# House battery... worth it?

I hope this image shows up as I really like these. It’s the Homer printout of the assumed consumption of the building I modeled.

I had input a stepped consumption graph across one day (such as 2kW from 9am to 6pm and 500W otherwise) and with less for weekends. By applying random variability the model has input into its calculations a more realistic pattern. The chart shows the assumed real consumption across each day as coloured bands. Each day is one vertical stripe, although in this small image the stripes bleed together. Black is no consumption and red is 7 kW per hour. You can see that the weekends are darker and that no 2 days are the same, even though I had entered them as the same. I think the crest factor shown is based on the amount of variability I asked it to range over.

If one had a year’s worth of hourly smart meter data (although it’s house consumption one needs and not import data) I guess this could be input instead, but do any models take such data? And do any others randomise their data like Homer?

It does another one of these randomised charts for generation. Then by subtracting one from the other hour by hour (might even be minute by minute, I can’t remember) the deficit or surplus is calculated. The batteries or grid need to fill these gaps. Without the variability going into the model the gaps are a lot smaller hour by hour even though on average across the year it’s the same. So I think you’d estimate a smaller battery and array without this variability.

One further benefit of a model like Homer is that it better accounts for the limited input/output of the inverter in a battery storage system. Mine is 3kW, as are many.

For instance in a medium complex hour by hour model I assume one would take the average house load at say 5pm in March and subtract average capability of the battery to deliver then, which will be 3kW as it will assume it always has got enough SOC then. Then it adjusts for average solar at that hour. So it never assumes an import at 5pm in March, as average consumption less solar is always less than 3kW then.

But in the randomised version on some days the load will well exceed 3kW and the solar may be zero with the battery flat. So it will import a lot. Other days there will be lots of battery to cover the lower load then assumed. This is more reflective of real life where cooking one’s dinner can use >>3kW for short periods as the oven thermostat, microwave and kettle turn on and off.

I am not connected to the Homer company so am not pushing them.

Another model I tried could not account for dual rate electricity and overnight top ups. Not certain that Homer can do top ups either as not tried it for this aspect, although it will do variable tariffs.

I guess one problem in Denmark is that your long dark winters generate little solar. All the more reason to store cheap overnight electricity if you can.

When I had a firm quote for a storage add on to my PV they recommended a particular battery size. But I could not get them to tell me how they calculated it. The payback was >20 years by my simple calculation but I was not accounting for cheap overnight charging.

yes, but as PVGIS told me i would need a 100 kwp install to get 1600 kwh in december, the cost of the install is about the same as we paid for the house… it kind of scared the hell out/in to me

yes… i would need 125kw system to get 1600 kwh in december, but that does not tell me what the system cost would be… where as i can get a 100 kwp system at 60000£ that is 6 times more than we want to spend

Yes but one should never aim to more than very partially cover your needs in winter without a lot of import. I don’t aim for this but I do buy it cheaply overnight for most of the winter.

Besides if you have a massive array to cover winter you will be giving away too much in summer.

The first few panels and batteries have good payback (ignoring the fixed cost of scaffolding etc for the install so a small system is uneconomic.) After that it’s a diminishing return for each one added.

so what would a reasonble % cover of the most consuming month be ?

or should i look to cover myself in the summer instead ?

i guess it might be something in between…

i could go ask an installer here, but sadly they tend to look for money before they lift a finger, hence doing some foot work myself

You’re glibly talking of a 100 kWp system as if it might be a real possibility. Is it? Quite apart from the cost, do you have enough space? Most people want to put their solar on their roof and most houses are not big enough even for a 10 kWp system, let alone a 100 kWp.

pvgis is pretty much the European standard. Everybody uses it and trusts it fairly well. So I’d suggest just using that to get solar data and accept the results will be a best guess.

Trying to match your generation to demand on a daily basis all year is a hiding to nothing. Heating requirements are greatest in winter when solar generation is at a minimum. It certainly won’t be an economic solution even if you find one that is technically possible.

Here in the UK we have restrictions on the size of solar (or wind etc) generation that we can connect to the grid without getting special permission to avoid problems for the distribution network operators. You perhaps have some similar rules in Denmark? That number is a good starting base to work out a possible solar installation. How much would it cost, where would you fit it, how much of your demand would it cover by itself or with a battery and what size battery would you need. After you’ve worked out an example system design like that you’ll have a much better idea of what’s involved and can think about whether you want a bigger system, or even a smaller one.

Good luck

I can’t really answer that, if only because no 2 houses are the same, let alone countries. I am in UK. I guess that on average only 10% of my consumption is solar generated in the middle of winter, with most of the rest coming from cheap overnight storage. But in Denmark, with your shorter winter days, you should aim for less. The only real way to know is to use one of the models mentioned here to do a cost benefit calculation.

I find the suppliers just make a guess at the best system for you. They are probably more interested in maximising their profit than giving you the most ideal system. And they can’t be bothered to take the time to enter all the right data into their models unless you pay them a lot to do it. For instance in Homer it takes maybe 10 man hours for a non-expert like me to build an effective model and that is a lot of money if done by a consultant.

If you are armed with a model’s output that is reasonably correct it puts you in a much better position to argue a deal with a supplier.

As far as I can see that model is just giving you the solar generation potential. Homer does the same using NASA satellite generated insolation data.

But what a full model really needs to do is perform a matching of the generation against the consumption and do a cost benefit of different potential systems, so you can work out the most cost effective way to spend your money, e.g. is it better to buy more batteries or more panels? How many of the models mention here do this? In the Africa case I modelled the batteries were to cover the frequent grid outages and the benefit was in buying more panels and batteries to reduce the expensive running of the backup generator. But I could not get the run time to zero without ridiculous amounts of batteries, so the model gave the ideal cost balance.

perhaps… if someone dropped some cash at my door

2000 sqr meters

that is what i found out… i dont have half the house value to spend on solar

in the area i live in wind is banned, its to protect the "view. they say

not that i know of, they who have the money to spend even build solar farms on farm land that can no longer be used for any other purpose

~60000£

roof and ground (house is only 150 sqr meter so not enough space there)

see that is now the tricky part, how much DO I WANT to get covered and when, and what size battery

my goal is to import as little and export as little i can get away with, of course as everything its a compromise

pvgis is ok for me as i can manpulate the kwp numbers until i get the output i want, i will try and go for 5% in december and see where that lands me

same thought here… we have a saying that roughly translates: if you have to do something well, do it yourself… or a more universal one: self done is well done

for those that wonder where i live etc the location was in the links for pvgis…

i dont care that people know this, there are always jobs i can ask people to do if they show up like 2000 sqr meter new lawn done by hand

5% in december almost means that may is 0… i consume 791 kwh but with a 5 kwp install i could potential generate 737 kwh

now just to figure battery size… would it be far off if i found the day with the most consumption in winter so that i can store that and use during the day?

Well people here build solar farms on farm land too, but they are commercial operations and don’t benefit from the domestic limits. They all have to apply for permission to connect to the grid.

Compromise indeed, the two goals are almost diametrically opposed! The minimum export is nothing from no panels and the minimum import is nothing from a system big enough to power your house on a cloudy day in winter with a battery big enough to store all the power. Or maybe a few times bigger than that to account for a run of several cloudy days, or panels being snowed over etc.

i could do that, ie create a company only to get arround the limits, we have a few cases where people have done that.

good you mention that, as we live near the coast very fine sand is a problem.
its so fine that when i wash our windows it form like a very fine paste. so freq. washing would be needed too…

but yeah, aiming to cover 5% in december and then charge batteries when electricity is cheap sounds like the best plan if not going to pawn my kidney and sell my soul to the devil