House battery... worth it?

i know that most say its not worth it

but here in DK we get paid very little for solar, the best we is a feeling of being bent over and f’ed from behind… ie we invest a lot and all we get are pocket money back…

so yes i agree with those that say use what you produce… but what if we dont produce when we consume? my house is heated by a heat pump, but that will consume most in evenning and night and early morning, hot water i can control, currently its doing that in middle of night when import is cheap, same in the first part of the afternoon.

the radiators is more like we dont need them when the sun is up where solar is peaking

so my thought was to charge batteries when sun is at work and then empty them when sun is away and do a little import if that does not cut it

but how do i calculate how many panels i need and how much battery i would need…
so far i have keept off as i think its very complicated, almost like religion (and i have to come up with what i will put faith in :smiley: )

any comments? dont be shy, i will not bite

If you do, I will - and I’m a moderator here. I’ve got bigger teeth. :rage:
:rofl: :rofl:

What I think you need, if you don’t have it, is data on which to base a decision - because every home is different: in construction, orientation, location, not to mention the way you live and use it. The obvious things are the energy you import, the amount you export intentionally or via losses (which will be weather-dependent), and any “free” gain you get from solar through windows and heating the outside, plus a few other things I’ve not thought about.

When you have all that, you can make a start. You’ll be able to see how much energy you consume at various times of the day, which will give you upper and lower limits on the battery capacity to supply that amount of energy. How much P.V. you need will depend on the weather, so should your battery carry you over one night, or a night, a day and a night, or more? What is the balance between “good” and “bad” P.V generation days, or between “good” and “bad” heat pump days?

Only you can throw “what-if?” numbers into the calculations and see where it takes you.

hmm… the amount i use and when is a simple task… i can login to a an hub and extract my data, all meters these days are remote read ones and they report their data to a hub that everyone with the access rights can read from… ie i have since i am the owner, but electricity company of course have it too…

then comes the location… well i know where i live etc, but how do i figure how much avg solar production i can get?

and would it be correct to aim for the worst production (in winter) and pair that up with my use in the winter too?

but yes the weather also have a play… the house leaked a lot of heat when we got it, i have since improved the vapour plastic that also to a great extent makes the house tight, i still miss a 1/4 of it to complete it on the first floor, downstairs i do a room by room ceiling upgrade and at the same time the vapour plastic done here too… and dont get me started on the floors, concrete but someone forgot to put the required 300mm styrofoam insulation under, so also here a room by room upgrade and converting to radiant heating instead of radiators

There are websites where you can enter your location, the direction and angle of your solar panels and they provide an estimate. How accurate the estimate is, I have no idea - but I would hope it is better than nothing (which is where you are now). Or you could buy a small solar panel (a few watts), put it where you might put your real panels and measure the output. Again, whether that would be an exact analogue of the full-size panels is questionable, but it would give you some idea. There are professional instruments at a professional price, I don’t know of any ‘budget’ ones.

Again, a decision for you. Do you want to cater for the absolute worst case conditions, or are you prepared to supplement the battery with mains power for a few exceptional days each year?

As you mention you will need your annual electrcity consumption and when during the day and week you consume. There will always be a mismatch between when a solar PV system generates and when you consume. This is both on a daily basis and on a seasonal basis.

Battery storage will time shift the Solar PV output on a daily basis. You can avoid much of the production being exported. These figures are based on a domestic UK property consuming around 4000kWh/a for non space heating purposes and a solar PV system with a capacity of around 4.5 - 5.0kWp. Typically without battery storage you will consume around 35-40% of the generated electricity. With battery storage you will consume between 60-70% of the generated electricity. This is dependent on the size of the battery and other capabilities.

In the UK we also have access to ‘time of use’ electrcity tariffs. These have peak, off-peak and standard times. One tariff is based on 24 hours ahead half hourly billing similar to a commercial tariff. Depending on the capabilities of the combined solar PV and battery system, in times of low production (winter months) the battery can be charged from the grid at cheap times and discharged during peak times to avoid the higher costs. Some tariffs also pay for export at a premium rate during the evening peak. If you have aheat pump you will need at least 10kWh and battery storage capacity to bridge the evening peak. We have 27kWh covering a 16kW heat pump.

So is it worth it? It all depends on how much you are paying for electrcity and how much the cost of the Solar PV & battery system cost. The solar PV system should have an expected life of 30 years with an inverter change at some point. The battery will last at least ten years and if properly designed and mangaed should last a lot longer.

Between the house, the heat pump and EV charging we use around 15,000kW/a. The solar PV produces around 5000kW/a. We have a very high utilisation and benefit from export payments. On a previous tariff for Tesla Powerwall owners costs netted off meaning we only paid for 10,000kWh of grid electrcity.

As to how big a system do you need, you require a Solar PV professional who can provide accurate estimates of system output in your location (using local irradiance data), and has the capabilities to run simulations through expert system software to give the kind of answers you require. Calculations have to take into account panel azimuth and orientation, and shading objects that will reduce output. If you have detailed consumption data, this can also be modelled. Be warned that even in the UK not that many Solar PV installers have the capability to undertake this level of analysis and design. Before I retired this is what we did for every single job. There are manual methods available to do this, but they are not as accurate. A simulation of a proposed system design will predict generation and utilisation.

To be honest, most customershave different buying motivations other than purely saving money. Here in Scotland there are interest free loans available from the Scottish Government for Solar and Battery storage, along with grants for Heat Pumps. People are taking advantage whilst they are available recognising that sooner or later they will have to make these kind of changes anyway. In the future such incentives may not be there.

These should help with estimating annual PV energy harvest for your location:

For a daily solar prediction:

i wish the danish goverment had the same bright people, sadly that is not the case. They want people to go green, but no help :-/

i have yet to find one that can do that without it cost me an arm+kidney. hence i will try and figure something roughly that can give me an pointer if i’m total off or it would make sense to go further

but well my consumption last year was just shy of 11000 kwh and the worst month is december with 1600 kwh

i have tried to use and figured that i would need 10 kWp installed to make an avg of 2000 kwh in december… the link with location details: Global Solar Atlas

not sure if that is the usefull to figure things out but its the best i could figure myself that did not cost anything

with same location and kwp its says 173 kwh in december, quite a difference

we have that to0, i use Market data | Nord Pool and each day at 13:00 we get the prices for the next 24h

dishwasher is running at night, and also clothes washer and drier run at night

heat pump heats water for shower etc at night and in the afternoon, that all keeps the use of electricity to where its cheapest.

on top of that heatpump drops temp for space heating when in the expensive hours, and now we have just changed to the summer schedule where drop temp from noon to dinner time, all the south facing windows provide enough, and even the conservatory helps if we open the door there, thou this can not be done on windy days as that will just suck air out of the building

forgot that one, as we all 3 in the house are pensioners there is not much difference in what day of the week and usage during the day stays pretty much contant with peaks in the morning (breakfast), then lunch, and in the evening. if we put effort in to it and in the summer there is almost no usage outside of that…

difference from what?

between the to sites… PVGIS Online Tool and Global Solar Atlas

PVGIS is about right. Sometimes import from PVGIS-SAF. Also use Meteonorm 8. Have never looked at Global Solar Atlas but sounds like complete fantasy.

is that a database ?

i only ask as that is not an option in PVGIS

but hmm if i want to cover my arse complete in december it means 100 kwp

that will cost me about 60000£ (give/take currency conversion)

need to think

It is one of the available databases in the simulation software I use, PVSol. The database is a licenced product where access is provided through the PVSol licence.

Did you try PVWatts?

I used one of the more sophisticated models HOMER - Hybrid Renewable and Distributed Generation System Design Software to calculate a 10kW system in Africa. The model is good in that is accounts for exact location and historic solar data and introduces day to day and hour to hour solar variability. But whilst that might give you a reasonably accurate daily/yearly generation, it is v difficult to match this against the variability in and pattern of your consumption to see where battery storage may benefit you. The model does though take a daily average and then apply random variability to it to see when import and export may happen. But it’s not precise as few people really know what their pattern is. I could set a rough pattern based on if the most usage is in the day or evening. Plus set off peak electricity tariff costs and system component costs.

For the mathematically inclined I think what it is doing is to perform Monte Carlo simulation on all the variables and system configurations. I.e. it applies an hour by hour random shift to all the non-fixed variables, like insolation and consumption, and covers lots of different panel and battery sizes. You can set how much your day to day usage might vary and set say weekends to be different. It calculates an electricity pattern for a one year period. After doing that 1000 times It then calculates the cost of each system against the benefits in electricity for each of the 1000. So you get 1000 different cost benefit calculations covering many different systems.Then it presents to you say the top 5 most cost effective systems, rather than printing them all out

As far as I am aware many of the other models are much simpler than this, so less accurate. i.e. they may be no more than an Excel worksheet with simple maths. Homer takes around 5 mins to calculate an answer, compared with instantaneous in Excel, so you can see the difference in sophistication. As it is based on seeding with random numbers a new run with the same input data will produce a slightly different list of ideal systems, but they are close.

I got a 30 day free trial of the off grid Pro version. Otherwise it is quite expensive. There is a Grid version for on-grid connected ones. Anyone else used Homer?

For my own 10kWh battery system I have no idea if it going to pay its install cost back, as never did any modelling on it. Probably will as I charge cheaply overnight if the forecast does not expect much sun the next day. But unless you can automate this process it’s going to be very tedious to guesstimate. Also I got it cheap through self-building it and getting some bits s/h.