Field note · Method + case study · Battery storage · Jun 2026

Did the new batteries actually work? Proving a storage upgrade with calculus

Mike had two battery packs added and asked if they were doing anything. The app's percentage can't tell you. So we ran one full discharge, integrated the power curve, and let the maths answer it.

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Very very helpfuland so quick. Made sure that a non expert like myself understood what the problem was and how to resolve it.

Declan S · Jun 2026

Contacted Solar Tech about my Givenergy battery storage system that wasn't working. Battery status was "idle". Given the company Givenergy had gone bust, I need independent quality technical help. Very patient and clearly very knowledgeable about battery systems Ron diagnosed possible issues and suggested several possible remedies. Worked our way through them and fortunately it began to work. The fault was very specific and only an experienced engineer would have thought to check. Suffice to say I'll be back if I need independent support again. Lastly you only pay if there is a solution. Outstanding.

Mr P · Jun 2026

My Givenergy battery stopped working nearly a month ago. After unsuccessfully reaching out to my installer, who looks like he's also busted, I found Solar Tech Support on a Google search. They fixed my issue in a couple of hours. Any frustrated Givenergy customers, I highly recommend these guys.

Sarma Amirapu · May 2026

Excellent response to diagnose a problem on our SolarEdge installation. Kept us informed at every step. Diagnosis quickly completed and solution implemented.

Les Bennett · May 2026

Prompt and useful support regarding my Sunsynk system lack of performance. Ronald simplified the technical issues to make them understandable. Many thanks, looking forward to follow up.

Patrick Keenan · May 2026

Amazing support, went out of his way to help try and get us back up and running

Paul Smith · May 2026

What a fantastic service. Had my fault diagnosed within minutes and actually managed to resolve the issue remotely within a few minutes more. This guy is like a “Solar Batman” helping consumers fix their problems using his extensive industry knowledge and expertise. Outstanding service. Thank you so much.

Andrew Palmer · May 2026

Excellent service from Solar Tech Support. Extremely quick to respond, easy to deal with and clearly very talented engineers. They were persistent throughout a complex GivEnergy battery issue and resolved everything completely. Highly knowledgeable, professional and reassuring support from start to finish. Highly recommended.

David Harris · May 2026

Ron responded very promptly regarding my GivEnergy battery issue, his knowledgeable diagnosis was spot on and resolved the issue on first attempt. Would recommend to any and all.

Simon Hill · May 2026

Ron made more sense in 20 mins than our installer has done over the last 12 months There is a jungle out there and you need someone like Ron to give a comprehensive overview and solution

David · May 2026

I have a GivEnergy system consisting of two batteries, two inverters and a controlling EMS (Energy Management System) which has not worked since Nov 2025. After six months I discovered Solar Tech Support, reached out to them and Ron phoned me back – how often do you get that service? Could not be more helpful – worked directly with me over the phone, outside what I would call normal working hours. Lucid explanations and we were able to discuss the issues and history using camera and email history. As this was a very rare setup, Ron was able to access an EMS expert in the field to confirm the solution. One sunny day in, I am now only paying for standing charge and a few pence for spikes in grid consumption while battery catches up with house demand.

Ian · May 2026

When my GivEnergy system had an issue, I was completely left without support and had honestly lost all hope. Thankfully, I searched online and found Ron, which completely turned things around. After sending him a message, he responded incredibly fast and called me to assure me that he would get the problem fixed. I really admire his dedicated, supportive nature and his determination to find a solution. With this kind of outstanding attitude and customer service, he has absolutely secured a future customer in me.

Sree · May 2026

Ron want out of is way to help, nothing was to much. He was very thorough in what he did Very knowledgeable I would highly recommend Ron and his company He did a fantastic job for me. if you have any problems, he'll do his best to help you out and resolve your problem. I wouldn't hesitate to recommend them

Dennis Brown · May 2026

Ron took me through a diagnostic to confirm my GivEnergy Inverter had a fault. A common one as it turns out with the AC Inverter. As GivEnergy is defunct there is no immediate fix, aside from sourcing 2nd hand replacement. It may be that a fix becomes available over the summer which would make a lot of GivEnergy customers happy (Again)

Tony Deacon · May 2026

Contacted Ron with a problem and he sorted it out quickly with no problems at all. Very knowledgeable on anything solar/ batteries. I would recommend him to anyone

Phill · May 2026

I've spoken to Ron a couple of times with issues with my Givenergy installation. Such a friendly knowledge guy very highly recommended. Thank you very much for resolving my issues

michael fairhurst · May 2026

Contacted Solar Tech Support in desperation. After explaining the issues I had with my system a diagnosis was made and a solution proposed. Fantastic service, even contacted a manufacturer to arrange replacement parts for me. Great communications, explained all they were doing and what I had to do, clearly and precisly. Followed up to confirm all was ok. Excellent service.

Mr Machin · May 2026

After GivEnergy went into liquidation, just my luck, my battery started playing up (internal board crashed). Contacted my installer - not interested! Found Solar tech support on a Google search. Sooo glad I found this company! Ron is extremely helpful and has plenty of experience. He soon confirmed what the fault was, and helped me to get my system up and running again. Now moved my GivEnergy account to Solar tech support, and will definitely use again if I have more issues. Unusual to find such a helpful company in these times, no morons reading scripts, just direct contact with the engineer.

Keith Ballard · Apr 2026

Contacted Solar Tech Support when trying to understand what my Givenergy inverter problem might be and what might be my options. Received good/honest advise which backed up my thoughts.

Hugh Speirs · Apr 2026

Ron is a super star. Two months ago my GivEnergy battery failed a firmware upgrade leaving it a brick. My installer couldn't/wouldn't fix it. GivEnergy couldn't/wouldn't fix it. Then they went into administration and all hope was lost. A flurry of emails later and Ron had diagnosed the fault (failed USB flash drive, something I'd suspected) and talked me through resolving it. Two months of nothing resolved in about 3 hours. It's great to work with someone who pays attention to the details, knows that they're doing (not just following a script) and gets stuff sorted without a fuss or up-charging.

Christopher · Apr 2026

I can add to the list of customers who had already 'given up' on GivEnergy due to their appalling customer service, and that was before they went into administration. So you can imagine my desperation when, having changed my ISP and my Inverter, predictably, proving to be the only device that didn't connect automatically to my new network, I found zero prospect of any customer support with GivEnergy having called in the administrators just five days earlier! The salvation came from Solar Tech Support. My IT advisor stumbled across their web site and some very helpful tips for beleaguered GivEnergy customers, as well as an offer to provide direct assistance. Nothing ventured, I decided to drop them an E-Mail, with very low expectations based on my experience of GivEnergy customer support. Within an hour Ron had responded with some pin point advice, and after a few exchanges of E-Mails he had nailed the problem, enabling the combined efforts of my IT advisor and solar installer to resolve it and reconnect my Inverter. Thank you Solar Tech Support, and Ron in particular, for coming to the aid of a deserted and despondent GivEnergy customer. Expert, razor sharp advice and first class customer service, even though I wasn't officially a customer.

customer · Apr 2026

This company are a rare gem, I had a very unusual problem following a failed firmware upgrade on my GivEnergy kit. I then found out GivEnergy were in administration and had dismissed all their support staff! None of the usual fixes to try and restore my inverter comms would work, and I looked everywhere, forums, GivEnergy youtube support videos - even AI couldn't figure it out. My installer was talking about huge sums for system replacements, and being vague / evasive about if they'd even install replacement GivEnergy inverter. Enter Solar Tech Support, reassuring and knowledgeable from the very start, I've learnt loads about my solar system though the friendly chat while my engineer worked as he diagnosed the problem and figured out a fix procedure that I've not found anywhere else - amazing. If you need solar system repairs - especially if you like me have been left high and dry by GivEnergy, I cannot recommend this company enough. Give them a call.

Andy Thomas · Apr 2026

I sent a message on their website regarding a problem I have on my Givenergy system. Although not supplied by Ronald, I thought it was worth an email. Within the hour on a Saturday, he phoned and we discussed the problem. He logged in remotely and gave excellent advice. I'm too far away for his on-site help but he did diagnose the problem and was happy also to chat through my thoughts about an upcoming solar/battery install I'm planning. Great bloke.... if only he was nearer!

Philip · Apr 2026

Very very helpfuland so quick. Made sure that a non expert like myself understood what the problem was and how to resolve it.

Declan S · Jun 2026

Mr P · Jun 2026

Sarma Amirapu · May 2026

Excellent response to diagnose a problem on our SolarEdge installation. Kept us informed at every step. Diagnosis quickly completed and solution implemented.

Les Bennett · May 2026

Prompt and useful support regarding my Sunsynk system lack of performance. Ronald simplified the technical issues to make them understandable. Many thanks, looking forward to follow up.

Patrick Keenan · May 2026

Amazing support, went out of his way to help try and get us back up and running

Paul Smith · May 2026

Andrew Palmer · May 2026

David Harris · May 2026

Ron responded very promptly regarding my GivEnergy battery issue, his knowledgeable diagnosis was spot on and resolved the issue on first attempt. Would recommend to any and all.

Simon Hill · May 2026

Ron made more sense in 20 mins than our installer has done over the last 12 months There is a jungle out there and you need someone like Ron to give a comprehensive overview and solution

David · May 2026

Ian · May 2026

Sree · May 2026

Dennis Brown · May 2026

Tony Deacon · May 2026

Contacted Ron with a problem and he sorted it out quickly with no problems at all. Very knowledgeable on anything solar/ batteries. I would recommend him to anyone

Phill · May 2026

I've spoken to Ron a couple of times with issues with my Givenergy installation. Such a friendly knowledge guy very highly recommended. Thank you very much for resolving my issues

michael fairhurst · May 2026

Mr Machin · May 2026

Keith Ballard · Apr 2026

Contacted Solar Tech Support when trying to understand what my Givenergy inverter problem might be and what might be my options. Received good/honest advise which backed up my thoughts.

Hugh Speirs · Apr 2026

Christopher · Apr 2026

customer · Apr 2026

Andy Thomas · Apr 2026

Philip · Apr 2026

Mike had two extra battery packs added to his storage system, and asked a fair question: are they actually doing anything? You can't answer that from the percentage on the app — it swings from 0 to 100% no matter how much real capacity sits behind it. So we did what an engineer does. We ran one full discharge, measured how much energy actually came out, and compared it to what four healthy packs should give. The tool for turning a wiggly power graph into a single, defensible number is the same definite integral you met in first-year calculus. Here is the method, the maths in full, and Mike's result.

The question the app can't answer

After you have storage added, the obvious instinct is to open the app and look at the battery percentage. The problem is that the percentage tells you almost nothing about capacity. State of charge is a normalised gauge: the battery management system (BMS) scales it so that "full" is 100% and "empty" is 0%, whatever the true amount of energy in the pack. Lose half a pack and the gauge still climbs to 100% at the top and falls to 0% at the bottom — it just gets there faster.

The only honest way to test capacity is to measure energy directly: charge to full, run the system flat under load, and count the kilowatt-hours that come out. Mike's system is two original packs — 8.2 kWh and 5.2 kWh — plus the two freshly-added 2.6 kWh packs. Four healthy batteries should deliver a specific, predictable amount of energy. If the new pair is doing nothing, the total falls short in a way we can calculate in advance.

Power is a rate; energy is the total

First, the distinction the whole method rests on. Power (in watts) is an instantaneous rate — how fast energy is flowing right now. Energy (in watt-hours) is the accumulated total. If the power were constant, you would just multiply: a steady 3 kW for 2 hours is 6 kWh — the area of a rectangle, base times height.

But a real discharge is not a rectangle. It holds a plateau near the inverter's maximum, then steps down as the cells empty. The "height" of the graph changes with time, so there is no single number to multiply by. To get the total energy you need the area under a curve whose height varies — and the exact tool for that is the definite integral:

E = ∫₀^T P(t) dt

In plain words: the total energy E is the integral of power P over the discharge, from the start (t = 0) to the end (t = T). The integral sign is, quite literally, an elongated "S" for "sum" — which is the clue to how we actually compute it.

From Riemann sums to the integral — the bit I enjoy

Here is the catch: the monitoring portal does not hand you a tidy formula P(t) to integrate. It hands you readings at instants in time. So we rebuild the area the way the integral is defined in the first place — as a limit of sums. Slice the discharge into thin strips of width Δt, take the power in each strip, and add up power times width:

E ≈ Σ_k P_k Δt

That is a Riemann sum. Make the strips thinner and, for any well-behaved signal, the sum converges on the true area. That limit is the definition of the definite (Riemann) integral — ever-finer pieces melting into the smooth area under the curve:

E = lim_Δt→0 Σ_k P_k Δt = ∫₀^T P(t) dt

Ordinarily the next move would be the Fundamental Theorem of Calculus: find an antiderivative of P(t) and evaluate it at the two endpoints. But the portal never hands us a formula for P(t) — only samples — so there is nothing to antidifferentiate. That is exactly why we compute the area numerically instead.

We can do better than flat-topped rectangles for free. Instead of pretending the power is constant across each strip, join consecutive readings with a straight line. Each strip becomes a trapezium, and its area is the average of the two end heights times the width:

Diagram of the trapezoidal rule: a smooth power curve with the area beneath it split into vertical strips. Each strip is a trapezium whose area equals one half times the sum of its two end heights times the strip width. — The trapezoidal rule. Each strip's area is ½(*P_k* + P_k+1) Δt. Because the straight chords hug a gently curving line closely, the sum of the strips is a very good estimate of the true area underneath.

E ≈ Σ_k ½(P_k + P_k+1)(t_k+1 − t_k)

Why this is exactly the right tool here: the trapezoidal rule is exact whenever the signal is a straight line across the strip. Mike's discharge is a near-flat plateau for most of its length, so over that stretch the approximation is essentially perfect. The only real curvature is at the step-downs near the very end, and those are short.

And on the smooth stretches the error is not a mystery — it has a known shape. Wherever the power varies smoothly, the composite trapezoidal rule carries an error bounded by:

|E − E_trap| ≤ (T / 12) · Δt² · max|P″(t)|

Two useful things fall straight out of that. The error shrinks with the square of the strip width — halve Δt and you quarter the error — and it is proportional to P″, the second derivative: the curvature of the line. Along the long, near-straight plateau P″ ≈ 0, so that whole section is captured almost exactly.

The one place that bound does not formally apply is at the sharp corners, where the power steps down and the second derivative is effectively undefined — it would be wrong to claim the formula holds there. But the corners are handled by where we put the samples: drop a reading directly on each corner, and every segment between two readings is itself almost a straight line — precisely the case the trapezoidal rule integrates with no error at all. So the corners, far from wrecking the estimate, are pinned down exactly. That is why a figure read by eye off this graph is trustworthy to within a few percent — and why we still quote a band rather than pretend to a single exact number.

An aside for the control-theory nerds: the percentage is an integral too

While we are here — the dashed state-of-charge line on these graphs is itself the output of an integration. The standard way a BMS tracks charge is coulomb counting: start from a known state and integrate the current flowing in and out over time.

SoC(t) = SoC(0) − (1 / Q_nom) ∫₀^t I(τ) dτ

It is a pure open-loop integrator, and that is its weakness: any small bias in the current measurement gets integrated along with the real signal, so the estimate drifts over hours and days. It is the textbook problem of integrating a noisy measurement without feedback. Real systems treat the 0% and 100% rails as anchors — fixed points where they re-zero the integrator — and the better ones blend in a voltage-based estimate, a simple observer, to pull the drift back out. Which is exactly why, for a capacity test, we trust the energy integral of measured power over the reported percentage. One is a direct measurement of energy leaving the cells; the other is an estimate that has been quietly accumulating its own error.

How we run the test

The procedure is deliberately simple. Charge to 100%. Put the house on the battery and let it discharge in one clean run down to 0%, logging Battery Power throughout. Then sample the curve at the points where it actually changes — the corners are what matter, the flat stretches you can take in a single long strip — and apply the trapezoidal rule.

One detail stops the result being argued with. The quantity the portal labels Battery Power is measured at the battery terminals, on the DC side. So its integral is the energy that left the cells — not the smaller amount that reached the house after the inverter took its cut. That means we can compare it directly with the batteries' usable kWh, with no fudge factor for inverter efficiency. Had we measured on the AC side instead, we would have to gross the number up for conversion losses, and the conclusion below would only get stronger.

Mike's numbers

Graph of Mike's battery system discharging from 100% to 0% over about five and a half hours. Battery power holds a plateau near 3.85 kW for roughly four and a quarter hours, then steps down to zero. The shaded area under the power curve is labelled as the integral of power, approximately 17.8 kWh. — Mike's system over one full discharge. Power held ~3.85 kW — the inverter's discharge ceiling — for about four and a quarter hours, then stepped down as the cells emptied, reaching 0% near 19:00. The shaded area is the energy delivered.

Running the trapezoidal rule over the logged points gives a measured total of about 17.8 kWh, with a sensible reading band of 17.2–18.5 kWh.

Now the yardstick. Usable capacity is each pack's nameplate multiplied by the depth of discharge (DOD) it is allowed to use:

8.2 kWh pack at 100% DOD = 8.20 kWh
5.2 kWh pack at 80% DOD = 4.16 kWh
2.6 kWh pack at 80% DOD = 2.08 kWh (the first new one)
2.6 kWh pack at 80% DOD = 2.08 kWh (the second new one)
Total usable, all four healthy = 16.5 kWh (nameplate 18.6 kWh)

So 17.8 kWh measured is about 108% of the 16.5 kWh you would expect from four healthy packs — full capacity, and a shade more, because the system ran the cells slightly deeper than the conservative 80% figures assume.

And here is the clincher, the part that actually settles whether the new packs work. The two original packs can supply at most 13.4 kWh between them — and that is only if you drain them stone dead, well past their normal limits. Mike's system delivered 17.8 kWh: about 4.4 kWh beyond anything the originals could produce on their own. That surplus has to come from somewhere, and the two new 2.6 kWh packs are rated at exactly 4.16 kWh usable between them. The arithmetic closes almost to the decimal. Had the new pair been dead, the system would have hit 0% at around 16:40 — more than two hours sooner — and handed over only about 12.4 kWh.

All four packs are pulling their weight. The two newly-added 2.6 kWh modules are delivering their full share — the measured energy is roughly 4.4 kWh more than the original two packs could ever provide alone, which is precisely what two healthy new packs should add.

The verdict, from one full discharge and a definite integral.

Why this beats "it looks fine on the app"

A capacity test like this is worth running after any battery addition, or whenever a system "doesn't seem to last as long as it used to". The app's percentage genuinely cannot see a weak or idle pack — it normalises the very thing you want to measure. The maths can, and it needs nothing more than the data the system already records plus one full cycle to look at. Power in, integrate over time, compare to what four healthy packs owe you. The curve does not lie.

Field note by Ron Vitova, Solar Tech Support. Customer name changed; figures read from the system's own monitoring and integrated by the method described above.

FAQ

Frequently asked questions

Can't the app tell me whether a battery pack has failed?

No. The percentage on the app is a normalised gauge — the battery management system rescales it so the top reads 100% and the bottom reads 0%, whatever the true capacity behind it. A weak pack and a healthy one can both show 100%. The only honest test of capacity is to measure the energy that comes out of a full discharge.

Why integrate — why not just multiply power by time?

Multiplying power by time only works if the power is constant — that's the area of a rectangle. A real discharge holds a plateau near the inverter's limit and then tails off as the cells empty, so the power changes with time. The area under a curve whose height varies is, by definition, a definite integral. In practice we approximate it with the trapezoidal rule.

How accurate is reading the energy off a graph?

Accurate enough to draw a firm conclusion. The trapezoidal rule's error shrinks with the square of the time-step and is proportional to the curve's second derivative — its curvature. Across the long, almost-straight plateau the curvature is near zero, so that stretch is essentially error-free. We still quote a band (here 17.2–18.5 kWh) rather than a single false-precision figure.

Do inverter losses affect the result?

No, and that is the neat part. The figure the portal calls Battery Power is measured on the DC side, at the battery terminals — so its integral is the energy that left the cells, not the energy that reached the house. That makes it directly comparable with the batteries' usable kWh, with no need to guess at inverter efficiency. Measuring on the AC side would only make the battery's figure look larger.

How do I get this test done on my own system?

It needs nothing more exotic than the data your system already logs and one full charge-then-discharge cycle to look at. If you have recently had storage added, or your battery does not seem to last like it used to, we can run the test and write up the result. It is the kind of check worth doing whenever capacity is in question.

Want to know if your battery is really delivering?

Tell me what you have and what changed — a recent battery addition, or a system that feels like it does not last. A full-discharge capacity test measures what is actually there.

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