Green Building Forum - Contemplating some storage for my grid-tied PV Mon, 25 May 2026 10:16:24 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 Contemplating some storage for my grid-tied PV https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269552#Comment_269552 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269552#Comment_269552 Wed, 25 Jul 2018 09:47:51 +0100 gravelld Posted By: djh
Presuming the relay/contactor was suitably rated that should be possible. But most diverters will divert arbitrary fractions of the power and connect and disconnect frequently and arbitrarily. I can't think of many inductive devices that are designed for an intermittent supply like that.We use a oil filled electric radiator like this to contribute to heating in the shoulder months.]]> Contemplating some storage for my grid-tied PV https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269555#Comment_269555 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269555#Comment_269555 Wed, 25 Jul 2018 12:37:05 +0100 DamonHD
Looking at our usage, the only time we have excess electricity generation over usage and have heating demand is maybe one month, eg March last year:

http://www.earth.org.uk/saving-electricity-2017.html

Maybe a bit of April and October in 2016 also...

Rgds

Damon]]> Contemplating some storage for my grid-tied PV https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269558#Comment_269558 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269558#Comment_269558 Wed, 25 Jul 2018 13:33:05 +0100 djh Posted By: gravelld
Posted By: djh
Presuming the relay/contactor was suitably rated that should be possible. But most diverters will divert arbitrary fractions of the power and connect and disconnect frequently and arbitrarily. I can't think of many inductive devices that are designed for an intermittent supply like that.
We use a oil filled electric radiator like this to contribute to heating in the shoulder months.
An oil-filled radiator is resistive, not inductive, isn't it?]]> Contemplating some storage for my grid-tied PV https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269559#Comment_269559 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269559#Comment_269559 Wed, 25 Jul 2018 13:43:02 +0100 DamonHD
Rgds

Damon]]> Contemplating some storage for my grid-tied PV https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269560#Comment_269560 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269560#Comment_269560 Wed, 25 Jul 2018 19:50:05 +0100 gravelld Posted By: djh
An oil-filled radiator is resistive, not inductive, isn't it?I might have misunderstood the discussion, but here I thought resistive/inductive was being used as a synonym for variable/fixed demand.

While the heating element in an oil filled rad is resistive, the electronics that drive it aren't and demand a particular draw.

I think I realise that I'm not talking about the fundamentals of what makes a resistive load resistive, rather the practicalities of these situations - I'm ready to be educated.]]> Contemplating some storage for my grid-tied PV https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269567#Comment_269567 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269567#Comment_269567 Wed, 25 Jul 2018 23:35:44 +0100 Peter_in_Hungary Posted By: gravelldWhile the heating element in an oil filled rad is resistive, the electronics that drive it aren't and demand a particular draw.

I'm not sure what sort of oil filled rad you have but any that I have used are about as far from electronics as my teapot. For mine you plug them in, the heating element heats up and when the set temp is reached a mechanical switch (driven by either a bi-metal strip or a capillary tube) switches of the heating element. 100% resistive plus a bit of electromechanics.]]> Contemplating some storage for my grid-tied PV https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269604#Comment_269604 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269604#Comment_269604 Sun, 29 Jul 2018 17:29:27 +0100 DamonHD
http://www.earth.org.uk/electricity-storage-whole-household-2018.html#SMAXCDW

Rgds

Damon]]> Contemplating some storage for my grid-tied PV https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269677#Comment_269677 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269677#Comment_269677 Wed, 01 Aug 2018 23:25:26 +0100 Jamster
https://octopus.energy/agile/

Their text:

"Plunge Pricing pays you to take excess energy off the grid

"Agile Octopus introduces Plunge Pricing – a world first that lets you take advantage of negative price events, and get paid for the electricity you use. Receive SMS alerts whenever prices drop below zero, or use our API to program your smart devices."

Don't know if its worth thinking about since you appear to have a fair idea what you are doing with these things!]]> Contemplating some storage for my grid-tied PV https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269678#Comment_269678 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269678#Comment_269678 Wed, 01 Aug 2018 23:45:38 +0100 DamonHD
That is very interesting, thanks! (And Octopus seems very interesting to keep an eye on in general.)

But (a) I'm doing this much more for my education than for money and (b) intend to stay with Ecotricity.

Time of Use tariffs are how life is going to be. The suggestion that I heard from an eminent prof is that those people who stay on all-day single-price tariff will be paying several times more than those who show some flexibility to support solar and wind which are already cheaper than new fossil generators...

Rgds

Damon]]> Contemplating some storage for my grid-tied PV https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269680#Comment_269680 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269680#Comment_269680 Thu, 02 Aug 2018 00:44:15 +0100 djh Posted By: DamonHDI've been having fun, creating a simulation of a single Emphase AC battery
Don't want to put a dampener on your fun, but have you priced the Enphase batteries?]]> Contemplating some storage for my grid-tied PV https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269685#Comment_269685 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269685#Comment_269685 Thu, 02 Aug 2018 08:58:59 +0100 DamonHD
Very very much more expensive by the usable kWh than my lead-acid off-grid storage, but wrapped in an interestingly elaborate control and monitoring system including BMS. Yes, I wish I was seeing that utopian US$100/kWh wholesale price of the near future!

Part of this experiment is (potentially) to see if systems from different manufacturers can be paralleled post hoc as things become cheaper. I have always detested having to junk older stuff prematurely because new stuff won't play nicely.

Rgds

Damon]]> Contemplating some storage for my grid-tied PV https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269690#Comment_269690 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269690#Comment_269690 Thu, 02 Aug 2018 12:41:52 +0100 djh
Overnight, it occurred to me that there's a simple way to price them.

Take the price of the battery and its capacity. For the Enphase that's £1,699 for 1.2 kWh according to https://www.which.co.uk/reviews/solar-panels/article/solar-panels/solar-panels-and-energy-storage#batterytable

Take the warranty, in this case 10 years. Then if we make the simplistic assumption that we want to recover the cost of the unit within it's warranty period, and also that we can fully charge and discharge it every day, we can calculate how many units we can save in the waranty period: 10*365*1.2 = 4,380 kWh and if we divide the cost by that number, we'll see how expensive the electricity has to be to make the unit cost effective: £1,699 / 4380 kWh = 39p /kWH

So in this case, there's no real possibility of the unit being cost effective. Now we can change the assumptions of course. Firstly, we'd probably want to earn twice as much to account for days when we didn't charge or discharge the battery completely and to account for the cost of money. But then we might make money from grid regulation payments, for example, or we might be able to charge more than once per day from the mains. But unless and until the tariffs change in nature, I'd say the whole thing is pretty uneconomic.]]> Contemplating some storage for my grid-tied PV https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269693#Comment_269693 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269693#Comment_269693 Thu, 02 Aug 2018 14:21:10 +0100 DamonHD
In some countries/markets the grid just won't take your exported power or may not let you connect if you have PV, or may charge a large fee to handle PV-related flows.

ToU tariff arrival here is going to make a huge difference.

But, for the record, I could nominally switch my gas heating to heat-pump and completely disconnect from the gas and electricity grids right now: I'm exergy-negative. Then I *just* have to store the energy seasonally. About 250kWh for the electricity alone, ignoring heat/heatpump. A sizeable chunk of the value of my house.

I know at least one person that does live in the Styx (in the US) without any grid connection and a huge battery. If the grid-connection costs are huge, storage at current prices looks like a bargain, and will go on getting better.

Rgds

Damon]]> Contemplating some storage for my grid-tied PV https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269697#Comment_269697 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269697#Comment_269697 Thu, 02 Aug 2018 15:53:33 +0100 fostertom
"... one person that does live in the Styx (in the US)" = an American hedging his bets?]]> Contemplating some storage for my grid-tied PV https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269699#Comment_269699 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269699#Comment_269699 Thu, 02 Aug 2018 16:03:32 +0100 owlman :wink:]]> Contemplating some storage for my grid-tied PV https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269700#Comment_269700 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269700#Comment_269700 Thu, 02 Aug 2018 16:05:27 +0100 gyrogear
gg]]> Contemplating some storage for my grid-tied PV https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269702#Comment_269702 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269702#Comment_269702 Thu, 02 Aug 2018 17:24:51 +0100 DamonHD Contemplating some storage for my grid-tied PV https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269704#Comment_269704 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269704#Comment_269704 Thu, 02 Aug 2018 19:04:25 +0100 fostertom Contemplating some storage for my grid-tied PV https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269709#Comment_269709 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269709#Comment_269709 Thu, 02 Aug 2018 20:44:13 +0100 DamonHD Contemplating some storage for my grid-tied PV https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269779#Comment_269779 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269779#Comment_269779 Tue, 07 Aug 2018 15:55:57 +0100 DamonHD
http://www.earth.org.uk/Enphase-AC-Battery-REVIEW.html

Yesterday evening the house basically ran off battery after sunset until we went to bed.

It'll be a couple of days before I start to get access to stats.

Rgds

Damon]]> Contemplating some storage for my grid-tied PV https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269844#Comment_269844 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269844#Comment_269844 Mon, 13 Aug 2018 17:09:13 +0100 DamonHD
http://www.earth.org.uk/electricity-storage-whole-household-2018.html#Data

Very good to import just 0.2kWh out of 2.2kWh demand even when roughly half the demand happens when the PV wasn't generating (second graph).

However, the effects are generally more subtle since the Enphase is not speced to prevent the big loads hitting the grid, just to take the edge off them.

Rgds

Damon]]> Contemplating some storage for my grid-tied PV https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269850#Comment_269850 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269850#Comment_269850 Mon, 13 Aug 2018 20:20:46 +0100 gravelld
I'm a little puzzled why the export W doesn't remain flat for a short while during the charging of the battery, e.g. from 6am to 8am in your 2018/08/11 graph. Shouldn't it be charging the battery rather than exporting, or is there a throttle on the charging?]]> Contemplating some storage for my grid-tied PV https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269852#Comment_269852 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269852#Comment_269852 Mon, 13 Aug 2018 21:24:20 +0100 DamonHD
Sorry about the colour scheme: that's gnuplot's default for the first two traces and I hadn't thought to override it. I'll have a think.

Rgds

Damon]]> Contemplating some storage for my grid-tied PV https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269858#Comment_269858 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269858#Comment_269858 Tue, 14 Aug 2018 07:07:41 +0100 gravelld Posted By: DamonHDThat dip before 8am is us using more electricity (for a short while) to make breakfast than the PV+battery can cover.No, I'm asking why are you exporting while the battery is charging - shouldn't all excess power be going into the battery and charging it faster?]]> Contemplating some storage for my grid-tied PV https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269863#Comment_269863 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=269863#Comment_269863 Tue, 14 Aug 2018 08:20:17 +0100 DamonHD
Rgds

Damon]]> Contemplating some storage for my grid-tied PV https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=270066#Comment_270066 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=270066#Comment_270066 Fri, 31 Aug 2018 15:28:57 +0100 DamonHD
So we imported 2.4kWh for the whole 15 days that we were away, even though daily consumption was ~1.6kWh/d over that time.

So far, so good.

Rgds

Damon]]> Contemplating some storage for my grid-tied PV https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=270068#Comment_270068 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=270068#Comment_270068 Fri, 31 Aug 2018 16:19:20 +0100 WillInAberdeen
IE work out:
A) the CO2 that could have been displaced if you had exported at time of generation during the day
B) the CO2 that was avoided when you drew battery instead of importing at night

Is the difference +ve or -ve, did you actually save some CO2 overall?]]> Contemplating some storage for my grid-tied PV https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=270069#Comment_270069 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=270069#Comment_270069 Fri, 31 Aug 2018 16:33:40 +0100 DamonHD
The argument about whether it's really a CCGT somewhere that spins up or down in response to marginal changes in grid demand is still there!

Note also, I'm nominally able to claim some possible savings by simply reducing flows across the grid (~5% distribution network, ~2% transmission network) if that's more than the round-trip through local storage.

Rgds

Damon

PS. Don't tell the rest of my family, but I still harbour an intention to do this "properly" with ~250kWh interseasonal battery storage to kill grid imports more or less entirely!]]> Contemplating some storage for my grid-tied PV https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=270070#Comment_270070 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=270070#Comment_270070 Fri, 31 Aug 2018 16:34:59 +0100 DamonHD Posted By: gravelldThanks! Any chance you could use colours other than red and green for the graph, I'm colour blind!

Would it help if I thickened up one of the lines so that you're not solely reliant on colour?

Like here: http://www.earth.org.uk/_off-grid-stats.html

Rgds

Damon]]> Contemplating some storage for my grid-tied PV https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=270467#Comment_270467 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15801&Focus=270467#Comment_270467 Sun, 30 Sep 2018 20:42:28 +0100 DamonHD http://www.earth.org.uk/Enphase-AC-Battery-REVIEW.html#Self

Self-consumption, ie (Net - Import) / Net, where Net consumption = Import - Export + Generation, rose from 48% in August 2017 to 72% in August 2018 (~0.5kWh/d reduced imports). The Enphase was only installed on the 6th, and August is a special low-consumption case.

September 2018 roughly halved daily imports compared to September 2017 (1.6kWh/d vs 3.2kWh/h), with self-consumption up to 69% from 37%. Some behaviour change and better PV generation may have helped.]]>