Looks like the party is over....

[JohnMo]

17 minutes ago, Beelbeebub said:

For a battery, your estimate of 6 to 7kwh in to get 5kwh out implies a round trip efficency of 70-80% which I think is too low it's more likely 80-90% or better.

You always have a reserve you can't touch, called depth of charge, my battery is rated at 13.5kWh and has a depth of charge of 100%, so in reality the actual battery is slightly bigger than that.  So you need to look at the data sheet and see what you are actually getting,  Bit like heat pumps, battery rating isn't always black and white

[Beelbeebub]

21 minutes ago, JohnMo said:

You always have a reserve you can't touch, called depth of charge, my battery is rated at 13.5kWh and has a depth of charge of 100%, so in reality the actual battery is slightly bigger than that.  So you need to look at the data sheet and see what you are actually getting,  Bit like heat pumps, battery rating isn't always black and white

i am assuming he is talking about round trip efficiency ie how many kwh it takes to charge a battery so that you can discharge 1kwh.  For lead acid batteries it can be 60% round trip ie you only get 6kwh out after putting 10kwh in.  LFP batteries are very good in these circumstances with efficiencies in the 95% plus range.  

 

This is independent* of the nominal and usable batter capacities

 

* nearly - i believe you get worse round trip efficiencies if you use all of the usable capacity vs a smaller subsection (eg 20-80%) but it's not a huge effect and i believe most data sheets assume a higher depth of discharge 10-100% 

[sharpener]

2 hours ago, Roger440 said:

 

Related, though not directly is the very long payback periods on some of this stuff. If you can see a return in 2 or 3 years, if you have the capital its a no brainer. But 10 years, not so much.

 

Original FIT scheme we signed up to cost £14k for just 3.7 kW but the rates on offer were so high the actual payback was under 7 years - which sounded good then and is still the gift that keeps on giving. I.e. it has paid for another 3.2 kW capacity plus the battery inverter system, and is now making a contribution to the cost of the HP.

 

 

1 hour ago, Beelbeebub said:

Without a battery the best you are likely to get is 15p all day, and the best import you can get is 25p because you can't time shift without a battery.  

 

 

Well in a sense you can bc you can do all the laundry and run the dishwasher on cheap rates, which with e.g. Cosy is under 15p for 8 hours in every 24.

 

1 hour ago, Beelbeebub said:

 

To be fair panels are so cheap now that *if you could get a really high export limit*, like 10kw or something you could have a huge array, big enough to export so much elecreicty in the day and summer months that you could build up enough credit to import "off sun" and still end up ahead. 

 

But you would need the space for the panel and the high export limit. 

 

 

It took some negotiating but the DNO finally agreed last year to an unlimited export capacity. So with a nameplate capacity of 6.9 kW we have exported about 5.3 MWh in the last 12 months and imported about 5.5 so we are more or less in that postition.

 

1 hour ago, JohnMo said:

You always have a reserve you can't touch, called depth of charge, my battery is rated at 13.5kWh and has a depth of charge of 100%, so in reality the actual battery is slightly bigger than that.  So you need to look at the data sheet and see what you are actually getting,  Bit like heat pumps, battery rating isn't always black and white

 

That is the fly in our ointment, the Pylontecs stop discharging when the SoC gets down to 10% so effectively we only have 90% of the capacity we paid for.

 

58 minutes ago, Beelbeebub said:

i am assuming he is talking about round trip efficiency ie how many kwh it takes to charge a battery so that you can discharge 1kwh.  For lead acid batteries it can be 60% round trip ie you only get 6kwh out after putting 10kwh in.  LFP batteries are very good in these circumstances with efficiencies in the 95% plus range.  

 

This is independent* of the nominal and usable batter capacities

 

* nearly - i believe you get worse round trip efficiencies if you use all of the usable capacity vs a smaller subsection (eg 20-80%) but it's not a huge effect and i believe most data sheets assume a higher depth of discharge 10-100% 

 

Then there is the little matter of the inverter/charger efficiency. When the battery current is at 70 amps everything gets quite warm and this accounts for losses which are possibly another 5% in each direction.

[SteamyTea]

38 minutes ago, sharpener said:

Then there is the little matter of the inverter/charger efficiency. When the battery current is at 70 amps everything gets quite warm and this accounts for losses which are possibly another 5% in each direction.

If everything is in the same room, and you know the thermal characteristics of that room, measuring the rise and fall in air temperature would give you a very good idea of those losses.

Edited 7 hours ago by SteamyTea

[Beelbeebub]

Yes inverters have efficiencies, depending on the operating range it can range from 90-97%. Even if you take the absolute worst cases (90% in, 90%out and 95% battery) you're still over 75% round trip. But at more general levels my system can beat 90% round trip efficencies. 

 

I think the drop in price of hybrid systems has made them a "no-brainer" for the majority of installations - not least because even without PV you can save by time shifting your demand.

 

Obviously there will be edge cases where it doesn't make sense - but the majority of those will be the result of tarrifs that may not hang around.

 

If the ratio of import/export prices widens or the price of imported electricity rises batteries will make even more sense.