If you already charge your electric car overnight, start with the electricity your home buys during the more expensive hours. Sizing a home battery from your entire annual bill can leave you paying for capacity you rarely use.
The useful question is: how much daytime and evening electricity could you move into a cheaper charging window? That gives you a starting point for storage capacity. Your charging window, power demand and any solar panels then shape the final design.
Your EV tariff changes the calculation
A suitable battery can charge from the grid and supply your home later. The Energy Saving Trust’s energy storage guide explains how storage can work with time-of-use tariffs, including homes without solar panels. Sunergy’s off-peak battery charging service follows this approach.
As checked on 7 September 2026, Intelligent Octopus Go advertises six overnight hours for the home, from 11:30pm to 5:30am, alongside smart EV charging from 8p/kWh. Eligibility depends on a compatible EV or charger and suitable smart metering. Octopus Go lists a 00:30–05:30 window and requires an EV charged at home. Check your own quote and terms; these tariffs are not interchangeable.
Start with the electricity still bought at the day rate
Use a full year of half-hourly readings if available. Split consumption by the price actually paid, and subtract each off-peak unit only once. Here is an illustrative household without solar or an existing battery:
| Electricity use | Annual amount |
| Total bought from the grid | 6,000 kWh |
| EV charging already on the cheap rate | 3,000 kWh |
| Other use already on the cheap rate | 600 kWh |
| Remaining higher-rate use | 2,400 kWh |
| Average higher-rate use per day | 6.6 kWh |
Illustrative arithmetic: (6,000 − 3,000 − 600) ÷ 365 = 6.58 kWh/day. This is a worked example, not a measured customer result.
Dividing the entire bill by 365 gives 16.4 kWh a day. Yet the average load available to shift is only 6.6 kWh. Check winter, weekends and busier days before choosing a size: an annual average hides peaks.
Compare usable capacity, reserve and power
Battery capacity in kWh describes stored energy. Output in kW describes how quickly the system can supply it. You need both figures to compare quotes properly.
Ask how much energy reaches your circuits after operating limits, backup reserve and conversion losses. Do not subtract a reserve already excluded from the quoted usable capacity. The Energy Saving Trust’s battery guide explains why storage involves losses.
For illustration, 8 kWh nominal capacity × a 90% operating allowance × 95% discharge efficiency delivers about 6.8 kWh. These are assumptions, not product specifications.
A larger battery does not automatically provide more power. With 6 kW household demand and 3 kW battery output, the grid supplies the balance in normal grid-connected operation. Flag appliances that run together.
Can it recharge in the cheap window?
If recharging needs 11 kWh from the grid, an average 3 kW charge takes about 3.7 hours; at 2 kW it takes 5.5 hours. Allow for charging limits and tapering.
Car and battery charging controls must suit the property’s electrical supply. Discuss our home EV charger service alongside your battery design.
What could that battery save?
Compare the same home and tariff, with and without a battery. Do not count existing overnight EV tariff savings as a new battery saving.
| Illustrative assumption or result | Amount |
| Daily energy delivered / round-trip efficiency | 6 kWh / 90% |
| Day rate / off-peak rate, including VAT | 30p / 8p per kWh |
| Day-rate cost avoided / recharge cost | £1.80 / £0.53 |
| Operating saving before other costs | £1.27 per day |
Calculation: (6 × £0.30) − ((6 ÷ 0.90) × £0.08) = £1.2667/day. Rates and performance are example inputs, not a supplier quote.
Repeating this daily gives about £462 a year, before standby use, degradation, maintenance, purchase or finance costs. It assumes sufficient higher-rate demand every day, no solar, unchanged rates and the same standing charge. Actual savings may be lower.
How existing solar changes battery sizing
Your bill shows grid purchases, not total household consumption. Combine it with generation and export data. Do not subtract solar already used directly from grid imports a second time.
Overnight charging can leave less space for tomorrow’s solar. Where exports are paid by the unit, storing solar gives up some export income. Compare both routes seasonally. Extra capacity needs enough useful charging and discharging opportunities to justify its cost.
Our battery storage service covers existing solar installations and new systems.
Frequently asked questions
Is a 10 kWh battery enough for my house?
It may be. Compare energy available after operating limits with demand outside cheap-rate hours, then check output and recharge time. Electric heating can substantially change the design.
Should I include overnight EV charging in the battery size?
Usually exclude EV charging already bought directly at the cheap rate when estimating load to shift. Model daytime charging separately. Passing cheap grid electricity through two batteries adds losses without automatically improving savings.
Can I use a home battery without solar panels?
Yes, with a suitable system and tariff. Savings depend on demand you can shift, losses and installed cost. Owning a battery alone does not qualify you for an EV tariff.
Will it keep the lights on during a power cut?
Only with an appropriate backup arrangement. Supported circuits, output and runtime depend on the design and charge remaining. An outage reserve reduces capacity available for everyday savings.
Get a battery sized around your home
Send Sunergy your latest bill, tariff details, EV charging pattern and any solar generation figures. Half-hourly usage data helps us compare the storage you could actually use with the cost of installing it.
Based in Nuneaton, we help homeowners across Warwickshire, Leicester and Leicestershire plan solar and battery systems around their property. Request a battery assessment or call 02477 752 574.
Tariff information checked 7 September 2026. Recheck supplier rates and eligibility before switching. All worked examples are illustrative.
