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Solar PV + battery - do I need a diverter?

Hi,

I am building a new house and am going to install 7.4kW of PV with a 5kW inverter, and a Tesla Powerwall 2 (with 13.5kWh usable storage).

In my current home I have 3.4kW of PV and a DIY PV diverter based on the EmonTX shield + Robins diverter sketch. It works great.

However I am wondering if the addition of a battery might allow me to simplify things and remove the PV diverter. My plan is as follows;

  • add a relay or timer to control my HWC element (3kW)
  • at 12am’ish turn on the HWC element for 2 hours
    • during this time any excess solar will be soaked up by the HWC
    • and any shortfall will be covered by the battery
    • by starting at 12am the battery should have had a chance to charge up a bit
  • once the HWC is up to temp, or 2 hours is up, any excess will go back into the battery
  • at 4am turn the HWC element on again for a few hours (if needed)
    • I get cheap night rates so this ensures I have hot water in the mornings

The general idea is to simplify the system by removing the diverter, and just using the battery as a buffer for when solar is limited, or the house load spikes.

I have a few other ideas also; by having two elements in the HWC (middle and bottom) and checking the solar forecast each morning…if high solar expected turn on the lower element, in order to maximise the amount of hot water, and if low forecast turn on the middle element, so we have enough for the evening but don’t risk discharging the battery too much if the solar is limited.

Anyone considered this approach? Have any thoughts about whether it is a good/bad idea?!

Cheers,
Ben

If there is loads of solar then the HWC should get a full 3kW without needing to touch the battery, unless the house load spikes, but any shortfall will be quickly replaced in the battery once the HWC is up to temp.

On a low solar day, with a PV diverter, I end up getting hardly any excess so the HWC remains cool. This means I have to do a boost heat in the late afternoon at peak rates, in order to have hot water for the evening.

With my plan above, on a low solar day, the HWC will still be heated from 12-2 (only the top half via the middle element), but this will come almost solely from the battery. The battery will have been charged overnight during off-peak rates, so that is relatively cheap power.

Seems to me this should work quite well, a much simpler system and potentially more cost effective on low solar days, while just as effective (as a diverter) on high solar days.

Hi ben,

We’re doing something similar - at least in terms of new house and PV. We’ll only put the maximum PV in that you can do without asking permission from the DNO, so a 4kW ground mounted array. We’re also installing an ASHP for UFH and DHW. DHW will be from a 300l tank with 2 immersion points and a diverter - we may actually install an Eddi this time rather than home brew.

We are still on an old couple of vehicles, so until the oldest dies and we switch to an EV, we won’t need to charge any vehicles, although we will install EV charging points. We’ll probably go onto one of the Octopus plans with v low night time rates and use that to heat the DHW tank if it needs it. Depends on the rate I guess, the ASHP should have a CoP of between 3 and 4, so you’d need a rate less than 18p divided by 3 or 4 otherwise it might be cheaper to run the ASHP for the DHW.

As far as any battery system goes, we’re going to wait for a year or so until we have the data about how all of the systems are working.

I’d be interested to hear how you get on.

Simon

Hey Simon,

Yeah I have been looking at the Eddie during my research - looks a great little device. I just got a quote from the guys doing my solar + battery install down here in New Zealand, and they want to charge $NZ1700 to supply + install the Eddie - there is no way I am paying that!

I did find another supplier here in NZ selling them for just under $NZ1000, so they are far from cheap down here.

I looked very seriously at a ASHP for DHW, but after talking to someone who installs them for a living, he suggested I would be better off with a simple HWC + element - especially since I am putting up so much PV. His reasoning was they are expensive to install, and expensive to repair, and they quite often need servicing. He argued a ASHP for space heating (very typical in NZ) makes sense as they are cheap to install and run, and really improve your comfort in the home, but for DHW there is no difference to your comfort whether heated by ASHP or element - therefore the extra cost with installing and servicing them quickly wipes out any cost benefits with the higher COP.

Interesting perspective from someone who installs ASHPs for a living!

If I was doing UF hyrdonic heating I would most definitely be using one however.

Anyone else got any thoughts about my conundrum above? Does a decent sized battery negate the need for PV diversion?

Interesting project. Funny how lots of simple systems suddenly can become one complex system - let us know how it goes.

Yep I do like the idea of removing the complexity of the PV diverter, but only if the remaining systems will function as well.

I was hoping some others on here might either have experience with this approach, or opinions on whether it is a good idea or not.

So your battery will act as a ‘time-shifter’ for charging your hot water storage? And only if, there wasn’t enough PV during the day.

If hot water demand is your largest electric use then this focus seems to make sense.

However, I still think a diverter will in practice help out. Maybe on intermediate intensity solar power days you might be able to heat the tank up totally on solar (without battery boost), but you will need to follow the PV system output in order to get your got water tank full. Without a diverter you might end up exporting where you still have a cold tank.

But it depends on price of that diverter…(always easy to add a diverter later aswell)

You raise a good point, but my thinking was that by the time the solar starts cranking up the battery will be at least partly depleted, after running the house from 7am when my cheap rates stop and there is no solar yet. So any excess solar before 12pm should be topping up the battery.

Then at 12pm when I switch on the HWC element, if it goes cloudy then I have already stored any excess in the battery ready for use. And if it stays sunny I just use that.

A key to making it efficient will be utilising the solar forecasting API and deciding whether to heat half or all of the cylinder each day (automatically) I think. That should prevent excess battery depletion or imports on rainy days.

But you are quite correct, that it would be easy to add a diverter later if I found I needed it.

Another point to consider Ben are the losses in the inverters. Unless you are charging the battery from the DC of the PV system, any usage of the battery to power the immersion will have been converted 3 times if the energy originated from the PV panels or twice if the power came from the grid.

Haven’t a clue how efficient the Tesla Powerwalls are or a typical solar inverter but you could ‘lose’ a lot in all the conversions back and forth between AC and DC.

Simon

EDIT - see here for some experience with a Powerwall and PV → Solar PV + Tesla Powerwall (or storage battery) - Photovoltaics (PV) - BuildHub.org.uk

1 Like

If you build this system, I think it might well be one of the most advanced in the World! Is everyone in NZ like you? LOL

Hi guys,
The prime purpose of the Mk2 PV Router is to identify any surplus energy which would otherwise be lost to the grid and then divert it for some suitable online use. By careful layout, the Mk2 can operate at a higher level in the heirarchy than all other control elements. Hence the Mk2 will only divert any energy to its load(s) after all other loads such as batteries etc. have had their fill.

This nifty arrangement was devised by Robert Wall a couple of years ago. The other controller(s) know nothing about the Mk2 Router, and the Mk2 sees everything else as baseload. It’s a simple setup which works a treat!

Thanks Robin - yeah that is my plan if I decide to stick with the diverter. I guess I am just questioning if having a battery in the system will sock up a large portion of any excess energy and render the diverter almost redundant?

I know it won’t be 100% but I am curious if anyone else has this arranged, sans diverter, and if they are still seeing a lot of export?

As I mentioned I think I can get a little clever and try to predict the solar for the day, and adjust the amount of hot water heating accordingly. I think it may well be a case of installing the system without the diverter, see how it performs and then add it in if I am seeing loads of solar export when the cylinder is not at full temp.

Appreciate all the comments!

Ha, I love this stuff!! Been automating my home for 10 years - and the prospect of building from scratch has me positively frothing at the possibilities!!

Hi Ben, We have a very similar setup but in the south of UK, maybe not that different to NZ in terms of solar strength?
Its been live since the start of the year & the findings are that the Tesla at this time of the year is full again before midday as the draw on it overnight is quite low. In the Winter the battery was being part charged (dependant on the previous days weather) on the overnight cheap rate.
We are completely dependant on elec as we have no oil/gas etc & the house & DHW is heated by GSHP.
I’m having the property re-roofed and in prep for electric car (original leaf possibly on openEVSE… if only V2G/V2H was available) & to try & improve the winter solar gain I’m adding another 3.5kw (our other arrays are on outbuildings).
In one outbuilding i’m considering adding a tank/thermal store to dump excess to in summer as I’ve a few projects that require DHW in the summer. for this I will possibly be using a MK2, but several steps before I get there.

Sounds like an impressive setup - GSHP are cost prohibitive here but I love the concept and wish it were more mainstream. I think I will go ahead without the diverter to start with, and see how things go.

Will be easy enough to add in later, and I will make sure the wiring is in place if needed.