Fortyeight

Questions to ask every battery installer

Print this, take it to each quote, and write the answers in. Quotes are only comparable when these are answered the same way. The full battery report includes this sheet.

Size and power

  1. What is the usable capacity in kWh, not just the nominal size on the brochure? Usable capacity is what you can actually shift; some batteries hold back 5–10% of nominal.
  2. What continuous charge and discharge power (kW) will the inverter allow? Power decides how much of a short cheap window you can capture and whether the battery can cover a kettle plus oven at teatime.
  3. Why this size for my home? Can you show the half-hourly data or assumptions you used? Size to your use in the expensive hours, not your annual total. Fortyeight’s report shows where extra kWh stop paying.
  4. Is the battery AC-coupled or DC-coupled (hybrid inverter), and does it work with my existing inverter or solar? Affects efficiency, what else needs replacing and whether existing solar warranties are affected.

Efficiency, warranty and wear

  1. What round-trip efficiency should I expect from grid to battery to home? Every kWh you store costs 10–15% in losses on many systems; it eats into the cheap/dear price gap.
  2. What does the warranty guarantee: years, remaining capacity at the end (for example 70% or 80%) and any energy-throughput cap? Warranties differ: Tesla’s UK Powerwall warranty is 10 years with 80% capacity for normal home use; others quote 70%.
  3. Does the warranty need the battery to stay online, or registered, or serviced? Some warranties require an internet connection for their full term.
  4. Who honours the warranty if your company stops trading? Manufacturer-backed or insurance-backed warranties survive the installer.

Connection to the grid

  1. Will you notify the network operator under G98, or apply under G99 before installing? Who files the forms? Up to 16 A per phase (about 3.68 kW single-phase) is G98: notify within 28 days of commissioning. Above that, G99 needs approval before installation.
  2. Does the battery inverter plus any solar take me over 16 A per phase? If so, will you fit an export limiter? Battery inverters count towards the limit, and solar plus battery often exceeds it.
  3. Will I get an Electrical Installation Certificate and Building Regulations (Part P) notification? You need them for insurance, for selling the house and for some export tariffs.

Tariffs and export

  1. Is the installer and the product MCS-certified, and will I get an MCS certificate? Export tariffs such as the Smart Export Guarantee and Octopus Outgoing need MCS (or an equivalent route) for solar; Flux needs solar, a battery and an MCS certificate.
  2. Can the battery follow my tariff automatically (Agile half-hourly prices, Go, Cosy or Flux windows)? Which apps or integrations does it support? Savings depend on charging at the right times every day without you touching it.
  3. Will exports from the battery be paid on my export tariff? On Outgoing and Agile Outgoing, Octopus only pays for exports from eligible generation, not from batteries. Flux pays for battery exports.

Price, VAT and timing

  1. Is the quote at 0% VAT, and will the installation be finished by 31 March 2027? Installed batteries are 0% VAT until 31 March 2027, then 5%. A battery bought without installation is charged 20%.
  2. What exactly is included: battery, inverter, gateway, isolators, consumer-unit work, scaffolding, commissioning, app setup? Compare quotes like for like.
  3. What deposit do you need, and what happens to it if you can’t install before the VAT change? Get the date commitment and the deposit terms in writing.
  4. Where will it be mounted, and does the location meet the manufacturer’s temperature, ventilation and fire-separation guidance? Lofts and some garages can be too hot or cold; location also affects insurance.

Sources: Electricity Safety, Quality and Continuity Regulations 2002 reg 22; Electricity North West G98/G99 guidance; HMRC VAT Notice 708/6; Octopus export tariff terms (V2.2); Tesla Powerwall warranty (EMEA, Rev 1.16). As of 18 October 2026. Run the battery calculator to know your price ceiling before you ask.