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Why your study and the vendor's quote disagree, and how to check either

When a feasibility study and a vendor's quote give different savings or paybacks for the same battery, the cause is almost always in the inputs, not the physics: a different load profile, a different reading of the tariff, nominal instead of usable capacity, an efficiency measured at another point, perfect-foresight dispatch, a cost covering a different scope, no ageing, a kWh counted twice, or different financial conventions. Put the two side by side input by input. The one that cannot show its inputs is the one to doubt.

7 min read · Updated 2026-10-30

Disagreement is normal

  • A storage maker's CTO described the gap between what estimation tools predict and how completed systems perform as a major hurdle for installers (Solar Power World, 2018).
  • In a German forum thread on home-storage tools, one owner noted the many tools giving different results; another reported actual values about 20% below the calculated ones (photovoltaikforum). That is a residential context, but the mechanism is the same.

1. The load data

A case built on a standard load profile, monthly bills or hourly data does not see the site's real 15-minute peaks. A German competitor's founder lists standard load profiles instead of real consumption data among the common errors in storage calculations (Solarserver, 1 Apr 2026). Ask: which file, which period, which resolution, and was it checked?

2. The tariff

  • A different tariff version, voltage level or price system. German Leistungspreise above 2,500 hours range from €103.84 to €148.05 per kW a year at medium voltage in just three 2026 sheets (RheinNetz, NEW Netz, Netze BW).
  • An input error can swing the result: in one public case a demand charge entered twice turned about $1,000 a year of savings into about $20,000 (SAM forum).
  • Ask: is the baseline bill rebuilt and reconciled with the invoices? See Reading the C&I bill and rebuilding it from the meter data.

3. Nominal or usable kWh

The sonnenPro Compact 261 has 261 kWh nominal and 245 kWh usable: sizing on the first overstates the energy by 16 kWh, about 6%. Ask which one the quote's kWh are, and which one its price per kWh divides by.

4. Where the efficiency was measured

"Up to 98%" for the battery alone (SMA) and 91.3% at the AC side including auxiliaries (Huawei) describe different things. A neutral default for LFP is about 86% AC to AC at the inverter (PNNL 2022). Ask for the AC-side figure and the auxiliary load. See Reading a battery datasheet.

5. Dispatch

  • Perfect foresight — a year known in advance — gives the most the battery could ever earn. A real controller earns less. Ask which one the savings come from.
  • A dispatch that ignores the bill can miss what the tariff charges: a SAM team member explained its peak-shaving algorithm cannot respond to ratcheted demand charges because it does not know the bill calculation (SAM forum).
  • See Rule-based vs optimal dispatch.

6. What the cost per kWh covers

A "price per kWh" can mean five different things. From Zorowatt's cost benchmarks:

  • Battery pack only: $70 per kWh for stationary storage, global average 2025 (BloombergNEF).
  • Turnkey system without installation or grid connection (battery, inverter, controls): $177 per kWh in Europe in 2025, mostly utility-scale (BNEF via Energy-Storage.news).
  • Complete C&I systems with inverter, before installation, in one German webshop: €173–434 per kWh, median about €285 (retailer's own figures, AceFlex, Oct 2026).
  • Installation and engineering alone for US C&I systems: $56–168 per kWh (Lazard, June 2025).
  • Installed, including installation, excluding VAT, per usable kWh, commercial solar-plus-battery in Germany: €450–800 per kWh in 2024 (Fraunhofer ISE).

Ask what is included: inverter, controls, installation, grid connection work, commissioning, maintenance, warranty extension.

7. Ageing and replacement

A case with no capacity fade and no replacement looks much better than one with them: in the worked example in NPV, IRR and payback for a C&I battery, the NPV without ageing, replacement or escalation is about €59,900 against about €25,550 with them. Ask for the fade assumption and the replacement year.

8. The same kWh counted twice

Energy kept back for the afternoon peak cannot also be sold at the evening price, and a battery full of morning solar has no room for cheap grid energy. If the quote adds peak savings, solar savings and arbitrage computed separately, it may count the same kWh more than once. Ask whether the value streams were simulated together.

9. Financial conventions

Simple payback or NPV; with or without price escalation; which discount rate; incentives inside or outside the headline. Two honest cases can differ on these alone. Put both on the same conventions before comparing.

A checklist to send the other side

  1. The load data used: file, period, resolution, and any gaps filled.
  2. The tariff: version, category, price system, and the rebuilt baseline against the invoices.
  3. Usable kWh, AC power, AC round-trip efficiency and auxiliary load.
  4. Dispatch: rule-based or optimised, and the threshold or reserve used.
  5. Cost scope, line by line.
  6. Fade, end of life, replacement year and cost.
  7. Discount rate, escalation, project life and any incentive.

The Workspace prints every one of these in the report, with the tariff version and its source, so the comparison can be made line by line.

Frequently asked questions

Which one is right, the study or the quote?
Neither by default. The one whose inputs are stated and checked — data, tariff, usable kWh, efficiency, dispatch, cost scope, ageing — is the one you can defend. Put both on the same inputs and you can see which input explains the gap.
Is it wrong for a vendor to show the optimised savings?
No, if it is labelled as the upper bound. It becomes a problem when it is presented as the expected saving.
Should I share my meter data with the vendor?
With the client's consent, yes: two cases on the same data are far easier to compare. Ask for their inputs in return.
What if the difference remains after aligning inputs?
Then look at the dispatch rules and the bill engine: how each one bills the peak, the contract and the losses. A line-by-line bill for both cases shows where the money differs.

Related guides

Sources

Last checked: 2026-10-30. Every figure in this guide comes from the source linked next to it, with the retrieval date Zorowatt's research recorded (28–30 October 2026), or from arithmetic shown step by step. Cost benchmarks as recorded in Zorowatt's data/batteries/cost_benchmarks.json (retrieved 28 October 2026); US dollar figures are not converted.

Stuck on something else? Ask the community.

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