Ev charging tariff rip offs

[jds1972]

What metric would you use for sizing a battery to soak up excess PV power ? I have heard 1.5 times daily use averaged over the year; does that sound about right ? But what happens if you go away for a week or two in mid summer, so use a minimal amount; once the battery is full, what happens to any excess ?

i dont install energy systems, but i would say depends on your system setup , i dont have solar but fairly sure any excess would go to the grid. i would say with battery that it should be based on your pv output not your usage. your daily usage could well be higher than what your pv system can generate. im sure someone with better experience of pv will help more

 

[Dave]

What metric would you use for sizing a battery to soak up excess PV power ? I have heard 1.5 times daily use averaged over the year; does that sound about right ? But what happens if you go away for a week or two in mid summer, so use a minimal amount; once the battery is full, what happens to any excess ?

Once your batteries are full any excess goes to the grid if you have a grid-tied system. If you have an off-grid system it is lost into the ether.

 

[Hairyfool]

Once your batteries are full any excess goes to the grid if you have a grid-tied system. If you have an off-grid system it is lost into the ether.

Er no!

All solar panels have a number of parameters which includes short circuit current (i.e. @0v), Maximum power voltage and current (the optimum) and the open circuit voltage (i.e. @0A)

With an off grid system once the batteries are fully charged and in theory demanding no current the output voltage rises to the OC voltage. IN practice batteries will always accept a small (maintenance) current but the panels will rise to pretty close to that OC voltage. Nothing is "lost to the ether" it's just not generated in the 1st place.

This is why solar charging MUST be done through a charge controller. In the case of a 12v system the OC voltage could be as much as 20V and without the controller that voltage would be applied to the battery once fully charged which would eventually cook the battery.

What happen with On Grid systems is a little more difficult to make sense of because all 3 systems are tied together therefore cannot be at different voltages. In principle if the charger (or house) is demanding current it would make the grid voltage drop slightly, the solar controller will try to maintain the voltage so will supply current to the charger instead of from the grid. When the charger (or the house) doesn't want current the solar controller would want to raise the voltage but because the grid won't allow it spare current goes to the grid instead.

It gets really complicated when you consider the house demands fluctuating, the sun coming out or going in and the fact that grid voltage fluctuates and is allowed to vary from -6% to +10% of 230v (216 to 253v AC)

 

[Dave]

“With an off grid system once the batteries are fully charged and in theory demanding no current the output voltage rises to the OC voltage. IN practice batteries will always accept a small (maintenance) current but the panels will rise to pretty close to that OC voltage. Nothing is "lost to the ether" it's just not generated in the 1st place.”

That’s what I meant. Thanks for the clarification.

 

[Hairyfool]

What metric would you use for sizing a battery to soak up excess PV power ? I have heard 1.5 times daily use averaged over the year; does that sound about right ? But what happens if you go away for a week or two in mid summer, so use a minimal amount; once the battery is full, what happens to any excess ?

Personally I try to have no excess PV power. Sunny days are for washing, tumbling, baking, other cooking and charging the car. For that reason the savings from having a battery would take several decades to recoup the outlay.

I tended to think that the fuel equivalent for the car was 0 pence per mile. Then it occurred to me that I get paid FiT for the power I generate not how much I feed into the grid. That means I get paid to charge the car so the PPM is negative (I just don't have the brains to work out how much).