The argument I can think of is that solar diverters precisely match the power draw to the available excess solar power, which heat pumps cannot do.  This means that with a heat pump you may be importing some or most of the power consumed, whereas with a solar diverter you are guaranteed not to be importing any. 

The financial equation depends also on your import/export rates so if, for example, you are not getting paid for export then a solar diverter may well make sense even if you have a heat pump.

One approach favoured by many is to time DHW heating at say noon, when you are likely to have solar (in the season) and also the heat pump is likely to be efficient because its likely to be warm.  If, alternatively, you have a night time import tariff which is less than your export tariff it makes sense (at least when there is any export available) to do it on the heat pump at night!

Spot on, @adamk. This is exactly why, when our diverter failed, we chose not to replace it. It was a bit of a lightbulb moment for me, and part of what led to me creating this calculator:

https://renewableheatinghub.co.uk/which-is-more-efficient-ashp-or-solar-diverter-a-comparative-calculator/

@jamespa raises some valid points too, and it’s always good to look at the wider picture, but in our case, it’s hard to ignore the numbers. To heat our 300-litre tank to 45C, our heat pump uses around 5.81kWh (on a 2.7 COP). The diverter, by comparison, would use 15.7kWh. That kind of efficiency gap makes the choice pretty straightforward for us.

I should probably add to my comments above that, whilst I can definitely see an argument for using a solar diverter in some cases - most particularly if you get no payment for export, I removed mine once I got the ASHP.  I currently get paid 15p for export and only pay 7p at night for import, so my cheapest option by far is to heat DHW at night on the ASHP, and export by day if there is excess solar available.

Posted by: @adamk

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am i missing something? was watching this podcast and they started suggesting solar diverters for HW even with a Heat Pump. why? heat with heat pump get maybe 1-3 ratio? or us immersion and get 1-1 ratio.

For me there are few factors why you may want to heat water via excess PV.

1. You don't get any money for export.

2. You do get money, but because you don't or can't have a smart meter you will only get 4.5p kWh 

3. I generally get a CoP of 3 or less doing DHW, so it costs me on my tariff more than either 4.3p or 10p to heat water per kWh of actual heating.

4. If you run legionella cycles you get them for free, mostly, with a diverter.

So for me, I have smart meter, but it cannot communicate due to poor signal area. We have self installed PV so don't get paid for export. Prefer to have cylinder heated to 70 degs whenever it's free to do so. Then night time (cheap rate) heating of cylinder doesn't occur, as cylinder is still hot enough, this saves a few pennies each time. Our cylinder today is still at 64 degs from being heated yesterday, so unlikely to need heating by heat pump today. Normal routine is heat twice per day, so no heat pump heating yesterday or today from one good solar day. 4 cycles of heat pump live saved. Do that 6 months of the year, you add a further year or two on the overall life of the ASHP.

If you have a battery a diverter isn't the most efficient, as battery has to full and excess energy going to grid. I prefer to use a big relay controlled by home assistant to leverage battery capacity just prior to battery being full and divert the full 3kW at the immersion. This ensures nothing goes to grid, if there is work to be done in the house.