Green Building Forum - Solar PV Grid Tied with Battery

Solar PV Grid Tied with Battery

quinnj3 — Mon, 14 Jun 2021 13:01:53 +0100

Hi All,

I've an interesting project to get stuck into and I'm currently looking at the most financially feasible solution. My background is a time served electrician plus 15 odd years in the design, manufacture and implementation of control systems so technically I can design and implement this system without issue. I have a local solar panel company willing to review, test and sign off on my implementation if necessary.

My plan is to have solar panels mounted to my garage roof which is facing south with a 15Â° pitch. I want to push down my current electric bill as much as is reasonable and need to strike a balance between 'Green energy' and cost savings. I was given a used set of 3kW Polycrystalline solar panels without mounting rails or inverter. I also have a Victron Multiplus II 3000VA power management unit.

What I would like to achieve is solar panel power stored in a battery system (approx 10-15kWh Lithium) for use as required as, like most people, our home is quiet through the day. I would configure the system for zero export / import and use the Multiplus to manage that. The multiplus will monitor consumption from the grid and use the energy in the batteries to balance that out to zero import or maximum output of unit. I am considering setting up an economy 7 tariff which if the batteries are below 50% at 1am will charge the batteries to 80% netting me half price electric when solar can't match my demand. Above 80% I would switch on a 1.5kW heating element to create domestic hot water.

As I already own some of the big ticket items I need to decide on the next options:
1) Use Victron MPPT chargers to take the solar energy to charge the batteries directly. The multiplus always draws from the batteries to reduce grid import.
2) Use a grid tied Solar panel inverter which will output when the solar panels are active. The Multiplus will take the excess energy not used by the dwelling and store in batteries.

My current consumption averages to 14kWh / day. My bill is approx Â£800 / year. My basic research into this estimates I could drive my bill down to approx Â£250 / year. As I can do all work by myself labour is minimal. Both options will cost approx. Â£4000 - Â£5000 to implement (mostly batteries and mounting rails). I'm located in sunny Northern Ireland so I'm a little pessimistic about my calculations. I know if I was purchasing all new equipment then at Â£8k+ it's not financially feasible.

My question is:- From people's experience is it realistic to expect these kind of savings from this system? If not what would be required to achieve the desired performance?

Any input or comments appreciated.

Jonathan

Solar PV Grid Tied with Battery

Ed Davies — Mon, 14 Jun 2021 14:43:00 +0100

You might find this blog interesting: https://greening.me.uk/ PV, batteries, agile tariff, EV, electric to replace gas water heating under some conditions, etc.

Solar PV Grid Tied with Battery

DamonHD — Tue, 15 Jun 2021 09:16:35 +0100

Could you reduce your base consumption at the same time to get an even better result?

We cut our gas and electricity usage more than in half, plus added PV and storage (on- and off- grid).

As a family of 4 our highest-ever (lockdown) electricity consumption was ~8kWh/d, more typical is 5kWh/d.

Bills for gas+electricity are under Â£600/y in total, mostly standing charges in fact, though more than covered by FiTs since we were in early.

Rgds

Damon

Solar PV Grid Tied with Battery

quinnj3 — Tue, 15 Jun 2021 10:22:40 +0100

It would be an ultimate goal. My wife is working from home now so PC is on for at least 8 hours a day at approx. 300w/h/day. I used to heat all our hot water from electric for a few years as I never got around to replacing a failed heating valve on our HW tank. I've now got that replaced and I'm using oil to heat the water which should bring down our electric bills. We were at 17kw/h/day but I've estimated now down to 14kw/h/day but that may be even lower. Our house is only 10 years old with a B energy rating. Heating bills are about Â£6-700/year on oil. Cooking is on gas at approx Â£75/year. If I can get hot water from solar in the summer and meet my electric needs mostly from solar in April to October I'd be very impressed. I do have several older appliances and tv's now however the cost of changing still exceeds the saving plus environmental pressures. If the solar worked out I would consider changing my cooker to electric.

How did you cut your gas bills? My wife is always cold so it was a compromise to get the heating set as low as 21Â°C in my house. Kids and myself are (nearly) always running around in t-shirts. Heating isn't required at all from end May to October though. We have a median temperature year round of approx 8Â°C so its quite cold all year really except for a couple of months in the summer.

Solar PV Grid Tied with Battery

DamonHD — Tue, 15 Jun 2021 12:57:34 +0100

Lots of things for gas and electricity:
* We turn heating on in November (or December last year) and off at the end of March, which trims demand significantly.
* We don't run around in t-shirts in the heating season since keeping a body warm is much cheaper and more green than keeping the whole house very warm.
* We soft-zone the house with Radbot so rooms that are less occupied automatically fall back to a lower temperature when vacant.
* Target temperature during occupancy in most rooms last year (as set on Radbot) was 17C, and a bit warmer in the living room where most of the family spends much of its time.
* We use laptops (~10W to 30W when in use). Even without lockdown the adults are working from home most of the time.
* We have installed 2x single-room MHRVs to retain heat better.
* Quite a lot of IWI.

See more at my site, eg:
* https://www.earth.org.uk/saving-electricity.html
* https://www.earth.org.uk/superinsulating-our-living-room.html
* https://www.earth.org.uk/MHRV-mechanical-heat-recovery-ventilation.html

Rgds

Damon

PS: full disclosure I invented and sell Radbot, which I suppose is "scratching an itch" for our energy savings!

Solar PV Grid Tied with Battery

quinnj3 — Tue, 15 Jun 2021 20:47:35 +0100

Thatâ€™s brill. Damon. Thanks for sharing. I donâ€™t think I could handle 17 degrees myself. We have a full house heat recovery system which works really well at keeping moisture out and helping us keep temps down. Iâ€™ll have to look into the Radbot, itâ€™s an interesting concept. I assume it works best if heating is on a time clock as normal set for the times the house is occupied. The radbots then ensure the heat is used where itâ€™s needed most minimising waste. I assume the boiler would be short cycling with this method? Iâ€™ve been considering a thermal store anyway as it could work quite well with a solar boost. Especially in the cross over months.

Solar PV Grid Tied with Battery

quinnj3 — Tue, 15 Jun 2021 20:51:05 +0100

Also if they work based on light in the room would there not be a lag in heating the occupied rooms? Or would this be reduced as there is less load in unoccupied areas so possibly better heating response than normal? Thinking out loud here!

Solar PV Grid Tied with Battery

DamonHD — Wed, 16 Jun 2021 19:27:25 +0100

I don't want to be doing too much of a Radbot promo here.

a) Radbot is happy to work with existing bolier clock timings, but will generally not call for heat overnight anyway (a 6C setback will apply) so it can work well without a boiler clock at all. <50% of English homes have all three of house thermostat/interlock, timer and TRVs according to a fairly recent English Homes Survey (DHCLG)!

b) As well as responding to live occupancy signals, Radbot predicts occupancy so that ideally if you walk into a room at a time you are usually there the room should be only just below target temperature as if you had an old-fashioned TRV.

(And yes we definitely expect better heating response in rooms where the heat is actually needed, and saw that in an unexpected way in a case study!)

Rgds

Damon

PS. Mod(s), kill this post if it breaks the rules.

Solar PV Grid Tied with Battery

jms452 — Sat, 19 Jun 2021 16:45:46 +0100

Posted By: quinnj3My wife is working from home now so PC is on for at least 8 hours a day at approx. ... My wife is always cold so it was a compromise to get the heating set as low as 21Â°C

Sitting around makes you feel cold - I'm rarely cold at home but if I'm working from home even I get cold.

Our fix is a 500W electric heater under the desk which keeps the office warm and the person at the desk very warm in the day meaning we can switch off the main heating - most of that is the solar PV for us.

We've also been experimenting with a 100W heated throw that sits at about 35C. Even if it's just over the office chair that also works pretty well.

Have you considered using the grid as your battery - I've attached the current outgoing octopus rates which many people are surprised by..

Personally if I was on oil I'd consider a heat pump before a battery, or at least use the battery to export 4pm to 7pm making you 15p/kWh.

Solar PV Grid Tied with Battery

djh — Sat, 19 Jun 2021 21:48:48 +0100

Feeling cold is largely a matter of wearing the right clothes. My wife originally came from the tropics and used to feel cold all the time. But if she wears the right layers she now tolerates the cold better than me. Socks are very important for feet for example. I never understand the popularity of jeans; cotton is a very poor choice in a damp climate. But I agree with John, local heating works well and we've tried a heated throw recently as well.

Solar PV Grid Tied with Battery

RobL — Sun, 20 Jun 2021 08:55:54 +0100

About 10 years ago I worked in a large air conditioned office, one of those with externally managed temperature using a BMS (building management system). Every now and then some bloke would wander round in charge of the bms, and if he got complaints, would move the temp up or down a bit. It was frighteningly low humidity due to forced air ventilation with no control - the fan was just on or off. The secretary we had used to keep a 3kW bar heater under the desk to keep warm! I bought a 50W 12v chair cover, intended for a car, but worked fine on an office chair. It didn't completely stop her using the bar heater, but cut it back a bit.
Im glad to not work there now, I hated the low humidity all the time.

Solar PV Grid Tied with Battery

revor — Wed, 23 Jun 2021 20:09:00 +0100

Looks like a nice little project. As mentioned by others try and get your usage down and when you are up and running make sure you can get your battery 100% full before solar stops so you are full overnight as breakfast can take a lot of juice out of the battery. Somewhere I guess someone will have made some relationship between annual solar, Electricity consumption, size of array, size of battery needed etc. I do not know if your balance is right but if you only have 3 Kwp and your usage is high I wonder if the battery to panels at 15KW may be OTT which made me wonder if there is an algorithm out there. The investment in a battery works out as purchasing your electricity over the lifetime of the battery at around 12 to 15 p per KWh. Add to that the 5 or 6 p to charge battery overnight you are not saving money over and above normal tariff. The battery model works best if you can save the free solar you have generated in the day.
My 6Kwp array and 10kw battery (7 weeks since commissioning) combo is working well this time of year may be very different come winter. We are achieving most days 98 to 100% self sufficiency on electricity but on the month is around 95 to 96% as loose the battery every 20 days as the BMS takes over and does it's maintenance, so we use about 2 to 3Kwh from the grid whilst this is going on. We like everyone else I guess try to use the good solar days for the heavy use stuff, washer dishwasher baking etc which can be a challenge as being on a smallholding we are outside when fine. Other things we have found is that when turning on a heavy load e.g induction hob not to put it on full straight away but start at low power and ramp up over a few seconds. This stops (on our system anyway) taking initial juice as a spike from the grid whilst the system works out what is going on and where to balance the power from. Putting the hob on several times a day to boil a kettle the spikes adds up. I noticed the other day that the panels were dusty and as we had very light drizzle all day today decided this evening to clean them. Before cleaning was getting 0.3 Kw after 0.5 Kw I do not believe they were that dirty but have concluded the performance was down because the rain had formed as thousands of tiny droplets which I think was causing the light to diffract.