How Many Solar Panels to Power a House? A 7-Step Sizing Checklist

Who this checklist is for

I'm the office administrator for a 30-person engineering company in Kidderminster. I handle energy equipment ordering, and I report to both operations and finance. I'm not an installer or an engineer. I'm the person who checks the quotes and signs the orders. This checklist is for anyone asking the same question I had to answer: how many solar panels to power a house—or a workshop.

In 2024, we went through a solar and battery project. It involved a Victron Energy solar system, three quotes, one missed deadline, and one big lesson about transparency. Here is the practical version of that lesson.

The short answer

If you are in the UK and your household uses about 2,700 kWh per year—Ofgem's medium figure for a typical home—you probably need only 8 to 12 panels to cover the annual bill. If you have electric heating, an EV, or a workshop, the number is more like 20 to 30 panels. But the panel count is not the hard part. The inverter, battery, protection, and the installer's assumptions are where most of my mistakes happened.

The 7-step checklist

Step 1: Pull your real electricity usage

Use 12 months of meter readings, not estimated bills. If you only have an annual total, divide by 365 to get a daily average. I nearly approved a quote based on an estimated bill that was 40% low. The installer was using the number the supplier printed on the invoice, and the supplier had not read the meter in months.

Checkpoint: you know your daily kWh usage within 20%. If you cannot get that number, you are not ready for a quote.

Step 2: Define what 'power a house' means

Do you want to offset all grid electricity over the year, or keep the lights on when the grid goes down? Those are different systems. A grid-tied system with a small battery is cheaper and probably enough for 90% of people. A full off-grid system is a different beast—solar alone rarely covers a UK winter without a generator or backup.

Write down one sentence: 'I want to offset ___% of my electricity.' Then decide if battery backup matters to you.

Step 3: Choose the inverter before you count panels

Panels do not directly power a house. They feed a solar controller or a hybrid inverter, and the inverter creates the AC power your sockets use. If you have a single-phase supply, a 5 kW or 8 kW inverter is common. For larger buildings or workshops, a 12kW 3 phase hybrid inverter might be the right class—one box that handles solar, battery and grid interaction.

In my opinion, a slightly bigger inverter is less painful than an undersized one. Check whether your supply is single-phase or three-phase. Check the inverter's DC input voltage range, not just the AC output rating. This is where the rookie mistake happens: choosing the inverter based on panel wattage, then finding out the array's cold-weather voltage is too high for the MPPT.

Step 4: Size the battery—or decide not to add one

If you arrived here from a 'home battery storage Kidderminster' search, this step will save you the most money. A battery does not need to match the panel array. It needs to match your evening load or your required backup time.

For self-consumption, start small: 5 to 10 kWh for a typical UK home is often enough. For backup, list the loads you genuinely need for 4 to 6 hours—fridge, internet, heating controls, a few lights. In a Victron Energy solar system, a battery monitor is worth more than additional capacity. Honestly, I would trust a system with a good monitor and a modest battery over a huge lead-acid bank with no visibility.

If your system is in a vehicle or boat, add a Victron Energy battery isolator so the starter battery is not drained by the house load. For a fixed home system, that role is filled by proper disconnects and a BMS. Same idea, different components.

Step 5: Now count the panels

Use the local annual yield, not the panel's theoretical output. According to the Global Solar Atlas, a south-facing 1 kWp array in the West Midlands can generate roughly 850 to 950 kWh per year.

Annual kWh used ÷ local annual yield per kWp = kWp needed.

If you use 2,700 kWh per year and assume 900 kWh per kWp, you need 3 kWp. With 400 W panels, that is 8 panels. If you use 10,000 kWh per year, you need about 11.8 kWp—that is 30 panels. Add roof geometry and shading into the calculation before you decide.

A rough daily formula for other locations: daily kWh ÷ (peak sun hours × 0.8) = kW DC. For Kidderminster, I trust the annual yield method more.

Step 6: Plan protection, isolation, and monitoring

This is the step most people ignore. Every system needs DC isolators, fuses, earthing, and a clear emergency shut-off. Lithium batteries need a BMS. Battery cables are not optional extras. If a vendor's quote does not mention protection and monitoring, ask why.

I made this mistake with DC cable runs. I knew I should check voltage drop on a long cable, but I thought 'what are the odds?' The odds caught up when the MPPT controller kept shutting down. It was not a solar panel problem; it was a cable sizing problem. The fix cost £300 more and two days of delay.

Checkpoint: ask for a single-line diagram. Make sure every fuse, isolator and disconnect is on it.

Step 7: Ask 'what's NOT included' before you ask 'what's the price'

This is the transparency lesson. I have learned to ask 'what's not included' before 'what's the price.' In 2024, we collected quotes from four suppliers for the same planned system. The lowest quote was £3,100 cheaper than the middle quote. It also did not include DNO notification fees, scaffolding, the second battery, or the official commissioning visit. By the time we added those items, it was £600 above the middle quote.

The vendor who lists all fees upfront—even if the total looks higher—usually costs less in the end. If a quote is one page with one total, it is not transparent. It is incomplete.

Common mistakes and notes

• Sizing by roof area instead of usage. The sun does not care how much roof you have. The panels only need to cover the energy you actually use.
• Buying the battery before the inverter. The inverter's charge output determines how fast the battery can recharge. Match the battery to the inverter's voltage and charging profile.
• Forgetting shading. One tree can cut string output by a surprising amount. If you have shading, ask for separate MPPT inputs or optimisers.
• Believing in zero maintenance. Panels need cleaning, fans need filters, and batteries prefer a stable temperature. There is no 'set-and-forget' system.
• Assuming the biggest array is the best. If your inverter is capped, extra panels can add cost without adding useful energy.

Bottom line

How many solar panels to power a house? Start with your real annual kWh, divide by your local yield, then ask the 'what's NOT included' question. A Victron Energy solar system—or any quality system—will only perform if the design basics are covered. I'm not an engineer, and you do not need to be one either. You just need a checklist and the confidence to ask for a line-item quote.

Sources: Ofgem typical domestic consumption values (2024, ofgem.gov.uk); Global Solar Atlas (globalsolaratlas.info, accessed January 2025). Prices and system figures are for general reference only—verify current numbers before ordering.