How Many Solar Panels Do I Need, And What Size Battery?

Step One: A Year Of Your Own Electricity Use

Your bills, or your supplier’s app, show the kWh your home used over the last 12 months. Start from that number. Two houses with the same number of bedrooms can sit thousands of kWh apart, depending on who is home in the day, how you cook and whether an electric car charges on the drive.

Without a year of bills to hand, use the government’s National Energy Efficiency Data-Framework (NEED). It gives the median electricity use for homes in England and Wales by number of bedrooms, from meter readings for February 2024 to January 2025, and half the homes in each group used more than the figure in the table below. Ofgem’s typical consumption values, revised from 1 July 2026, put a medium household at 2,500 kWh a year, a low user at 1,600 kWh and a high user at 3,800 kWh.

Step Two: What A kWp Makes At Your Address

Kilowatt peak is a panel’s rated output in a factory test, with 1,000 W of light falling on each square metre at 25°C. Your roof sees light that strong on clear summer days at best, so a 4 kWp system makes less than 4 kW through most of its working hours. The figure you need is the kWh each kWp gives you over a year where you live.

The European Commission’s PVGIS tool works that out from satellite sunshine records for 2005 to 2023. For a south-facing roof pitched at 35° in New Mills, with 14% allowed for system losses, it gives 853 kWh a year for each kWp. PVGIS also models the hills on the horizon, which counts in a town built down the sides of a valley. That 853, rounded, is the 850 in the sum above.

Move south and the divisor rises. PVGIS gives a south roof in central London 987 kWh per kWp, so a London home needs about 14% less system for the same use.

Step Three: Panel Wattage Today

Panel count is the system size divided by the wattage of one panel. SolarWatt’s own UK range shows how far wattage has climbed: its Panel vision GM 3.0 reaches 365 W, and the new Panel vision M 5.5 starts at 465 W. The M 5.5 frame measures 1,762 mm by 1,134 mm, about 2 m², and weighs 24.8 kg. SolarWatt makes longer panels at 515 W and above, and pitches the bigger frames at larger commercial roofs.

A 4 kWp system therefore takes ten 400 W panels or nine 465 W panels. Ask any quote to give you the kWp as well as the count, so you compare like with like.

How Many Solar Panels Do I Need? The Worked Table

Our arithmetic: the NEED median for each group, divided by 853 kWh per kWp (PVGIS, south-facing at 35° in New Mills), then by 465 W a panel and rounded up to a whole panel. Panel area counts the panels alone at 2 m² each, and an installer leaves clear space at the edges and below the ridge, so the slope has to be bigger than that.

For the three-bedroom question, the table gives about 3.1 kWp, seven panels and 14 m² of panel area. A home that uses 4,000 kWh needs about 4.7 kWp whatever its bedroom count, which is why your own bills beat the median.

Matching a year’s use on paper leaves the seasons out. For each kWp on a south roof in New Mills, PVGIS gives 109 kWh in June and 23 kWh in December. In summer the panels make more than the house uses through the middle of the day and the surplus goes to the grid, and in winter the grid supplies most of what you use. A battery moves the afternoon surplus into the same evening.

Roof Space And Direction Set The Limit

Count the panels your roof can take before the sum goes any further. Measure the slopes you’d use, allow 2 m² a panel plus clear space round the edges, and take off chimneys, dormers and roof windows. If the roof holds fewer panels than the table asks for, the design fits what the roof takes, and the battery and the tariff do more of the work.

Direction changes the divisor. PVGIS gives an east or a west slope in New Mills about 695 kWh per kWp a year, about a fifth less than south, so a home on an east-west roof divides its use by about 700 instead of 850. Splitting panels across both slopes, slate fixings, shade from stacks and gables and the case for a north slope each get their own section in our guide to solar panels on slate and north-facing roofs.

What Size Solar Battery Do I Need?

A battery earns its place by holding the surplus your panels make in the day and handing it back once they stop, so its size follows the electricity you use between late afternoon and the next morning. Four steps get you there:

1. 1

   Find your after-dark use. With a smart meter, many suppliers show half-hourly readings in their app or online account. Add up the kWh from about 4pm to breakfast on an ordinary weekday, and check a winter day as well as a summer one.

2. 2

   Cap it at a day’s use. A battery bigger than the electricity you use in a day won’t empty on an ordinary day, and the extra capacity sits idle. The daily averages in the table above give you a ceiling.

3. 3

   Check the panels can fill it. A 3 kWp array in New Mills makes about 11 kWh on an average June day and about 2 kWh on an average December day (PVGIS). In winter a battery fills from a cheap overnight tariff more than from the roof, so the tariff belongs in the sizing too.

4. 4

   Compare usable capacity. A quote should state the kWh you can draw out. SolarWatt’s Battery vision 2.0 comes in packs of 4.3 kWh usable, from 8.6 kWh to 25.9 kWh, and the Battery vision 1.0 in packs of 2.6 kWh, from 5.2 kWh to 18.2 kWh.

Take the median three-bedroom home at 7.3 kWh a day and say half of that falls after dark: 3.7 kWh. A 5.2 kWh battery covers that, with room for an evening when the oven and the washing machine both run late. A 13 kWh battery on the same house could hand back no more than the 3.7 kWh the house uses on a night like that. Your own half-hourly readings replace the “say half”.

We size the battery from your usage and your tariff, and the payback workings go into the written quote. For some houses the straight answer is to wait, and our guide on whether solar and battery storage is worth it covers the payback side. The battery storage installation page sets out how we fit one, including a battery added to panels you already have.

A Heat Pump Changes The Sum

Moving from a gas boiler to a heat pump puts your heating on the electricity meter, so a solar design for a home with a heat pump on the way needs the bigger number.

The sum takes a year of gas use and divides it by the heat pump’s SCOP, the seasonal coefficient of performance: the units of heat it gives back for each unit of electricity over a heating season.

Our arithmetic: the NEED median gas use for each group (gas year mid-May 2024 to mid-May 2025) divided by 3.5, added to the electricity medians in the first table, then sized the same way. The gas figure includes cooking on a gas hob and the heat a boiler loses up the flue, so the heat a heat pump has to supply sits lower and these added figures run high. SCOP 3.5 is the bottom of the 3.5 to 4.0 our heat pump designs aim for where the house allows it, and your heat loss survey replaces the whole column with your own rooms.

Fifteen panels for the three-bedroom home means about 30 m² of panel area. Measure your best slope against that figure before you plan on covering a heat pump from the roof.

The calendar sets the harder limit. Your heating runs hardest in December and January, when a south roof in New Mills makes 23 kWh per kWp a month. Panels on a heat pump home earn most from April to September, heating water and running the house, and the winter heating draws on the grid, where an off-peak tariff and a battery charged overnight bring the cost down. SolarWatt builds its battery to grow by adding packs, and names a heat pump fitted later as a reason to add them.

If a heat pump is on your list, plan it now, so the inverter, the battery and the wiring get sized for the larger load and nothing needs redoing. Our heat pump installation page covers the room-by-room design behind the SCOP figure, and the full renewables package puts the heat pump, the panels and the battery under one design.

Solar Sizing Within About 20 Miles Of New Mills

Our surveys run within about 20 miles of our New Mills base: Manchester and Stockport, the High Peak from Glossop and Marple to Whaley Bridge and Buxton, and the Hope Valley toward Sheffield. The divisor moves across that patch. PVGIS gives a south roof at 35° about 813 kWh per kWp a year in Glossop, 850 in Stockport, 861 in central Manchester, 873 in Buxton and 900 at Hope.

That gap matters to your quote. A home in Glossop needs about 5% more panels than the same home in Stockport for the same output, and the design accounts for it before anyone names a price. PVGIS models the hills, while the gable next door and the tree at the bottom of the garden need someone on site, so the survey checks the horizon from your own roof. Our pages on solar panels and battery storage in the High Peak and solar panels and batteries in Stockport cover each area.

How First Point Sizes A System

Your usage sets the size and your roof sets the layout, and you see both numbers before anyone talks about panel count.

If the numbers don’t work for your house, you hear that before you spend anything.