Circular reasoning (Rolfe's solar energy system)

[Gomev]

You need to treat yourself to one of these...

https://www.argos.co.uk/product/4821261

Only draws 1kW

It still takes the same amount of energy to heat a set amount of water whether you draw 1kW or more. It just takes longer. So lower draw is useful if the output is restricted, but it does not actually save anything.

 

[Gadget Geek]

I have a Vektra thermos kettle. You can set the temperature and it keeps the water hot for when you are ready for another cuppa. If it's cooled down too much, it's just a quick reboil as the water is still relatively hot.

Best kettle I've ever had.

 

[GaryMG4]

It still takes the same amount of energy to heat a set amount of water whether you draw 1kW or more. It just takes longer. So lower draw useful if output is restricted but does not actually save anything.

I think the idea is that if you have 1.2kW excess that is being clipped then there is no cost as you don't use the grid and you don't get paid for that clipped excess.

 

[Rolfe]

It's not quite like that. In order to harvest the generation that would otherwise be clipped I have to leave the inverter set to export the battery down to 100%. Which is very counterintuitive, but that's how it works. That way the battery only charges when the generation is over about 5.3 kW. Anything below that is exported. This is important because without this setting the battery would charge from the solar preferentially, get to 100% pretty quickly, and then I'd lose the clipped generation.

When the battery is on this setting it won't discharge to power the house. If there is enough solar to boil the kettle then that's fine, it will use that rather than exporting it. But if there's a cloud over the sun at that precise moment, so that the solar isn't enough to meet the kettle load, power will be taken from the grid. At the daytime rate.

So Everest is quite right. A kettle with a lower power setting would solve this by bringing the kettle draw down to what the solar can cover. I'm just too impatient. I want my coffee  now. It's quite easy just to turn off the export setting, boil the kettle, and turn it back on again.

 

[T1 Terry]

It still takes the same amount of energy to heat a set amount of water whether you draw 1kW or more. It just takes longer. So lower draw useful if output is restricted but does not actually save anything.

Not actually correct, unless you have a device that is so well insulated there is zero heat loss.
An example, those cigarette lighter elements that you drop into the cup to boil up the water for a cuppa, really struggle to reach boiling point even if the inside of the car is warm, leave it outside in winter and the water will never boil, the heat loss is greater than the thermal input and the closer to boiling point, the greater the differential between the heated water and the outside temp, so the greater the heat loss .... I used these a lot when balancing big battery packs during that initial conditioning charge required for long life when used as house batteries

Another example, more than once I've had to create a blanket around a slow cooker so it would actually cook, and it isn't that cold over here

T1 Terry

 

[Everest]

It still takes the same amount of energy to heat a set amount of water whether you draw 1kW or more. It just takes longer. So lower draw useful if output is restricted but does not actually save anything

Technically a lower power kettle will actually use more energy to heat the same amount of water due to heat losses during the heating process. But the point was about power, not energy demands.

So Everest is quite right. A kettle with a lower power setting would solve this by bringing the kettle draw down to what the solar can cover. I'm just too impatient. I want my coffee  now. It's quite easy just to turn off the export setting, boil the kettle, and turn it back on again.

We used to turn things (like dishwasher) off when we had only a 3.6kW inverter (a few years ago) if we wanted to boil a kettle... but got fed up doing that - hence used a low power kettle for a while. But I agree it's a pain with the time it takes to boil!

 

[GaryMG4]

I want my coffee  now. It's quite easy just to turn off the export setting, boil the kettle, and turn it back on again.

My coffee machine uses about 1.3 kW and only heats enough for the cup so I'm sorted. Mind you the price of the coffee pods far exceeds the energy 'saving'

 

[Rolfe]

There's more cloud again today and even the morning had high light cloud, but the battery has already put on 26% (from 39% to 65%) and it's charging at over 2 kW whenever the sun comes out.

 

[Rolfe]

It worked properly today. It's worth posting the graph. I've taken out the grid import because it was messing with the scale, but that doesn't matter so much.

At midnight the home battery is charging from the grid. The dishwasher is also on and is running at the same time, also drawing from the grid. Once the home battery is full, the Eddi heats the water for up to an hour, while the battery is held at 100% by still being on charge so again the Eddi draws from the grid. After that the battery discharges to export for an hour and a half, bringing its SOC down to 39%. (If I want it less than that I need to schedule a slightly longer discharge window.)

That's the end of the off-peak tariff. The battery is now free to supply the house until the solar takes over, but it only dropped by 1% for about ten minutes doing that, because there's not much house load at that time and the solar was beginning to ramp up.

Sunrise, and the solar quickly takes over the house load and starts to export. The battery can't take the solar because it's set to export to 100% (yes, I know) from sunrise. There was some high cloud so generation wasn't what it might have been, but by about ten o'clock there was enough for the export to be clipped. The battery then took the excess over and above that, and did pretty well until lunch-time when larger cumulus clouds started to come over. I ran the microwave at lunchtime without thinking about it, but even though generation was in a trough there was still enough to cover it.

The afternoon was cloudier, but direct sun during three or four breaks in the cloud let the battery get right up to 78%. Overall it harvested 3.9%, which is about 4 kWh. By about 3.30 pm when I made coffee the generation had fallen off a lot, and was at another nadir. I flipped the battery export off as I turned the kettle on, then back on again when it had boiled, just to stop the kettle drawing from the grid. Then the export continued.

There was still a bit of export by six o'clock when the export slot finished and the battery put on a couple of per cent, with a slight wobble as I boiled the kettle again. As the solar faded the battery took up the house load, starting to discharge at about eight. Having looked at what was left I set the battery to start exporting again just before ten, so that it finished discharging at about 11.30. Back to the beginning again, except the forecast for tomorrow is so bad that I'm not going to bother discharging the battery in the early morning as I don't think there will be enough clipping to make it worthwhile.

The last green peak of load is just because the car started to charge. I had plugged it in but then decided not to as it was already at 67%, so I turned it back off again.

So this works, it's only a minor inconvenience to keep an eye on what's happening during the day, and even on a day where there wasn't a lot of direct sun it gained enough to be worthwhile. I exported 10.3 kWh from the battery as well as 31.8 kWh solar generation. (The battery is only 9.5 kWh, so doing it in two stages like this with some charging in between definitely paid off.)

 

[Gomev]

Not actually correct, unless you have a device that is so well insulated there is zero heat loss.
An example, those cigarette lighter elements that you drop into the cup to boil up the water for a cuppa, really struggle to reach boiling point even if the inside of the car is warm, leave it outside in winter and the water will never boil, the heat loss is greater than the thermal input and the closer to boiling point, the greater the differential between the heated water and the outside temp, so the greater the heat loss .... I used these a lot when balancing big battery packs during that initial conditioning charge required for long life when used as house batteries

Another example, more than once I've had to create a blanket around a slow cooker so it would actually cook, and it isn't that cold over here

T1 Terry

Not to mention of course that the day has an T in it, you are wearing a woolly hat and wellies and it's between 2 and 3pm.