Battery or no Battery - Please help me decide!

Niv

Hi All,

I am getting solar panels installed soon and am still in two minds about a battery and wanted to gather thoughts from here to help me decide one way or the other.

The system is a 6.4kW split array with a 5kW inverter, I am also getting a immersion controller installed to divert excess to the water tank.

I have looked at the octopus cheap off-peak tariffs (go and intelligent) and it appears you can only have them if you have an EV so I wouldn’t be able to utilise the cheap period to charge a battery at low rate (I have a friend that does this and no longer pays peak rate for electricity but he has an EV too which is probably why he can get the tariff).

 

Advantages of battery

·         Potential for island mode (would need to double check this bit)

·         If I get an EV in future I could utilise cheaper off-peak electricity

·         During the lighter months off setting excess electricity to use into the evening

·         Ability to use own generated electricity rather than sell at cheap rate and buy back a higher rate

 

Disadvantages of battery

·         Cost / payback (quoted £5k for 10kW battery) not sure it will ever payback

·         Immersion diverter taking a portion (majority?) of excess

·         No potential for island mode in event of a power cut

 

I am particularly interested if anyone has a immersion controller and finds that as a result there is little excess electricity in any case because if this is the case a battery seems like it is not worth doing. I have oil to heat water so the immersion controller should pay for itself in the first summer.

I see island mode as a nice to have rather than essential as it would only really be for ‘national’ scheduled blackouts as we only get fairly small blackouts currently (typically only a few minutes).

 

Please help me decide!

Thanks,

Reed_Richards

If you want "island mode" you need to ensure that you have a inverter that will support it and you need a cut-over switch that will ensure that you really do have an "island" that is isolated from the mains.

In summer I usually have enough solar power to charge my 6.5 kWh battery and divert power to my hot water cylinder if the sun shines.  And that's with only 4.8 kWp of solar panels. 

Magnitio

I have a 6.4kWp solar array with 5kW inverter and 10kWh of battery. I have found it quite difficult to calculate the ROI for the battery based on the 9 months I have had it so far, mainly due to the volatile of electricity prices. I use both Agile Octopus and Agile Outgoing, so prices vary every half-hour during the day. Over the last few months this has provided a number of benefits that have been easier to take advantage of by having the battery e.g.

• during summer (in particular August) the battery quickly charged, after which excess was exported at decent rates and could be supplemented by partial discharging of the battery between 5 and 7 at rates between 50 and 80p/kWh. The remaining battery capacity was then sufficient for the relatively short number of hours when panels were not producing.
  
• during the last month, when solar generation is poor, batteries have been charged at the cheapest overnight rate available. Last night that was between 0.97 and 2.21p/kWh. Some days it is no lower than 15p.
  
• The only time we pay peak prices (34p/kWh) is when using more than 5kW in the evening for cooking, but that is rare.
• On cloudy non-summer days, the output from the panels can vary considerably from one minute to the next. Without a battery, this means exporting one minute and importing the next. We have a low power immersion heater (1.5kW), but combined with the base load for the house this is often more than the panels are producing; again, the battery ensures this load is not taken from the grid.
• we have the battery set up to provide power to 4 sockets in the event of power outage. Could be useful this winter!
  

The big issues with the battery are the cost, availability, longevity and the environmental impact of production and transport.

Another reason why my calculation of ROI for the battery is so complex is that I don't know what decisions I would make if I didn't have it e.g. cooking at mid-day, choosing a different tarrif, heating water with gas rather than immersion etc. I'm glad I chose to get it, but I suspect that if I had decided to go without, the panels and inverter would have paid for themselves quicker.

Alnat1

Simply, I haven't heard anyone who has a battery say they wish they hadn't bothered. The satisfaction of seamlessly moving onto battery power as the sun sets and back on to solar at sunrise can't be beaten.

powerful_Rogue

6.4kWp system installed a few weeks ago. Was quoted with a 6.5kWh battery, but I upgraded to 13kWh. No regrets at all.

I check the solar forecast for the next day and base my overnight charging on that. The solar during the day then tops it up.

I'm paying around £1.15 a day for electricity including standing charge.

Magnitio

Just to add to my comment above, Octopus Agile rates are negative tonight, so actually being paid to charge the battery!

ispookie666

Definitely yes for battery. It helps in load shifting in addition to maximising your solar consumption. I have an iBoost, which is a kind of dumb device. But if you like connectivity, maybe look at Eddi. 

Used to have solar panels in our old house (2015) always regretted not having batteries, it was not economically viable at that time. 

Petriix

Definitely no for a battery, unless you can show some calculations which clearly add up to a reasonable return on investment. I'd also get rid of the hot water diverter (with the same caveat).

You can get 15p per kWh SEG payments so exporting is almost certainly the best option. Each kWh you store in the battery will save you ~ 18p: 35p capped import rate, minus the 15p you would have earned for exporting it, then deduct 10% for losses.

At £5k, you'll need to save £500 per year for a 10 year ROI. That's 2,778 of those 18p kWh or likely around half your generation. The trouble is that much of your generation will be condensed into summer days where you'll generate up to 35kWh and struggle to use even half of it without being deliberately wasteful. Your battery will still be half full the following morning. In the winter you'll find that the battery doesn't get full very often. 

The numbers would look a little better with off peak charging (on Go if you had an EV) but you need to have a clear plan for how you're going to make the required savings to recover your initial outlay.

For hot water, current gas prices are lower than current SEG payments. You're better off exporting the electricity and buying gas. Ironically that actually saves more gas overall because a domestic boiler is more efficient than the round trip efficiency of a gas power station. 

I have seen some use cases for both batteries and hot water diverters. They are always very high users, sometimes with no mains gas, sometimes on the old FIT scheme with deemed exports.

You need to show some sound figures before anyone can confirm whether it's worth it or not. 

Alnat1

Keep reading, and you'll see it's as I said, the only people who say no to batteries are the ones who don't have them 

ecraig

I have just one 6.5kwh battery. I wish I had two...

Petriix

Alnat1 said:

Keep reading, and you'll see it's as I said, the only people who say no to batteries are the ones who don't have them 

And that's because the only people who have batteries are the ones who don't bother accurately assessing the ROI, or have very unusual usage. 

Believe me I would have batteries if they made financial sense. I cancelled my Givenergy order (at under £4k for 8.2kWh installed) because the numbers couldn't possibly add up for me.

However, my advice is to do the sums and make an informed decision.