Most batteries are what is known as DC-coupled, which means that they use the solar inverter to convert their DC voltage to AC used in your house. So the maximum power you can get from panels and battery is your inverter's limit. 

What size is your heat pump? Our 8kW Mitsubishi only uses 1.5-2.5kW during DHW in Eco mode.

@robs the Mitsu is 16kW. It consumes a little under 3kW when heating DHW but this is an average over an hour on the Shelly Energy device, so the maximum will be higher.

I am still undecided whether to take advantage of the cheaper night time rate, or opt for the greater efficiency of the ASHP at the hottest time of the day, but have some overshoot on the 4kW available, that has to come from the grid at the most expensive time.

I wonder how Havenwise would deal with this!

- Just checked instantaneous consumption and it is 4.9kW. I don't know if that reduces as hot water temperature increases.

@davidalgarve

Much bigger heat pump than us but still a surprising amount of power, do you have it in Eco mode? Or use quiet mode?

If it uses 3kW onaverage over an hour, then do you typically generate 3kW or more during the warm part of the day when doing DHW? If the peak power draw (the 1.27kW from the grid in your image) is only for a short period of time then running it on solar is almost certainly cheaper. Unless that is your export rate is greater than your import cheap rate, then you may be better off exporting instead.

No idea about Havenwise.

How long does the 4.9kW level of power use last? Generally, the hotter the water the greater the flow temperature needed and so the greater the power output (and input) required of the heat pump. Ours usually ramps up to about 1.5kW quite quickly and then slowly increases up to about 2kW by the end of the DHW heat up.

Similar situation here, although slightly lower numbers as I have a 12kW ASHP and a 3kW solar inverter.

In the summer*, I try to limit the power draw to keep it within the 3kW solar output as much as possible. My system will start off pulling around 2.5-3kW and this can rise to 4.5kW as the flow temp increases. My Samsung ASHP has a 'quiet' mode which limits output to ~70%, or a max current draw of around 3.3kW, keeping the current draw at or below 3kW for most of the cycle.

As the system is running in a 'lower and slower' mode, it's also more efficient in that it uses less energy overall (lower power for longer period, but also less overall and more of it covered by solar than drawn from grid).

All else being equal, of course you'd try to run the DHW cycle at the warmest and sunniest time of day. But as you say, you need to also factor in whether it would be more cost effective to sacrifice some efficiency by running overnight when it's colder but at a cheaper rate. When I was on Agile, I did a back of an envelope calculation to work out how much cheaper I'd need the rate to be to offset against the decreased efficiency due to lower temperature. As you have a battery, the cost element is presumably irrelevant as you can always charge your battery at cheapest rate / from solar, and then deploy at your leisure (assuming you have sufficient capacity). So for you I think the key factor becomes keeping the power draw largely within the capacity of the inverter, and this is where any form of quiet or eco mode can play a part to reduce the peak power draw keeping (more of) it within the limits of the inverter.

* Actually, not just summer as it's more efficient (costs less) to run the ASHP in quiet mode (low and slow) when heating DHW regardless of the time of year.

@old_scientist Thanks for your input. Firstly I have never considered "Quiet Mode" because I didn't realise this affected current draw, but I may try this now.

I have just done a check on power at 5 min intervals between 12:00 and 13:00. with ambient air temperature of 15C.

At 12:00 the water temp was 32Deg. and the power was 1.7kW . This rose steadily so that at 12:25 it reached a max of 5.03kW with a water temperature of 45.5Deg.

 It then started to decrease so that by 12:35 the power had reduced to 3.96kW and the water had reached 54.5Deg.

Frustratingly, my battery was 45% at the beginning, but nothing could be extracted to top up the 4kW inverter limit.

I believe I was producing around 4kW during this time, but missed checking it at the same 5min intervals, so will try that tomorrow     

Posted by: @davidalgarve

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If I have a 4kW inverter and panels producing 4.6kW; with a battery charged to 77% and then my ASHP demanding 4.5kW for DHW and 1.5kW for incidentals, should I be drawing anything from the grid?

You can only get 4kw out of the inverter, regardless of where it comes from.

So you are producing 4.6kwh from the panels, the inverter will put 4kwh to AC and 600wh to DC battery (excluding any loss to make it easy to read). If battery is full 600wh will be clipped.

You HP  is asking for 4.5kwh to run and 1.5kwh in the house, total use 6kwh.  Therefore you will get 4kwh from the inverter and 2kwh from the grid, Nothing from the battery.

The only time the battery will be used is if you produce less than 4kwh from the PV, then the battery will make it up to max inverter output of 4kwh.

It makes no difference what size / SOC of the battery, its the inverter that's restricting it all and that then depends on your DNO instructions.

Only way around it is to increase the size of the inverter output to be greater than over all usage. That may be limited by DNO

@uk_pete_2000 Yes I got the message that the max usable is dictated by the Inverter rating. I blame myself and my contractor (who installed both the DHW and the solar) for not recognising that the ASHP would consume much more power than the specified inverter.

I don't think I can justify discarding this one and buying a bigger one, so my problem is how to decide which is the most economical way to operate what I have, i.e. nighttime on lower tariff or daytime with a proportion from solar and the remainder from the grid at the higher rate.

I will do one more check today to see what difference "Quiet" setting makes, but I am not expecting a significant reduction in demand. 

What's the make/model of that solar inverter @davidalgarve

Is the storage battery connected to it directly?
or does the battery have its own mains power connection?

Any chance of you posting a photo here please? 🙂 

@transparent Hi Transparent. I can't tell how the battery is connected. The Main isolator just has 4 circuit breakers, which I assume are the 3 phases + earth

Photos attached

Just tried to select "Quiet Mode" in advance of doing another 1 Hr, check whilst water heating and it reports "Invalid". Is there anything I can control that would unlock that?

I've done a few checks on the Growatt MOD4000-TL XH inverter.

It costs around £550 in UK, and is a typical basic 3-phase design with a battery port.
Contractors get a better deal if they buy an matching Growatt Ark battery to connect to it.

It's not a 'hybrid' inverter.
So the battery cannot be recharged from the grid when power is cheap.

Nor can MODn000-TL units be connected in parallel to increase capacity. 😥 

It doesn't offer any limitation on export to the grid by default.

If you want the battery to only provide power to appliances within the home, the installer can fit an optional Current Transformer or Energy Meter.
You may or may not have that facility.

If you didn't have the heat-pump, then this solar-inverter and battery combination would be reasonable choice for the home.

But it doesn't appear to meet your aspirations.

I will note that the physical installation has been arranged neatly by using 50mm square PVC trunking, albeit taking up a lot of wall space.

However, the same trunking contains

• 3-phase AC mains cables (440v between phases)
• Ark DC battery cables (600v - 950v)
• solar panel input wires, nominally 600v
• monitoring and control data cables (5v and 18v)

That wouldn't be permitted in UK.