Calculator
Battery Backup Runtime and Autonomy Calculator
Backup is the reason many households buy storage, and it is the specification most often misunderstood. The limiting factor is rarely energy; it is the inverter's continuous power rating and whether the system can restart the array once the grid is gone.
This calculator separates the two questions: can the system carry the load at all, and if so, for how long.
Describe the outage scenario
Fridge, router, lighting, controls.
Cooking, pumps, washing.
Zero for a night outage; enter expected generation for daytime.
Usable energy at outage start
5.5 kWh
80% start, 20% reserve
Essential load per day
13.8 kWh
Net daily draw after solar
13.8 kWh
Runtime on base load alone
12.3 hours
Days of autonomy
0.4 days
Peak power headroom
800 W
Inverter can carry the peak
How the calculation works
Energy available is the usable capacity multiplied by the difference between the starting state of charge and the reserve the system keeps back, then reduced by discharge and conversion efficiency. Most systems reserve 10 to 20% to protect cell life and to keep a black-start margin.
Load is split into a continuous base and a daily intermittent allowance, because the two behave differently: the base load determines the floor runtime, while intermittent loads consume the reserve in bursts.
The power check is separate and often binding. A battery with 10 kWh of energy is useless to a 5 kW load if its inverter is rated at 3.6 kW continuous. Surge ratings cover motor starting for a few seconds only.
If the array can operate during the outage — which requires a backup-capable hybrid inverter and a properly configured island mode — daytime generation extends autonomy indefinitely whenever it exceeds daily consumption.
The formula
E_avail = C_usable × (SoC_start − SoC_reserve)/100 × η L_day = P_base × 24/1000 + E_intermittent Net = L_day − E_solar Hours = E_avail × 1000 / P_base Days = E_avail / Net (infinite if Net ≤ 0) Headroom= P_inverter − P_peak
Assumptions behind the numbers
- The system has a true backup output with automatic transfer. A grid-tied inverter without island capability produces nothing during an outage regardless of battery state.
- Reserve state of charge is respected by the system and not available for load.
- Solar input during the outage assumes the inverter can run the array in island mode, which some products limit to a fraction of the array capacity.
- Efficiency covers battery discharge and DC-AC conversion but not standby consumption of the backup circuits, typically 20 to 60 W.
- Peak demand is the simultaneous total, not the sum of nameplate ratings — few appliances run at rated power at the same instant.
How to read the result
For most households the meaningful specification is one to two days on essential loads, not whole-house autonomy. Moving the fridge, lighting, router and boiler controls onto a backup sub-panel makes a 10 kWh battery last two days instead of eight hours.
If peak headroom is negative, no amount of extra capacity helps. Either raise the inverter rating, exclude the offending load — usually an electric shower, oven or heat pump — or accept manual load management during outages.
With daytime solar recharging and a modest essential load, autonomy becomes effectively unlimited in summer and drops sharply in December, when the same system may have only a few hours of margin.
Worked example
A 10 kWh battery with a 20% reserve is at 80% charge when the grid fails at 18:00. The essential circuits draw 450 W continuously and 3 kWh a day for cooking and pumps. The backup inverter is rated 3,600 W continuous, and the largest simultaneous demand is 2,800 W.
Available energy is 10 × 0.60 × 0.92 = 5.5 kWh. Daily load is 0.45 × 24 + 3 = 13.8 kWh. With no solar, the battery lasts about 0.4 days — around ten hours — or 12.3 hours if only the base load runs.
Add 8 kWh of daytime solar and the net draw falls to 5.8 kWh a day, extending autonomy to nearly a full day and refilling the battery on any bright morning. Peak headroom is 800 W, so the household can run the cooker but must not start the pump at the same time.
Mistakes that skew the answer
- Assuming any battery provides backup. Most grid-tied installations shut down in an outage for anti-islanding compliance unless a dedicated backup interface is fitted.
- Sizing on energy and ignoring the continuous power rating.
- Backing up the whole consumer unit instead of an essential-loads sub-panel, which drains the battery in hours.
- Forgetting the reserve state of charge, which removes 10 to 20% of the apparent capacity.
- Assuming the array works during an outage. Many string inverters need the grid as a reference and will not run in island mode.
Questions readers ask
- How long will a 10 kWh battery power my house?
- On whole-house loads averaging 1 kW, about eight hours after reserve and efficiency. On an essential-loads panel drawing 300 to 500 W, twelve to twenty hours, extending to days if daytime solar can recharge it.
- Do I need a special inverter for backup?
- Yes. You need a hybrid or backup-capable inverter with an automatic transfer switch and an island-mode capability, wired to a separate backup distribution board. Retrofitting this to an existing grid-tied system usually means replacing the inverter.
- Can my solar panels charge the battery during an outage?
- Only if the inverter supports island-mode PV operation. Many do, but some limit array output during islanding, and AC-coupled retrofits often need frequency-shift control to throttle the existing inverter. Confirm this in the product manual, not the brochure.
- What loads should go on the backup panel?
- Refrigeration, lighting, a few sockets for phones and laptops, the internet router, heating controls and circulation pumps, and any medical equipment. Keep electric showers, ovens, immersion heaters and EV chargers off it.
- Why does the battery keep a reserve?
- To protect cell life, to guarantee enough energy for the system to restart itself, and in grid-connected mode to keep a defined backup floor so an outage never finds the battery empty. Most systems let you set that floor between 5 and 30%.