Solar PV - Battery Storage

[UKSolarGuy]

Hi,

I'm looking to get some solar panels fitted to the house & would like to use a battery to store excess energy generated during the day and then feed it back into the system overnight to offset the usage from the grid. The aim being to reduce the power consumption over 24 hours to as low as practically possible.

The Solar system is being fitted separately, so the battery system has to be independent of the solar system. I appreciate its easier to connect it to the solar system, but this isn't possible.

Any advice would be appreciated. I guess I need batteries!! But apart from that and how to connect it into the house system I haven't got a clue.

As a guide we use around .5kwh each hour overnight. The batteries would need to feed the power back to the house over 8 hours, so I don't need to have the capacity them to last for days, just 12 hours at max. Ideally it shouldn't feed back more into the house than is used, either it needs to be smart and know what is being consumed, or it simply feeds back say, .3KWH over the course of an hour. No point in feeding back more than used as I can't get any money for the energy being fed back into the grid.

It doesn't need to cover the usage 100%, simply offset some of the usage. The golden rule is that it must not feed back more than is being used.

That is of course if it could feedback more than it uses. I don't know how battery systems works, thinking about it, shouldn't it simply supply what is demanded anyway?

As I said, any help would be gratefully received

[Sunking]

Hi,

I'm looking to get some solar panels fitted to the house & would like to use a battery to store excess energy generated during the day and then feed it back into the system overnight to offset the usage from the grid. The aim being to reduce the power consumption over 24 hours to as low as practically possible.

Well that is not going to happen, unless you like throwing money away and paying 10 times more than you do now.

What you want is a grid tied system and use the utility as your virtual battery. During the day you sell the utility your excess capacity to build credit. At night you use some of your credit.

[UKSolarGuy]

Well that is not going to happen, unless you like throwing money away and paying 10 times more than you do now.

What you want is a grid tied system and use the utility as your virtual battery. During the day you sell the utility your excess capacity to build credit. At night you use some of your credit.

As I said above, the solar panels will generate more energy than is used. I can't sell this back to the utility as I wouldn't receive the money (its an odd situation I have). But what I want to do is charge the batteries during the day (obviously if a sufficient surplus is being generated) and then use them over the night to reduce the overnight consumption.

The batteries will then have been charged from the Solar panels and will release their energy overnight when the solar panels are idle.

[Mike90250]

straight grid tie, you sell excess in the daytime, and use it at night. After a week, the meter shows no net usage. You don't pay them, they don't pay you.

If you need backup power for a couple days a year, a small auto-throttle inverter genset (2,000 watts) is what you need.

Adding batteries is a mess, as it's a much less efficient technology.

[UKSolarGuy]

As I said above, the solar panels will generate more energy than is used. I can't sell this back to the utility as I wouldn't receive the money, someone else would (its an odd situation I have).

straight grid tie, you sell excess in the daytime, and use it at night. After a week, the meter shows no net usage. You don't pay them, they don't pay you.

If you need backup power for a couple days a year, a small auto-throttle inverter genset (2,000 watts) is what you need.

Adding batteries is a mess, as it's a much less efficient technology.

As above, I cannot sell the excess back, hence why I want to store some / or of it and then use it in the evening & overnight when the solar panels are idle.

Its not backup power I need it for, I simply want to store some of the excess during the day and then use it to reduce the electricity bills in the evening & overnight.

[Sunking]

You do not understand. There is no excess power to put into a battery with a Grid Tied System period. The utility is your virtual battery All excess power you generate goes out on the Grid and is used by your neighbors. If you do not have Net Metering, you are not a candidate to use Grid Tied Solar period because you will not receive credit for excess power you generate.

However your electric company will be very pleased if you do install a grid tied system as they will take your excess electricity for free and sell it to your neighbors for a nice profit at your expense.

If you go Off-Grid (battery) means you have no grid connection, and pay 10 times more for electricity for the rest of your life.

Understand?

[UKSolarGuy]

I'm obviously not explaining it clearly enough, I have tried to be a bit obscure because if enough people do it, its going to get found out.

In the UK we have a number of companies that offer to install a Solar Power system for free. In exchange, you get the energy generated during the day, the excess is sold back to the utility company and the company who install the setup get that money, I don't.

Therefore, the idea was to store a bit of the excess generated during the day and use this excess over night to offset the nocturnal energy usage.

Thus, I need a battery store to store the excess I want to keep and then feed it back into the system overnight to help keep bills for the whole 24 hours down.

Hopefully now its a bit clearer. Apologies for being a but obscure before, but I was trying to keep the details hidden really so that it doesn't become a common thing to do. As really it is a little bit cheeky and I'm sure these companies wouldn't be too chuffed if a lot of people did it.

[Sunking]

I'm obviously not explaining it clearly enough, I have tried to be a bit obscure because if enough people do it, its going to get found out.

You have explained it just fine, I understood what you were saying from the start. But you do not understand what a grid tied system is, how it works, or what it does. All excess goes to the utility period. No two ways about it. There is NO piece of equipment that can separate out what you use for consumption, from excess, and send the excess to a battery, well at least none your contractor is going to set you up with. They know better and what you would be up to. Instead of them screwing you over, they would get screwed over by you cheating them out of there free money in credits, and that is not going to happen.

The only person or group who is going to make any money from this deal is the company selling you the bill of goods. You and your neighbors (tax payers) get stuck (swindled) with the bill.

[Mike90250]

A 500VDC grid tie system cannot be connected to any kind of battery, in a non-industrial setting.

A hybrid system using a Xantrex xw6048 could possibly do what you want, but would the cost of a new battery bank every 7 years, be paid for by your energy savings ? Not very likely.

[Sunking]

A hybrid system using a Xantrex xw6048 could possibly do what you want, but would the cost of a new battery bank every 7 years, be paid for by your energy savings ? Not very likely.

He did not mention a hybrid system, but they do not work that way. On hybrid system normal mode (utility connected) the batteries are charged by the utility power, and only go on line in Emergency mode when commercial power is off and disconnected.

However let's entertain the idea for a minute and shine the ole truth light on it. I have to make a few assumptions mainly the daily average use. Here in the USA the monthly average use is 920 Kwh, so daily = 920 Kwh / 30.5 days = 30.16 Kwh /day. I doubt that in the UK is that high, I know it is not, so lets go with 20 Kwh per day.

OK in the UK the first major problem is solar insolation or Sun Hour Day; you don't receive a lot of sun. On the southern end of Britain, the sunniest part of the island, you only receive 2.9 Sun Hours per day on a yearly average. So the batteries have to supply power for a lot more than just night. But tell you what, I am feeling kind tonight, lets stack the numbers in your favor so it sounds good and cheap. Llets say the batteries only have to store and supply 1/2 of the 20 Kwh per day which is 10 Kwh..

Now here is where it gets fun, real fun.

The weight of the battery bank needed is around 3500 pounds, about the size of a sofa, and cost around $7000. That does not include any hardware, charge controller, permits to hold all that toxic and corrosive acid, nothing just the price of the batteries. All the other stuff is extra, but we will not include that $1500 or so dollars since we are stacking the numbers in your favor.

As Mike said you have to replace them about every 5 years at even higher cost. So lets say you do use 20 Kwh per day, 600 Kwh per month, 36.000 Kwh for 5 years. I went up to 5 years to show you how much all that free electricity will cost. Half of it comes from batteries, so half of 36,000 Kwh is 18,000. Here is the ole truth light, ready for it? $7000 / 18,000 Kwh = 40 cents per Kwh or .26 pounds sterling. Now go look at your electric bill and see what your electric company charges you for a Kwh. But here is the real fun, your electric company charges you monthly. Go with your idea, you get to cough up more than $7000 up front in cash. Wait 5 to 7 years, do it again. Fun huh?

Edit:

Just occurred to me I used US prices for batteries. You cannot buy them as inexpensively as we can. Suspect from what I gather from other UK posters you guys pay about twice as much for batteries than we do. Something to do with taxes and being on an island.

[russ]

I think he was talking about having a totally separate system - not associated with the solar system at all but just a charger connected to the electrical system with the batteries capable of returning power to the system at night.

1) Expensive and not worth it

2) Hate to burst your bubble but the solar system provider has already thought of this. If you see a contract I expect there are several ways for them to catch you out. If caught it would be court time and bad news.

[UKSolarGuy]

I think he was talking about having a totally separate system - not associated with the solar system at all but just a charger connected to the electrical system with the batteries capable of returning power to the system at night.

1) Expensive and not worth it

2) Hate to burst your bubble but the solar system provider has already thought of this. If you see a contract I expect there are several ways for them to catch you out. If caught it would be court time and bad news.

Thanks for the replies.

It was just an idea and the above was exactly what I was thinking of. If its not worth it, its not worth it. As I said, it was just an idea.

Your point #2 is spot on. I suspect there would be terms in the contract to prevent it.

It was just a wild thought to see if it was even feasible in the first place, and it appears not!

[russ]

@ UK Solar Guy - Never stop thinking!

Some ideas are better than others and some work better than others.

However, if one stops scheming and thinking none of the good and usable ideas can come!

[MarineLiner]

@ UK Solar Guy - Never stop thinking!

Some ideas are better than others and some work better than others.

However, if one stops scheming and thinking none of the good and usable ideas can come!

[solarpower112]

Well, it is worth the idea

Yeah, It has already been thought of...If you wanted you could do something with wind power, that is still new, but the solar and wind systems are also intertwined as well, but I would recommend that if you have the wind and the budget to go for a combo set up. And then get credit back from power company.