is $0.20 per kWh. I did not completely answer your question because my circumstances are significantly different than yours. My costs are before Federal Investment Tax credits. It all depends on where you are standing.

[kingocat]

Amster

I have an 8 KW system and thinking about installing 2 powerwalls. Im currently paying sce about 15/month to delivery electricity to me.
I'm thinking with the Powerwall, i wouldn't have to pay 15/a month.
Is this right?

[RichardCullip]

Amster

I have an 8 KW system and thinking about installing 2 powerwalls. Im currently paying sce about 15/month to delivery electricity to me.
I'm thinking with the Powerwall, i wouldn't have to pay 15/a month.
Is this right?

I'm not sure about SCE billing but here in SDG&E territory you can't escape the minimum daily charge of ~$10/month plus any non-bypassable charges accrued when drawing power from the grid.

[J.P.M.]

Assuming 2 things:, 1.) you have some form of time dependent rate tariff in effect with your power provider (that is, some form of net metering that also includes different per kWh power energy rates as f(time), and 2.) emergency backup is not your primary consideration, savings gained by using stored energy - which I'd assume is either solar generated and stored, or stored by taking from the power provider when rates are low(er) and using when rates are higher - can be estimated for any period by multiplying the kWh so stored and used by the differential rate per kWh for the period in question.

If the differential rate is large and/or the energy stored and used is large, the amount of money saved will be greater than if those quantities are small(er).

An example: On a, say, 10 kWh battery, and using U.S. $ and the somewhat draconian California electricity rates for my POCO and a T.O.U. rate tariff for residential PV system customers, Summer on peak time rates are $ 0.63729/kWh and $0.31245/kWh for super off-peak times (midnite to 6 A.M.). Winter rates for 7 months of the year are $0.41142/kWh and $0.30630/kWh respectively.
Now, say I manage to generate and store (and use over the same 24 hr. period) 8 kWh/day average into that 10 kWh battery as it's pretty sunny here, and for the sake of simplicity those summer rates are in effect 365 days/yr.
Daily savings will be, roughly: ($0.63729/kWh - $0.31245/kWh) * 8 kWh/day = $0.32484/kWh *8 kWh = $2.60/day, or $2.60*365 days/yr. = $948/yr., or $948/yr./10 kWh storage battery size = $94.80/yr savings per kWh of nominal battery capacity.

If I use the moron method of financial analysis, and a 10 kWh battery costs me, say, $US 5,000, that'll be a $5,000/$948/yr = ~ 5.3 yrs. payback but probably longer as I assume no battery degradation or maintenance.

I'm also assuming/using for the sake of keeping the example simple that all of the use of the stored energy will be at super off peak rates which is probably unrealistic as some of it will probably be used at peak rates (saving nothing), with most of the stored energy most likely being used at semi off peak rates making that summer differential rate = $0.24858/kWh and the winter differential rate = $0.04724/kWh. More analysis with consideration of other facets of the situation are necessary that impact savings and so the cost analysis for my situation. For example, that "on peak" summer rate I used to calculate savings is only in effect for 765 hrs./yr. out of (24)*(365) = 8,760 hrs. in a year.

Things get complicated around here which makes things worse when put together with the sad fact that most folks want one word answers to situations not amenable to simple answers.

Numbers are for example only. Your results will be different but the basic logic ought to hold - that is, that the savings made possible by storing generated energy or purchasing and storing energy when rates are lower are primarily dependent on your utility's rate structure and how/if/to what extent your power provider uses differential rates as f(time of day, season) to manage their demand. Weather also plays a part but the cost analysis for storage is mostly f(POCO rates, tariffs, policies).

[RichardCullip]

Hi guys - I am in the UK thinking about installing a 10KWh battery to my 4KW solar panel set up.

Could anyone who has done similar (we are East Midlands) break down exactly how much money you think you save per month, what you paid, and whether or not you'd do it again?

Thanks!

Not sure you would save anything but Ican't answer your question without details about your electrical use and the rate structure of your electric provider. In addition, I doubt you would have enough excess solar generation to keep that 10kWh battery charged with only 4kW of solar panels.

[Ampster]

Amster
....
I'm thinking with the Powerwall, i wouldn't have to pay 15/a month.
Is this right?

I agree with RichardCullip that there is little you can do about the Minimum Delivery Charge. I have seen reports from other forums that PowerWall owners are getting compensated by joining the Tesla Virtual Power Plant and those payments may offset the MDCs.