Your 6-Month Battery Install Plan: Start Now Before VAT Hits

Your electricity bill could drop by 75%. Here's exactly how to do it before March 2027 — and why waiting costs you thousands.

VAT Deadline: March 31, 2027
Why it matters: VAT on solar batteries rises from 0% to minimum 5% (likely 20% shortly after)
Cost impact: £800–3,000 more for a typical install if you miss the window

The 75% Savings Story

If you install a battery and charge it on an off-peak tariff (7p/kWh) instead of paying standard day rates (30p/kWh), your electricity bill drops by roughly 75%. Add an electric vehicle to the equation, and you're charging your car for less than the cost of gas.

This isn't theoretical. It's immediate. Month one after installation, you'll see it on your bill.

Before You Start: The EV Question

The first decision is binary:

Do you have an electric car (or planning to buy one)?

YES: A battery is immediately profitable. You're replacing 30p/kWh grid power with 7p/kWh off-peak charging. The payback happens in 4–5 years, and your car fuel costs drop dramatically.

NO: A battery still makes sense for home loads (heating, hot water, appliances), but the ROI is longer. You're betting on grid instability and tariff arbitrage rather than EV savings. Consider carefully, or wait until you have an EV planned.

Stage 1: Size Your Battery (2 Weeks) — September

Step 1: Review Your Energy Bill

You need to understand what you actually use. Most people guess wrong. Use our AI tool to analyze your energy bill and get a battery recommendation.

🔋 Battery Sizing Calculator

Upload your electricity bill. Our AI will recommend the right battery size, inverter power, and off-peak tariff.

Step 2: Pick a Battery

Recommendation: Fogstar bundles. Massive value for money. They come as battery + inverter, which simplifies everything.

Why Fogstar? Two critical features that cheaper batteries skip:

Fire suppression: Built-in fire extinguishing system. If a cell fails (rare), it doesn't burn your house down.
Battery warmer: Lithium batteries don't work in cold. If it's −3°C outside in January and you need to charge overnight, a cold battery will refuse to charge or discharge. Fogstar's warmer keeps it operational year-round.

These aren't luxuries. In a UK winter, they're essential. A cheap battery without a warmer is a brick in January.

Visit Fogstar battery bundles and choose based on your energy bill analysis.

Step 3: Match Your Inverter to Your Off-Peak Window

GOTCHA: A 32 kWh battery with a 3.6 kW inverter cannot fill in a 5-hour off-peak window. Math: 32 ÷ 3.6 = 8.9 hours. Your battery never reaches full charge during cheap hours, so you buy peak power to top it up. ROI evaporates.

Do this instead: A 32 kWh battery needs at least a 6–7 kW inverter (ideally 10 kW). This fills the battery in 3–5 hours, well within your off-peak window.

Step 4: Lock In an Off-Peak Tariff

Your battery is useless without cheap off-peak charging. Options:

Octopus Energy (Octopus Flux): 7p/kWh off-peak (typical), charged between 11pm–7am
EDF Heating & Power: Similar rates, less flexible windows

Get quotes from both. The difference between 7p and 10p/kWh compounds over 5 years.

Stage 2: DNO Approval (2–12 Weeks) — October–December

This is the bottleneck. It's why you start now.

What Is G99?

G99 is a grid connection application. You're telling the DNO (Distribution Network Operator — your local grid authority) that you're installing a 10 kW inverter. They need to check if your local grid can handle it.

Call an MCS-Certified Fogstar Installer

Don't DIY this. Find an installer from Fogstar's network (they're MCS-certified, which matters for export payments and building regs). They will:

Do a site survey
Submit the G99 application on your behalf
Handle all the DNO back-and-forth

Timeline: 2–12 Weeks

Fast track: 2 weeks (rare, but happens)
Normal: 4–6 weeks
Slow track: 12 weeks (grid congestion, staff holidays)

Your risk: If you call an installer in January, you're praying for 2 weeks. If it's 12, you miss the VAT deadline.

What Can Go Wrong?

DNO Rejection (rare): The grid can't handle 10 kW export. They might ask you to downsize to 6 kW. Negotiate if possible—it's unusual.

Export Limit (common): DNO approves your 10 kW inverter but limits export to 4–5 kW. This is normal and doesn't break the battery economics. You're charging your EV and running your house; export is a bonus, not the main ROI.

Conditions: DNO might require an automatic changeover switch (which you're installing anyway) or specific cable runs. Nothing insurmountable.

While You Wait for G99

Don't sit idle. Your installer can do a site survey, run electrical checks, and prep the installation. They just cannot start installation work until G99 is approved.

Stage 3: Installation + Backup (1–3 Days) — January–February

The Install

Once G99 is approved, your installer books a day (or two, depending on your electrical setup). They will:

Mount the battery (usually in a garage, outhouse, or internal wall)
Install the inverter
Run cables from battery to inverter to consumer box
Install an automatic changeover switch (ACS)

Total time: 1–3 days. Cost varies by location and your existing electrical state, but labour is not the expensive bit—the kit is.

The Automatic Changeover Switch (ACS): £1,000 Decision

Critical gotcha: A battery + inverter does NOT give you home backup during a power cut. If the grid goes down, you're down, even with a full battery.

To get backup, you need an automatic changeover switch. This:

Detects when the grid fails
Instantly isolates your house from the grid
Switches to battery power (you don't notice the outage)
Costs ~£1,000 installed

Our view: Install it. Grid instability is increasing as large power stations retire. Outages will become more common. The ACS is resilience insurance, and it should be installed at the same time as the battery (cheaper than retrofitting later).

ACS Installation & Setup

Do NOT install the ACS separately. Tell your installer to include it in the G99 submission and wire it during the battery install. Reasons:

Cheaper (one site visit, one electrical job)
Simpler wiring (all connected at once)
No second approval needed (it's already in the G99 submission)

ACS recommendation: EcoESS with monitoring. UK-made, meets all UK electrical standards, works with VolksDA inverters (and others). The monitoring feature (optional but worth it) sends you texts/emails when grid fails or power is restored, plus a dashboard to check status.

Typical Stage 3 costs:
Battery (32 kWh) + Inverter (10 kW): ~£8–10k
ACS + monitoring: ~£1.2–1.5k
Labour (install): ~£1–2k (varies)
Total: £10–13.5k before VAT (0% until March 2027)

Stage 4: Commissioning Tests (Critical) — Before You Pay

Most people skip this. Don't. Installers often do a quick 30-minute sign-off. You need 2–3 hours of proper testing, on-site, with the vendor present.

Charge Test

Measure the charge rate into the battery. It should be 8–9 kW, not 0.5 kW. Why? Because if it's 0.5 kW, your battery will never fully charge in the 5-hour off-peak window, and you'll be buying peak power to top up. ROI dies.

Discharge Test

Turn on high loads: ovens, electric heater, multiple appliances. Watch the inverter output. It should show 8–9 kW discharge (matching your 10 kW inverter). If it shows 2 kW, something is configured wrong.

Grid Isolation Test

If you have an ACS installed, manually disconnect from the grid (or ask your installer to do it). Verify that your house stays powered. Leave it isolated for 10 minutes, then reconnect. Automatic changeover should happen instantly—you shouldn't notice the switch.

Overnight Charge Cycle

Let the battery charge fully overnight (7 hours on off-peak tariff). Next morning, verify it's at 100%. Then run your house on battery power alone (ACS disconnected from grid) for 2–3 hours. Watch consumption, check the inverter output, ensure nothing trips.

Payment Hold

CRITICAL: Do not pay the full invoice until all tests pass. Payment strategy:

1. Pay when kit arrives on-site (not upfront)
2. Hold back 10–15% of contract value until commissioning tests complete
3. Only pay final 100% after you've verified charge, discharge, grid isolation, and overnight cycle

This gives you leverage if something is wrong. Most installers expect this and will cooperate.

Stage 5: Go Live & Month 1 Savings

Activation

Once tests pass, your installer configures the system for live operation. You'll activate your off-peak tariff with Octopus/EDF (usually online, takes 5 minutes). The battery will start charging automatically every night at the cheap rate.

First Bill Impact

Expect a ~75% drop in your electricity bill within the first month. If you usually pay £100/month for electricity at 30p/kWh, you'll pay ~£25/month at 7p/kWh (off-peak charging, plus some daytime grid use). This is not gradual. It's immediate.

Next Steps: Hot Water & Solar (Optional)

Immersion Heater: If you have one, you can heat your hot water tank overnight on cheap off-peak power instead of gas or peak electricity. Saves another 10–20% on energy bills. (Separate deep-dive needed here.)

Solar Panels: Optional, not required for the payback. If you want to add solar later:

Your 10 kW inverter can handle it (no inverter replacement needed)
But you'll need scaffolding, roof inspection, structural checks
Winter install is hard; better in spring/summer
Solar has the same VAT deadline (March 2027), but layering it now adds complexity
Decision: Battery-only first (simpler, faster install), add solar next year if ROI makes sense

The Timeline: Why You Start Now

September 2026: Size battery, pick installer, apply for G99
October–December: Wait for DNO approval (2–12 weeks is the range)
January–February 2027: Installation (1–3 days) + tests
March 2027: Go live, lock in 0% VAT savings

Miss this window? If you start in January 2027, DNO approval might not clear until April. You'd pay 5% VAT (or more). That's £400–650 extra on a £10k install.

Common Questions

What if my roof isn't suitable?

Batteries don't require a roof survey. They're installed indoors (garage, outhouse, utility room). Solar does require roof checks, but solar is optional.

What if DNO rejects me?

Rare. They'll either approve, impose an export limit (which is fine), or ask you to downsize. Worst case: 6 kW instead of 10 kW. Still profitable, just slower charge rate. Negotiate before accepting.

Can I do this without a gas boiler?

Yes. The 75% savings apply to all house loads: heating, hot water (via immersion or heat pump), cooking (induction hob), appliances. If you're all-electric or planning to be, this is even more valuable.

What if I don't have an EV?

Battery still works, but ROI is longer (5–7 years instead of 4–5). You're betting on grid instability and tariff arbitrage, not EV fuel savings. Still worth it, but consider whether an EV makes sense first—it changes the calculus dramatically.

The Decision: Start Now

Six months is tight. DNO approval alone can take 12 weeks. If you wait until January, you're gambling with the VAT deadline. There's no downside to calling an installer in September—surveys are free, G99 applications cost you nothing upfront, and you can walk away if it doesn't feel right.

But if you want 0% VAT and a 75% electricity bill drop, start today.

🔋 Size Your Battery Now

Upload your electricity bill to our AI adviser. Get a personalized recommendation in 2 minutes.

Recommended Installers

Thames Valley area? SPJ Electrical is personally recommended. They installed the Fogstar system at Rose Cottage and deliver quality work.

For installers elsewhere, start with Fogstar's installer network and look for MCS certification.