"Savings" calculation for solar PV with storage batteries?

[bhm1712]

Another thing to consider with battery storage (at least with ours) is that we benefit from economy 7. If the batteries are not fully charged during the lower light winter days, we can charge the batteries overnight using the lower rate economy 7 electricity and then use the battery and solar in the daytime. At present the economy 7 tariff is 11 pence cheaper for the night rate. We generally use around 13kwh in the daytime and if that’s all coming from the battery, that’s a £1.43 saving per day. In reality we are finding that we are saving ourselves the purchase of around 200 day units a month in winter months compared to pre solar and battery install. Our system was designed to export as little as possible to the grid since they’re only paying about 5p per unit generated and they’re selling it on for circa 30p a unit which in my opinion isn’t cost effective.

 

[JohnD]

"200 day units a month in winter months"

I presume you mean 200kWh per month in winter (about 6.6kWh per day on average), and if you have a battery of that or greater size you can probably use most of it. Obviously most of the generation is at times you need it least.

Looking at my month end solar meter records, I have the actual monthly generation figures for a year 2020/2021 (calculated as average per day, for each month). I have bolded the winter months. This is a fully south-facing roof in a sunny district on a 3.4kW nominal roof. If you are getting a winter average of 6.6 per day that seems surprisingly high. What is the nominal output of your installation?

June sol 14.6
July Sol 11.6 Low
Aug sol 13.8
Sept sol 11.7
Oct sol 6.6
Nov Sol 4.8
dec sol 3.4
Jan sol 3.3
feb sol 3.7
mar sol 9.7
apr Sol 14.3
may sol 11.3

 

[bhm1712]

It’s an 8kw Solar PV system with 2 6.5kwh battery units.

 

[JohnD]

must have cost a good bit.

 

[JohnW2]

"200 day units a month in winter months" ... I presume you mean 200kWh per month in winter (about 6.6kWh per day on average).... If you are getting a winter average of 6.6 per day that seems surprisingly high. What is the nominal output of your installation?

Given that the post was primarily about the (cost) benefit of being able to charge the batteries with off-peak E7 electricity (when solar generation was not adequate to fully charge the batteries), this ....

... we are saving ourselves the purchase of around 200 day units a month in winter months compared to pre solar and battery install.

... presumably means what it says (a reduction in day unit usage due to the combination of solar generation and charging from off-peak E7 electricity - not just solar generation), and therefore 'does not mention' that this 'saving' is appreciably reduced by having to buy more off-peak energy during the period in question.

Kind Regards, John

 

[SimonH2]

Total fiction, batteries purchased today are typically rated for 6000 cycles, or 15-20 years with daily use, after which the capacity has reduced to 80%, so they are still usable after that.

All I will say about that is ... I remain to be convinced that is achievable in "real" use environments.

There can be other benefits such as being able to operate at least some of the house in the event of power failure.

I do wonder how many of the inverters are capable of doing that. The majority of grid-connected inverters explicitly cannot operate standalone, and I suspect it would be a "brave" vendor that made it easy to over-ride the anti-islanding protection because the quality of the needed interlocks would be outside of their control.

 

[winston1]

Also the batteries have a loss. I don’t know the actual figures but for nicads you get around 700w out for every 1000w in. I guess those figures are typical.

 

[JohnW2]

Also the batteries have a loss. I don’t know the actual figures but for nicads you get around 700w out for every 1000w in. I guess those figures are typical.

Yes, there is bound to be a loss, and a further loss in the inverter subsequently converting it to AC. However, I suppose that 'some loss' is better than 'total (effective) loss'.

 

[Agile]

It would not be difficult to have a simple relay with the coil powered by the mains supply so that when it is de-energised it connects the house ( or certain elements like freezers and computers ) to a simple inverter operated from the battery.

 

[JohnW2]

It would not be difficult to have a simple relay with the coil powered by the mains supply so that when it is de-energised it connects the house ( or certain elements like freezers and computers ) to a simple inverter operated from the battery.

True, but I'm not sure what that has got to do with what is being discussed here.

Kind Regards, John

 

[JohnD]

It would not be difficult to have a simple relay with the coil powered by the mains supply so that when it is de-energised it connects the house ( or certain elements like freezers and computers ) to a simple inverter operated from the battery.

though you are required to have a changeover switch which disconects from the network before connecting to your alternative source

 

[Agile]

But the relay would be doing that.

 

[SimonH2]

Well you would certainly need "something" to isolate the system from the mains supply before you powered it up from an inverter.
Could be a manual setup - manual changeover switch with aux contact to allow the inverter to fire up. If you do it with a relay/contactor, then unless you put some effort in, it's going to use lecky all the time the mains is present which will negate some of your savings.
And as I mentioned above, I suspect very few standard PV inverters are capable of both grid-connected and standalone operation - so the extra standalone inverter is going to add to the cost as well. If you do find an inverter that does both modes, I suspect it'll cost a fair bit more than a basic grid-connected model.

I can certainly see the attraction of being able to use the PV+battery setup to also act as an off-grid supply for when the grid goes away*, but I suspect the cost and practicalities might make it an expensive luxury.
* The way we're going, it'll be "welcome back to 1974" before long But unlike 1974 when we just got out the candles and played games, these days it would be "OMG, no internet"

 

[crappy]

The unit I have takes care of all of this. Using either a ct or more advanced sensor on the incomer, it will manage its output to try maintain zero grid draw. When battery below threshold or islanding detected, it will stop generating output. While there is no mechanical disconnect, this is all type-approved for the US and Europe (and oddly, South Africa specific mention).

There is an additional output which will maintain output regardless of islanding. This is designed as an emergency or "ups" output should the grid fail. This cannot be tied to the main house wiring for this reason. For me, I have it as a single plug circuit to use when I deem necessary.

 

[SimonH2]

The unit I have takes care of all of this. Using either a ct or more advanced sensor on the incomer, it will manage its output to try maintain zero grid draw.

OK

When battery below threshold or islanding detected, it will stop generating output. While there is no mechanical disconnect, this is all type-approved for the US and Europe (and oddly, South Africa specific mention).

So it doesn't take care of it all then

There is an additional output which will maintain output regardless of islanding. This is designed as an emergency or "ups" output should the grid fail. This cannot be tied to the main house wiring for this reason. For me, I have it as a single plug circuit to use when I deem necessary.

Which is enough for a lot of uses.